Patent Publication Number: US-2022217053-A1

Title: Organizing execution of distributed operating systems for network devices

Description:
This application is a continuation of U.S. patent application Ser. No. 15/637,809, filed 29 Jun. 2017, which claims the benefit of U.S. Provisional Patent Application No. 62/479,804, filed 31 Mar. 2017; U.S. patent application Ser. No. 15/637,809, filed 29 Jun. 2017, claims the benefit of U.S. Provisional Patent Application No. 62/437,369, filed 21 Dec. 2016, the entire content of each application is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates to network devices and, more particularly, to operating systems for network devices. 
     BACKGROUND 
     Operating systems for network devices, such as routers, function to provide an execution environment in which various applications (such as network protocols, maintenance interfaces, virtualization applications, etc.) may execute. One function of the operating system of a router is to establish a way by which state information may be propagated to the various components or, in other words, computing nodes of the router to allow the router to correctly route network traffic within a network. 
     A router may, for example, maintain state information representing the current state of the interfaces between the router and the network. Such state information may include information representing the state of one or more IFCs, such as the current configuration of the IFCs. As additional examples, a router may maintain state information representing the state of one or more packet forwarding engines (PFEs), one or more routing engines, or other resources within the router. 
     A control node referred to as a “routing engine” operating within the router may execute an instance of the operating system to propagate the state information (and changes thereto) to various other processes or components within the router. These other processes or components are sometimes referred to as “consumers,” because they receive and utilize (or, in other words, “consume”) the state information maintained by the instance of the operating system. These consumers make use of the state information when performing their various functions. 
     As the complexity of conventional networks has increased in recent years, management of the state information within a router or other network device has likewise become a significant challenge. Some existing methods for managing state information involve caching the information within the instance of the operating system, and issuing state update notification messages to consumers executing within the router. In response, the consumers retrieve the state information from the instance of the operating system. 
     To increase reliability, some routers may include a primary routing engine and one or more standby routing engines, each of which may execute a separate and distinct instance of the operating system to manage the state information. In the event that the primary routing engine fails, one of the standby routing engines assumes control of the routing resources to continue operation of the router. The process of switching control of routing functions between the primary and standby routing engines is often referred to as failover. In some instances, to assume proper control and ensure operation, the standby routing engine is forced to “relearn” the lost state information from each resource, e.g., by power cycling the router resources to a known state. This causes an interruption in packet forwarding while the router resources restart operations as the instance of the operating system executed by the standby routing engines rebuilds the correct state information. 
     Routers have not only developed to be more reliable, but also to meet increasing bandwidth demands. One way to meet increasing bandwidth needs is to use multi-chassis routers, i.e., routers in which multiple routing devices are physically coupled and configured to operate as a single router. For example, a multi-chassis router may contain multiple line card chassis (LCCs), which include one or more IFCs, and a central switch card chassis (SCC), which forward packets between the LCCs and provides top-down management of the multi-chassis router. Because multi-chassis routers combine resources of multiple routing devices, multi-chassis routers typically have much higher bandwidth capabilities than standalone routers. The use of multi-chassis routers can simplify and improve routing on a service provider network by consolidating routing functions onto fewer routers. 
     However, multi-chassis routers may result in a large number of different components (such as routing engines) each executing a different instance of the operating system that is required to correctly maintain the state information and communicate changes to the state information to downstream consumers. That is, the multi-chassis router may include, in addition to multiple routing engines that each execute a different instance of the operating system, SCCs and LCCs that also include control nodes that execute yet another instance of the operating system, all of which require at least some portion of the state information and propagation of the state information to some if not all of the various consumers. 
     SUMMARY 
     Techniques are described for providing a distributed operating system for network devices that may allow for dynamic expansion or contraction (or, in other words, “elasticity”) of underlying hardware resources while also potentially providing robust convergence of state information across producing components (so-called “producers”) and consuming components (so-called “consumers”). The operating system may be distributed across computing nodes (which may also be referred to as “hardware computing nodes,” “computing nodes” or “nodes”), which may include routing engines, interface cards, packet forwarding engines, as well as non-networking nodes, such as processors, central processing units (CPUs), application specific integrated circuits (ASICs), graphical processing units (GPUs). The computing nodes may initially coalesce, starting from a kernel, detecting one another via an object flooding protocol (OFP) premised upon topology discovery similar to link state routing protocols, and organizing via a management process (referred to as “SysEpochMan”) to execute the distributed operating system. 
     The distributed operating system may, once booted across the computing nodes, allow for real-time (or, near-real-time) construction of a synchronization tree for synchronizing databases of state information maintained by the distributed operating system. The operating system may synchronize the databases using OFP, while also potentially pruning the databases and reducing bandwidth requirements. The operating system may handle coherence among the computing nodes executing instances of the distributed operating system using a systematic process, referred to as a “system epoch” so that coherent state information may be maintained by each instance in the event of various connectivity or failures of the instances of the distributed operating system. 
     The distributed operating system may avoid redundant execution of separate instances of the same operating system, while simplifying propagation of state information by way of flooding in the form of multicast delivery of state information. Furthermore, the distributed operating system may be resilient to computing node failure allowing for individual computing nodes supporting the distributed operating system to fail without requiring the reboot of the remaining computing nodes supporting execution of the single instance of the distributed operating system. 
     Computing nodes of a network device executing respective instances of the distributed operating system may be configured to store state information in respective data structures, such as tree data structures. The computing nodes of the network device may represent the state information as message fragments, where each message fragment is stored in a tree node of the tree data structure. Furthermore, the computing nodes of the network device may synchronize the tree data structures using OFP, by flooding objects representative of the message fragments to the other computing nodes. For example, when one of the computing nodes of the network device receives updated state information, the one of the computing nodes of the network device may update its local tree data structure, then flood the updated state information to the other computing nodes of the network device in accordance with OFP. In this manner, the computing nodes can maintain synchronization between their respective data structures for storing state information for the distributed operating system and/or applications executed in an application space provided by the distributed operating system. 
     In one example, a method comprises receiving, by a first computing node, implemented in circuitry, of a network device that executes a first instance of a distributed operating system, updated state information for at least one of the distributed operating system or an application executed in an application space provided by the distributed operating system. The method also comprises updating, by the first computing node of the network device, a local data structure of the first computing node of the network device to include the updated state information, the local data structure storing a plurality of objects, each of the objects defining a portion of state information for at least one of the distributed operating system or the application. The method further comprises synchronizing, by the first computing node of the network device, the updated local data structure with a remote data structure of a second instance of the distributed operating system executed by a second node, implemented in circuitry, of the network device. 
     In another example, a network device comprises a first hardware node implemented in circuitry, and a second hardware node implemented in circuitry. The first hardware node is configured to execute a first instance of a distributed operating system, and maintain a first data structure that stores a plurality of objects, each of the objects defining a portion of state information for at least one of the distributed operating system or an application executed in an application space provided by the distributed operating system. The second hardware node is configured to execute a second instance of the distributed operating system, and maintain a second data structure that stores synchronized versions of the plurality of objects. The first hardware node is further configured to receive updated state information for at least one of the distributed operating system or the application, update the first data structure to include the updated state information, and synchronize the updated first data structure with the second data structure through execution of the first instance of the distributed operating system. The second hardware node is further configured to synchronize the second data structure with the updated first data structure through execution of the second instance of the distributed operating system. 
     In another example, a non-transitory computer-readable storage medium has stored thereon instructions that, when executed, cause a first processor of a first computing node of a network device to execute a first instance of a distributed operating system to receive updated state information for at least one of the distributed operating system or an application executed in an application space provided by the distributed operating system, update a local data structure of the first computing node of the network device to include the updated state information, the local data structure storing a plurality of objects, each of the objects defining a portion of state information for at least one of the distributed operating system or the application, and synchronize the updated local data structure with a remote data structure of a second instance of the distributed operating system executed by a second computing node of the network device. 
     In another example, a device comprises a plurality of hardware computing nodes configured to execute a protocol by which to discover a topology of the plurality of hardware computing nodes, and determine, based on the topology, a subset of the plurality of hardware computing nodes to manage execution of a distributed operating system. The determined subset of the plurality of hardware computing nodes are further configured to execute a communication bus by which to synchronize operating system state information between the subset of the plurality of hardware computing nodes. The plurality of hardware computing nodes are further configured to execute, based on the operating system state information, the distributed operating system to provide an execution environment in which one or more applications execute. 
     In another example, a method comprises executing, by a plurality of hardware computing nodes, a protocol by which to discover a topology of the plurality of hardware computing nodes, and determining, by at least one of the plurality of hardware computing nodes and based on the topology, a subset of the plurality of hardware computing nodes to manage execution of a distributed operating system. The method also comprises executing, by the determined subset of the plurality of hardware computing nodes, a communication bus by which to synchronize operating system state information between the subset of the plurality of hardware computing nodes, and executing, by the plurality of hardware computing nodes and based on the operating system state information, the distributed operating system to provide an execution environment in which one or more applications execute. 
     In another example, a non-transitory computer-readable storage medium has stored thereon instructions that, when executed, cause one or more of a plurality of hardware computing nodes to execute a protocol by which to discover a topology of the plurality of hardware computing nodes, determine, based on the topology, a subset of the plurality of hardware computing nodes to manage execution of a distributed operating system, execute a communication bus by which to synchronize operating system state information between the subset of the plurality of hardware computing nodes, and execute, based on the operating system state information, the distributed operating system to provide an execution environment in which one or more applications execute. 
     In another example, a network device comprises a plurality of hardware computing nodes configured to execute a distributed operating system, at least one of the plurality of hardware computing nodes configured to determine whether one or more of the plurality of hardware computing nodes has failed and is no longer supporting execution of the distributed operating system. The at least one of the plurality of hardware computing nodes are further configured to determine whether remaining ones of the plurality of hardware computing nodes exceeds a quorum threshold, and restart, when the remaining ones of the plurality of hardware computing nodes is less than the quorum threshold, the distributed operating system. 
     In another example, a method comprises determine, by at least one of a plurality of hardware computing nodes included within a network device, whether one or more of the plurality of hardware computing nodes has failed, determine, by the at least one of the plurality of hardware computing nodes, whether remaining ones of the plurality of hardware computing nodes exceeds a quorum threshold, and restart, by the at least one of the plurality of hardware computing nodes and when the remaining ones of the plurality of hardware computing nodes is less than the quorum threshold, the distributed operating system. 
     In another example, a non-transitory computer-readable storage medium has stored thereon instructions that, when executed, cause one or more processors of a network device to determine whether one or more of a plurality of hardware computing nodes executing a distributed operating system has failed, determine whether remaining ones of the plurality of hardware computing nodes exceeds a quorum threshold, and restart, when the remaining ones of the plurality of hardware computing nodes is less than the quorum threshold, the distributed operating system. 
     The details of one or more aspects of the techniques are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the techniques will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is block diagram of an example network computing environment in which a service-provider network includes a multi-chassis router configured to operate in accordance with the distributed operating system techniques described in this disclosure. 
         FIG. 2  is a block diagram illustrating an exemplary multi-chassis router configured to operate in accordance with the distributed operating system techniques described in this disclosure. 
         FIG. 3  is a block diagram illustrating an example node of multi-chassis router shown in  FIG. 2  configured to operate in accordance with various aspects of the distributed operating system techniques described in this disclosure. 
         FIG. 4A-8B  are block diagrams illustrating various aspects of node operation within the multi-chassis router shown in  FIG. 2  in addressing various scenarios that may impact execution of the distributed operating system maintained in accordance with the techniques described in this disclosure. 
         FIG. 9  is a flowchart illustrating exemplary operation of the node of the multi-chassis router shown in  FIG. 3  in performing various aspects of the distributed operating system techniques described in this disclosure. 
         FIG. 10  is a conceptual diagram illustrating an example tree data structure for storing state information in accordance with techniques of this disclosure. 
         FIG. 11  is a flowchart illustrating an example method for synchronizing state information between different instances of a distributed operating system executed by respective computing nodes of a network device in accordance with the techniques of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram illustrating an example computing environment  2  in which service provider network  6  includes a multi-chassis router  4 . In this example, multi-chassis router  4  communicates with edge routers  5 A and  5 B (“edge routers  5 ”) to provide customer networks  8 A- 8 C (“customer networks  8 ”) with access to network  6 . In one embodiment, multi-chassis router  4  includes a central switch card chassis (SCC) that operates as a control node and one or more line card chassis (LCCs) that operate as packet routing devices. The LCCs may contain all the physical interfaces for coupling to other devices within network  6 , while the SCC controls and routes traffic between the LCCs. 
     Although not illustrated, service provider network  6  may be coupled to one or more networks administered by other providers, and may thus form part of a large-scale public network infrastructure, e.g., the Internet. Consequently, customer networks  8  may be viewed as edge networks of the Internet. Service provider network  6  may provide computing devices within customer networks  8  with access to the Internet, and may allow the computing devices within customer networks  8  to communicate with each other. Service provider network  6  may include a variety of network devices other than multi-chassis router  4  and edge routers  5 , such as additional routers, switches, servers, or other devices. 
     In the illustrated embodiment, edge router  5 A is coupled to customer network  8 A via access link  9 A and edge router  5 B is coupled to customer networks  8 B and  8 C via access links  9 B and  9 C, respectively. Customer networks  8  may be networks for geographically separated sites of an enterprise. Customer networks  8  may include one or more computing devices (not shown), such as personal computers, laptop computers, handheld computers, cellular phones (including so-called “smart phones”), tablet computers, workstations, servers, switches, printers, or other devices. The configuration of network  2  illustrated in  FIG. 1  is merely exemplary. For example, service provider network  6  may be coupled to any number of customer networks  8 . Nonetheless, for ease of description, only customer networks  8 A- 8 C are illustrated in  FIG. 1 . 
     Multi-chassis router  4  may provide for failover by including a primary routing engine as well as one or more standby routing engines. For example, an SCC may contain primary and standby master routing engines, and one or more LCCs may contain primary and standby local routing engines. Primary master routing engine may propagate state information to the standby master engine prior to forwarding the state information to the primary local routing engines in the LCCs. Similarly, the primary local routing engines propagate the state information to one or more standby local routing engines prior to forwarding the state information to consumer components (which may be referred to as “consumers”) within their chassis. In this manner, multi-chassis router  4  enforces a synchronization gradient when communicating state information throughout the multi-chassis environment. 
     In the event a primary routing engine fails, a standby routing engine in the same chassis assumes control over routing resources and routing functionality for that chassis. Moreover, because state information is propagated to a standby routing engine prior to forwarding the state information to a consumer, a standby routing engine can take up forwarding state information to consumers at the same place where the primary routing engine left off. U.S. Pat. No. 7,739,403 titled “Syncronizing State Information Between Control Units”, filed Oct. 3, 2003, describes techniques for a synchronization gradient within a standalone router and is hereby incorporated by reference. U.S. Pat. No. 7,518,986 titled “Push-Based Hierarchical State Propagation within a Multi-Chassis Network Device,” filed Nov. 16, 2005, describes techniques for a push-based state synchronization within multi-chassis routers and is hereby incorporated by reference. In this manner, the primary and standby routing engines synchronize their respective state information to allow the standby routing engine to assume control of the router resources without having to relearn state information. 
     In instances where the primary master routing engine pushes state information to each consumer, each of the consumers receives any state information produced by the primary master routing engine. As networks become larger and more complex in terms of the number of services offered, the primary master routing engine may likewise produce more state information concerning the additional services, which has to be consumed by a potentially larger number of consumers (especially in the context of distributed systems, such as software defined networks having hundreds of computing nodes acting as consumers, or in large scale data centers having potentially hundreds of computing nodes acting as consumers). Producer components (which may also be referred to as “producers”) may refer to any of the above noted components that produce state information, such as the primary master routing engine, primary local routing engines, and the like. Consumers may denote any of the above noted components that consume state information, such as primary local routing engines, interface cards, and the like. 
     In these circumstances, consumers may become inundated with state information that may or may not be relevant to the role of the consumers in multi-chassis router  4 . Consumers may therefore receive a large amount of state information that must be processed to determine whether such state information is relevant, discarding any state information that the consumer does not require in order to perform the operations for which the consumers are configured to perform. Distribution of state information according to the push model where consumers passively receive all state information produced by producers (e.g., the primary master routing engine) may not adapt well as networks grow and become increasingly more complex (in terms of services offered, protocols executed, and the like). 
     Furthermore, the push-model for propagating state information is premised upon the producers and at least some of the consumers (e.g., subordinate routing engines, such as the primary routing engines of the LCCs and any of the standby routing engines) each executing a distinct instance of the operating system. Each distinct instance of the operating system may require some subset (up to and including, in some instances, all) of the state information. Any one instance of the operating system that may fail (e.g., due to hardware failure, loss of power, corrupt memory, etc.) may impact operation of the multi-chassis router, potentially resulting in loss of packets or an interruption of packet forwarding. 
     In accordance with the techniques described in this disclosure, multi-chassis router  4  may be configured to execute a single instance of an operating system  10  across all computing nodes (which may collectively refer to all producers and consumers) of multi-chassis router  4 . The operating system described in this disclosure may be referred to as a distributed operating system  10  (“DOS  10 ”) in that execution is distributed across all computing nodes. Each of the computing nodes may self-organize, coalescing so as to execute the single instance of distributed operating system  10 . The computing nodes may include hardware computing nodes (such as routing engines, hardware forwarding units—which may include application specific integrated circuits, and interface cards) and virtual computing nodes executed by hardware control units (e.g., one or more processors, one or more application specific integrated circuits, field-programmable gate arrays, etc.). 
     As such, when a computing node within multi-chassis router  4  fails, the remaining computing nodes may continue to execute the single instance of distributed operating system  10 , potentially without impacting packet forwarding or other operation of multi-chassis router  4 . In other words, the number of computing nodes supporting execution of the single instance of distributed operating system  10  may expand and contract without, in some instances, impacting operation of multi-chassis router  4 . In this sense, the distributed operating system may be considered to be fully elastic as the number of computing nodes may grow or shrink (to some extent). 
     After coalescing to execute the single instance of distributed operating system  10  (which may be referred to as the “distributed operating system”), the computing nodes may execute a distributed application suite within the execution environment provided by the distributed operating system. Applications, in the context of multi-chassis router  4 , may include network protocols, such as routing protocols, management protocols, management interfaces (graphical user interfaces (GUIs), command line interfaces (CLIs), etc.), communication protocols, and the like. 
     Distributed operating system  10  may distribute application among the computing nodes supporting execution of distributed operating system  10  based on computing node capability and/or role within multi-chassis router  4 . Distributed operating system  10  may manage propagation of state information in support of the execution of distributed operating system  10  and/or the applications executing within the execution environment provided by distributed operating system  10 . 
     Distributed operating system  10  may utilize a hierarchical naming scheme for propagating state information from producers to consumers. Rather than push all state information produced by every producer to each and every consumer, multi-chassis router  4  may establish the hierarchical naming scheme so as to associate objects (which may refer to a discrete portion of state information) with a scope of distribution, which results in distribution of the objects only to those consumers that have requested the particular object. Using the hierarchical naming scheme, a consumer within multi-chassis router  4  may request any scope of state information up to and including all of the state information produced by any producer within multi-chassis router  4 , and down to an individual object. More information regarding the hierarchical naming scheme and how the hierarchical naming scheme may more efficiently propagate state information using an object flooding protocol (OFP) can be found in U.S. application Ser. No. 15/198,912, entitled “HIERARCHICAL NAMING SCHEME FOR STATE PROPAGATION WITHIN NETWORK DEVICES,” filed Jun. 30, 2016, the entire contents of which are hereby incorporated by reference as if set forth in its entirety. 
     In operation, each of the computing nodes of multi-chassis router  4  may first execute the same infrastructure to support execution of distributed operating system  10 . For example, each of the computing nodes of multi-chassis router  4  may execute a kernel  12 , such as a Unix® kernel. Execution of each instance of kernel  12  is considered “separate” at this point only because the computing nodes have not yet coalesced to support execution of distributed operating system  10 . After self-organizing (or, in other words, coalescing), the computing nodes may execute a single distributed kernel  12  to the extent that kernel  12  is aware of applications and/or other processes executed by other computing nodes. Executing a uniform kernel  12  across all of the computing nodes may improve reliability in that kernel  12  may only rarely be updated, allowing for upgrades further up software stack  11  to occur without reboots (as the kernel may not change between upgrade releases). Separating kernel  12  from other aspects of distributed operating system  10  may also decouple the update cycle for kernel  12  from that of other processes or features of distributed operating system  10 . 
     After executing the separate instances of kernel  12 , each computing node may next execute OFP  14 . OFP  14 , as noted above, may propagate the state information between the computing nodes of distributed operating system  10 . As noted above, OFP  14  may provide a subscription model for state information propagation, thereby allowing for potentially more efficient propagation of state information compared to a push-model of state information propagation. OFP  14  may allow for a multicast type of state information propagation that may reliably deliver state information to multiple computing nodes of distributed operating system  10  concurrently. 
     OFP  14  may also allow for self-assembly, where OFP  14  provides a mechanism by which to discover computing nodes available to participate in execution of distributed operating system  10 , and the link interconnecting the computing nodes. OFP  14  may generate a graph data structure representative of a topology of the computing nodes and links, with edges of the graph data structure representing the links interconnecting computing nodes, and the graph nodes of the graph data structure representing the computing nodes available to support execution of distributed operating system  10 . The graph nodes are referred to as graph nodes to distinguish from the nodes of the graph data structure from the computing nodes supporting execution of distributed operating system  10 . Reference to “nodes” in this disclosure is intended to refer to the nodes supporting execution of distributed operating system  10  and not the graph nodes of the graph data structure unless explicitly noted elsewhere or clearly implied by context. OFP  14  may also provide node reachability services to determine liveliness of nodes. 
     After initializing OFP  14 , each of the computing nodes of multi-chassis router  4  may next execute a system epoch management (SysEpochMan) process  16 . SysEpochMan process  16  may organize the (up to this point, distinct and separate) computing nodes to support execution of single distributed operating system  10 . SysEpochMan process  16  may also monitor distributed operating system  10  to ensure integrity should one or more computing nodes fail. SysEpochMan process  16  may provide for transitions from the previous system state to the new system state in the event of, to provide a few examples, changes to the number of computing nodes, interruption in inter-node connection, the organization of the computing nodes, and/or changes in computing node roles. 
     SysEpochMan process  16  may establish (and maintain) a Zookeeper® plane (where Zookeeper® refers to the Apache Zookeeper® project) and the OFP domain (which may refer to an OFP domain for use by distributed operating system  10  to propagate state information particular to distributed operating system  10  and not related to applications). While described with respect to Zookeeper®, the techniques of this disclosure may be performed with respect to any inter-process communication bus or mechanism. As such, Zookeeper® is referred to throughout this disclosure more generally as an inter-process communication bus  18  (“IPCB  18 ”). 
     IPCB  18  may differ from OFP  14  in that OFP  14  is an asynchronous communication protocol (meaning that OFP may guarantee eventual object delivery without ensuring ordered delivery of the objects) while IPCB  18  is a synchronous communication protocol (meaning that IPCB  18  may ensure delivery with proper ordering of changes, or in other words, all computing nodes receive the changes in the order the changes occur). IPCB  18  may execute within the SysEpochMan process  16  to coordinate services such as leader election (within the computing nodes) and namespace allocation. 
     After forming IPCB  18  (and assuming OFP  14  is operational), the computing nodes of multi-chassis router  4  may effectively communicate with one another to coalesce and execute distributed operating system  10 . The computing nodes may next execute a system manager (“SysMan”) process  20  that coordinates the execution of applications within the execution environment provided by the distributed operating system. Each of SysMan processes  20  may elect a SysMan master instance (e.g., using IPCB  18 ), which may be responsible for execution of applications on particular computing nodes according to, as one example, a policy engine. 
     The SysMan master process may communicate (e.g., via IPCB  18 ) the application decisions to the local SysMan processes that then act on the application decisions to execute the applications. The local SysMan processes monitor the executing of the applications and provide a status of the application to the SysMan master process to allow the SysMan master process to monitor the status of the applications. When the status indicates that an application has failed, the SysMan master process may, to provide a few examples, reinitiate execution of the application (by the same or a different computing node) or activate a standby instance of the application. 
     The computing nodes of multi-chassis router  4  may also execute a distributor process  22  as part of distributed operation system  10 . Distributor process  22  (which may also be referred to as the “distributor  22 ”) may form an object daemon data store (DDS) and coordinate with individual applications for delivery of state information. Distributor  22  may operate as a client to OFP  14 , and deliver objects between distributors executed by the different computing nodes. 
     As noted above, distributed operating system  10  executes to provide an execution environment in which the applications may operate. From the perspective of the distributed operating system, the computing nodes are all uniform and only distinguishable by which applications each computing node executes. Applications may refer to any process that is not described above with respect to distributed operating systems  10 , including Unix® daemons, and PFE applications (or, in other words, software) other than low level drivers and/or firmware. 
     SysMan  20  may distribute applications across multiple computing nodes, using objects to communicate the state information associated with these types of distributed applications. For example, multi-chassis router  4  may execute an application including the routing protocol daemon (RPD) and a collection of one or more PFE route handlers. 
     SysMan process  18  does not bind applications to particular hardware, thereby allowing application mobility (which may also be referred to as “process mobility”). SysMan process  18  may transfer applications between processing units or other hardware within a given computing node or between computing nodes to provide for failure recover, load balancing, and/or in-service system updates (ISSU). 
     As noted above, distributed operating system  10  initially executes OFP  14  to determine a topology of computing nodes that allows for coalescence and execution by the computing nodes of the single instance of distributed operating system  10 . OFP physical topology discovery may occur in a manner similar to that of link state protocols. OFP  14  may “discover” the links to which the particular instance of OFP  14  is coupled by configuration. That is, a network administrator or other operator may configure OFP  14  with the links to which each particular instance of OFP  14  is coupled. OFP  14  discovers computing nodes using an announcement protocol by which each computing node periodically multicasts the computing node identity on each link to which that computing node is connected. 
     OFP  14  classifies each computing node as a primary computing node and a secondary computing node. An administrator may configure computing nodes as either primary or secondary, where an example of primary computing nodes may include a routing engine (or, in other words, a computing node supporting a control plane), and an example of a secondary computing node may include a line card (or, in other words, a computing node supporting the forwarding plane). In some instances, the primary computing nodes may refer to any computing node with enhanced processing and/or memory capabilities in comparison to secondary computing nodes. OFP  14  may attempt to offload as much processing to primary computing nodes given the enhanced processing and memory capabilities. 
     The primary OFP computing nodes may send the announcement to all computing nodes participating in distributed operating system  10  (meaning all primary and secondary OFP computing nodes). The secondary OFP computing nodes may send the announcement to all primary OFP computing nodes (and not, in some examples, all of the secondary OFP computing nodes). Although described as not transmitting announcements to primary OFP computing nodes, secondary OFP computing nodes may, in some examples, transmit announcements to the one or more secondary OFP computing nodes. 
     Each OFP computing node that receives the announcement configures the physical topology graph data structure to identify the announcing computing node as a neighbor. Assuming that secondary computing nodes only transmit announcements to primary OFP computing nodes, secondary OFP computing nodes cannot become neighbors with one another as the secondary OFP computing nodes never receive an announcement from another secondary OFP computing nodes by which to establish the neighbor relationship. OFP  14  constructs the graph data structure representative of the topology of primary and secondary computing nodes interconnected with one another by the links based on the announcements. 
     SysEpochMan  16  may, after OFP  14  constructs the graph data structure representative of the topology of primary and secondary computing nodes, elect an epoch manager master from among those computing nodes configured to execute as epoch managers. As one example, a network administrator may configure computing nodes capable of executing inter-process communication bus  18  (IPCB—e.g., Zookeeper®) as epoch managers. The elected epoch manager master may elect one or more of the epoch managers (including the elected epoch manager master) to act as epoch managers. Each of the epoch managers may then execute IPCB  18 . 
     IPCB  18  forms a network of servers and clients. The servers may be referred to as an IPCB ensemble. IPCB  18  may utilize a quorum system in which a majority of servers (e.g., more than (N/2)+1, where N represents the number of servers/epoch managers) are connected and functioning for IPCB  18  to continue successful operation. IPCB clients represent computing nodes that utilize IPCB  18 . The IPCB clients may interface with any IPCB server to utilize IPCB  18 . Utilizing IPCB  18 , the IPCB clients may interact with a shared file system to write data to and/or read data from the shared file system, while also being able to configure notifications with regard to changes to the shared file system. In this way, the techniques may allow for separate (or in other words individual) computing nodes to coalesce for purposes of executing distributed operating system  10 . 
     Upon successfully launching distributed operating system  10 , distributed operating system  10  may present another OFP domain for use by the applications in propagating state information from producers to consumers. For example, the computing nodes of multi-chassis router  4  may synchronize state information for distributed operating system  10 , the applications, or other elements of multi-chassis router  4 . In particular, each of the computing nodes may instantiate a respective data structure that stores a plurality of objects, where each of the objects defines at least a portion of the state information, e.g., for distributed operating system  10  and/or for one or more of the applications. The computing nodes may synchronize the respective data structures according to the OFP domain, by executing OFP  14 . Furthermore, the computing nodes may use the synchronized data structures for configuration, e.g., of themselves and/or other components of the computing nodes. 
     A radix trie is a tree that is structured by its keys, for which every interior trie node has at least two children. To locate a trie node with a particular key, the tree is walked by examining the contents of the key, starting at the left-hand side. A radix trie is a minimal tree in that there are no internal trie nodes with only a single child. A Patricia Trie is a particular form of a radix trie. 
     In some examples, the computing nodes may instantiate the data structures for storing the plurality of objects as tree data structures, such as radix tries. For example, the computing nodes executing various instances of distributed operating system  10  may instantiate one or more topics arranged in a hierarchical manner (i.e., according to the tree data structure). The hierarchically arranged topics may have various levels of scope with topics situated above another topic in the hierarchy being inclusive of any state published to the topics under a topic situated above the underlying topic. The computing nodes may therefore instantiate tree data structures to store the hierarchically arranged topics, where the computing nodes executing respective instances of distributed operating system  10  may form tree nodes of the tree data structure to topics. 
     For example, topic “/a” may be an aggregate of topics “/a,” “/a/b,” and “a/b/c.” Topic “/a/b,” as another example, may be an aggregate of topics “a/b,” and “a/b/c.” Thus, a first tree node of the tree data structure may correspond to topic “/a,” a first leaf tree node of the first tree node may correspond to topic “/a/b,” and a second leaf tree node of the first leaf tree node may correspond to topic “/a/b/c.” In this manner, a topic string may be obtained from a tree node of the tree data structure and leaf tree nodes from the node. The topic string may correspond to a string representation of the topic, which in this case happens to be the topic hierarchy itself. In some examples, the hierarchically arranged topics will have only one root topic (which may be referred to as a “root topic tree node”) with multiple hierarchies under the root topic tree node, similar to the tree data structure. 
     Other nodes, applications, or components of nodes may act as consumers of these topics. The consumers may, once these topics are instantiated, receive updates to a local topic database informing the consumer of the new topics. The consumers may then subscribe to the new topic such that the objects published to the topic are distributed only to the subscribing consumers. The consumers may then consume the objects to update local state information without having to filter or otherwise discard objects that are not relevant to the operation of the consumer. 
     The producers may instantiate a topic within the hierarchy through interactions with an application infrastructure. The application infrastructure may tag each topic with one or more 32-bit scope IDs (which are collectively referred to as a scope vector) identifying the scope to which the corresponding tagged object is to be delivered. Each consumer subscribes to one or more scope IDs (via requests for the corresponding topic), and the application infrastructure automatically delivers the objects having the corresponding scope IDs to the consumers that requested such topics. The various units responsible for mapping scope IDs to objects and distribution of the objects is described in more detail with respect to a single computing node of multi-chassis router  4  as shown in  FIG. 3 . 
     In OFP, the leaves of a tree data structure represent individual message fragments, and the key is the fragment ID. Thus, any internal tree node represents a fragment ID prefix, and thus a range of fragment IDs. The root tree node may represent a zero-length prefix, and the “range” of all possible fragments. Each tree node carries a hash value that represents a digest of all fragments covered by the prefix. 
     The leaf tree nodes are degenerate examples—the digest is the Contribution of the fragment, and the fragment ID prefix is the fragment ID itself. In accordance with OFP, computing node  200  (or a processing unit thereof) calculates the contribution from the (logical clock, checksum) tuple described earlier, and positions the corresponding tree node in the tree data structure according to its fragment ID. By adding a reference to the fragment itself to the leaf tree node, the tree data structure can be used for looking up fragments as well. 
     The maximum fanout of a radix trie node is VS, where V is the number of possible values of a symbol and S is the symbol count. In OFP, a symbol is a single bit (the fragment ID is viewed as a bit string) so a value of S may be selected to provide an appropriate amount of fanout. A strictly binary tree would have V=2 and S=1, resulting in a very deep tree. For OFP, S is typically a small value greater than 1 (e.g., 4), which makes the tree a bit branchier and less deep. 
     The tree data structures may be immutable. Immutability of the tree data structures may facilitate scaling, as it means that all operations (including extractions of arbitrary subtrees) can be done in Log time, save for a traversal (which requires N*log(N) time). OFP  14  may set the tree data structures as immutable, which may improve scalability. 
     Tree nodes of the tree data structure may represent a hierarchy of “digests” (which are similar to checksums). The digests may comprise, for example, a scalar value (such as a modified Fletcher-64 checksum) representative of the content stored by leaf tree nodes of the tree data structure that is accessible by the respective one of the tree nodes of the tree data structure. Nodes supporting execution of distributed operating system  10  may store message fragments in the tree data structure, arranged by respective fragment identifiers (fragment IDs). OFP  14  may separate messages into a series of fragments, each of which fits into a single packet. OFP  14  may label the fragments with a fragment ID, which includes a tuple (Scope, Message ID, fragment number), as well as a logical clock value from the original, separated message. A reliability model for OFP operates on individual fragments (thereby reducing the impact of loss of a packet). As such, computing nodes supporting execution of distributed operating system  10  may separate a message into constituent fragments, and store each fragment as a tree node in the tree data structure, arranged by fragment IDs for the respective fragments. 
     Furthermore, computing nodes supporting execution of distributed operating system  10  may form interior tree nodes of the tree data structure to represent a block of fragment IDs (in the form of a prefix) and to include digests that represent all of the fragments in the blocks they represent. Thus, the root of the tree data structure, which represents a zero-length prefix, includes a digest that covers all messages in the topology. As such, the cost of determining that two tree nodes have the same topology contents is reduced to O(1) (as long as the contents are identical). 
     Whenever one of the computing nodes of distributed operating system  10  modifies one of the message fragments, the one of the computing nodes of distributed operating system  10  also incrementally updates the digests of all of the message fragment&#39;s ancestors, back to the root of the tree data structure. 
     In this manner, two or more computing nodes of distributed operating system  10  may synchronize their respective tree data structures by comparing respective digests of tree nodes of the respective tree data structures. When the digests for corresponding tree nodes of the tree data structures match, the computing nodes of distributed operating system  10  may determine that the tree data structures are synchronized. However, when the digests for corresponding tree nodes do not match, the computing nodes of distributed operating system  10  may determine that the tree data structures are not synchronized. Accordingly, the computing nodes of distributed operating system  10  may exchange messages (e.g., in the form of message fragments) to synchronize the respective tree data structures. Thus, two tree data structures may be described as synchronized when the tree data structures have a common arrangement and interconnection of tree nodes within each of the tree data structures and when the digests of corresponding tree nodes of the tree data structures match. 
     For example, the computing nodes of distributed operating system  10  may initially determine that two corresponding tree nodes of their respective tree data structures are not synchronized. The computing nodes of distributed operating system  10  may then determine which of the two tree nodes of the respective tree data structures includes a higher (i.e., more recent) logical clock value. The tree node of the tree data structures having the more recent logical clock value may be considered most current, and therefore correct. Accordingly, the computing node of distributed operating system  10  having the tree node of the tree data structure with the more recent logical clock value may send the corresponding message or message fragments for the tree data structure to other computing nodes of distributed operating system  10 . The other computing nodes of distributed operating system  10  may update their corresponding tree data structures using the message or message fragments, thereby synchronizing at least these branches of the tree data structures. 
     Computing nodes of distributed operating system  10  may further add, modify, or delete message fragments. To add or delete a message fragment, the computing nodes of distributed operating system  10  modify the respective tree data structures to add or delete corresponding tree nodes to or from the tree data structure. To modify a message fragment, the computing nodes of distributed operating system  10  update the contents of the appropriate tree nodes of the tree data structures. Furthermore, in response to adding, modifying, or deleting message fragments, the computing nodes of distributed operating system  10  walk the corresponding tree data structures from the leaf tree nodes to the root, incrementally updating the digests of the tree nodes of the tree data structures along the way. Since the digest value at any tree node is a contribution, the old digest is subtracted (as defined above) from its parent&#39;s digest, and the new value is added, and the process recurses upward toward the root. 
     In examples in which the tree data structures are radix tries and the digests are Fletcher-64 checksums, adding or deleting a leaf tree node may cause the creation or deletion of interior tree nodes. The contribution of a nonexistent tree node may be zero (due to the use of Fletcher), so that value is used as tree nodes are created or destroyed. 
     The worst-case cost of updating the tree data structures in these examples is O(log F N), where F is the maximum tree node fanout and N is the number of message fragments. In practice, this may be quite small—with one million objects and a fanout of 16, the cost is O(5), for 16 million objects it is O(6), etc. In this way, the techniques may efficiently maintain state synchronization between the various computing nodes for execution of either distributed operating system  10  or application, or both distributed operating system  10  and applications. 
     After forming the quorum and establishing IPCB  18  by which the clients may interface with the shared file system and thereby execute distributed operating system  10  to facilitate the exchange and synchronization of state information, IPCB  18  may monitor the IPCB servers to determine whether connectivity between one or more of the plurality of computing nodes has failed. For example, when an IPCB epoch manager fails or a link fails (which may be generally referred to as a “connectivity failure”), the remaining IPCB epoch managers may determine whether the quorum of epoch managers exists. 
     The remaining IPCB epoch manager may determine whether the quorum of epoch managers exists by comparing the number of epoch managers still operational (denoted by the variable “N”) is greater than, or greater than or equal to, a connectivity failure threshold (e.g., (N/2)+1). The connectivity failure threshold may also be referred to as a “quorum threshold.” When the number of remaining epoch managers exceeds the connectivity failure threshold, the remaining epoch managers may maintain the quorum and continue operating, potentially adding to the quorum new epoch managers that were not elected as epoch managers during the formation of the quorum. When the number of remaining epoch managers does not exceed the connectivity failure threshold, the remaining epoch managers may restart distributed operating system  10  (which may not require restarting multi-chassis router  4  or kernel  12 , but only restarting one or more of those layers above kernel  12  in software stack  11 , such as OFP  14 , SysEpochMan  16 , IPCB  18 , SysMan  20 , and/or distributor  22 ). 
     In this way, distributed operating system  10  of multi-chassis router  4  may coalesce from a number of different computing nodes of various different types and capabilities. OFP  14  may execute to discover the computing node topology, allowing IPCB  18  to form so as to establish the quorum by which to ensure sufficient resources to continue successful execution of distributed operating system  10 . The quorum may ensure that sufficient resources are available to allow for successful propagation of the state information, while also, as described in more detail below, allowing for mechanisms by which to overcome split-brain situations in which the computing node topology is separated into two different execution environments. 
       FIG. 2  is a block diagram illustrating an exemplary multi-chassis router  120  configured to operate in accordance with the techniques described in this disclosure. Multi-chassis router  120  routes data packets between network devices across a network. In this example, multi-chassis router  120  comprises four substantially identical LCCs  128 A- 128 D (“LCCs  128 ”) and SCC  122  that operates as a central control node. In other embodiments, a multi-chassis router may include more or less LCCs. SCC  122  provides centralized switching and control for multi-chassis router  120 . LCCs  128  provide interfaces to a network using IFC sets  134 A- 134 D (“IFCs  134 ”). 
     SCC  122  includes switch fabric  124  and a master routing engine  126 . Although not shown in the example of  FIG. 2 , SCC  122  may include a standby master routing engine when multi-chassis router  120  is configured as a high-availability router. Switch fabric  124  provides a back-side connection, i.e. a connection separate from the network, between switch fabric  125  of LCCs  128 . Functions of master routing engine  126  include maintaining routing information to describe a topology of a network, and using that information to derive forwarding information bases (FIBs). Routing engine  126  controls packet forwarding throughout multi-chassis router  120  by installing the FIB in LCCs  128  via communication with local routing engines  130  over cables  137 . A FIB for one of LCCs  128  may be the same or different than an FIB for other LCCs  128  and SCC  122 . Because cables  137  provide a dedicated connection, i.e., separate from a data packet forwarding connection provided by cables  136 , between SCC  122  and LCCs  128 , FIBS in LCC routing engines  130  can be updated without interrupting packet forwarding performance of multi-chassis router  120 . 
     LCCs  128  each contain one of local routing engines  130 A- 130 D (“routing engines  130 ”), one of switch fabrics  125 A- 125 D (“switch fabric  125 ”), at least one packet forwarding engine (PFE), shown as PFEs  132 A- 132 D (“PFEs  132 ”), and one or more IFCs  134 . In some examples when multi-chassis router  120  is configured to provide high-availability, LCCs  128  may also include one of standby local routing engines in addition to one of local routing engines  130 , which may be referred to as primary local routing engines  130  in the high-availability configuration. 
     Multi-chassis router  120  performs routing functions in the following manner. An incoming data packet is first received from a network by one of IFCs  134 , e.g.,  134 B, which directs it to one of PFEs  132 , e.g., PFE  132 B. The PFE then determines a proper route for the data packet using the FIB provided by the primary local routing engine, e.g., routing engine  130 B. If the data packet is destined for an outbound link associated with the one of IFCs  134  that initially receive the packet, the PFE forwards the packet to the outbound link. In this manner, packets sent out by the same PFE on which they were received from the network bypass switch fabric  124  and switch fabric  125 . 
     Otherwise, the PFE sends the data packet to switch fabric  125 , where it is directed to switch fabric  124  and follows a route to one of the other PFEs  132 , e.g., PFE  132 D. This PFE, e.g., PFE  132 D, sends the data packet across the network via one of IFCs  134 , e.g., IFC  134 D. Thus an incoming data packet received by one of LCCs  128  may be sent by another one of LCCs  128  to its destination. Other multi-chassis routers that operate in a manner consistent with the techniques described in this disclosure may use different switching and routing mechanisms. 
     Local routing engines  130  control and manage LCCs  128 , but are subservient to master routing engine  126  of SCC  122 . For example, after receiving state information updates from master routing engine  126 , local routing engines  130  forward the state information update to consumers on LCCs  128  using the hierarchically-ordered and temporally-linked data structure. For example, consumers that receive state information updates from local routing engines  130  include PFEs  132  and IFCs  134 . Local routing engines  130  also distribute the FIB derived by primary master routing engine  126  to PFEs  132 . 
     Routing engines  126  and  130  may operate according to executable instructions fetched from one or more computer-readable media. Examples of such media include random access memory (RAM), read-only memory (ROM), non-volatile random access memory (NVRAM), electrically erasable programmable read-only memory (EEPROM), flash memory, and the like. The functions of multi-chassis router  120  may be implemented by executing the instructions of the computer-readable medium with one or more processors, discrete hardware circuitry, firmware, software executing on a programmable processor, or combinations thereof. 
     As described above, nodes may include routing engine  126 , routing engines  130 , PFEs  132 , and IFCs  134 . Links may include switch fabric  124  and cables  136  and  137 , as well as other cables shown but not enumerated for ease of illustration purposes. The various nodes may perform aspects of the techniques described below in more detail with respect to a single node of multi-chassis router  120  shown in  FIG. 3 . 
       FIG. 3  is a block diagram illustrating an example computing node  200  of multi-chassis router  120  configured to operate in accordance with various aspects of the techniques described in this disclosure. As examples, computing node  200  may represent routing engine  126 , one of routing engines  130 , switch card chassis  122 , or one of line card chassis  128 . 
     As shown in  FIG. 3 , computing node  200  executes kernel  12  that enables interaction with the hardware of computing node  200 . Once kernel  12  is operational, computing node  200  may execute OFP  14 , by which to determine a topology  202  of computing nodes executing within multi-chassis router  120 . Topology  202  may represent the above noted graph data structure including graph nodes representative of the computing nodes of multi-chassis router  120 , and edges interconnecting the graph nodes representative of link interconnecting the computing nodes of the multi-chassis router  120 . 
     OFP  14  may discover or otherwise determine topology  202  through receipt of announcements  204 . OFP  14  may receive announcements  204  from each of the other computing nodes supporting execution of distributed operating system  10 , either as producers or consumers of state information. Each of announcements  204  may specify a computing node and one or more links directly coupled to that computing node. OFP  14  may be configured (e.g., by a network administrator) with links directly coupling to computing node  200 . From announcements  204  and link  206 , OFP  14  may construct topology  202 . OFP  14  further includes state  208 , representing a state data structure, such as a tree data structure, in accordance with the techniques of this disclosure. 
     From links  206 , OFP  14  may also generate an announcement  204 , transmitting the generated one of announcements  204  via the links identified by links  206  so that OFP  14  executed by adjacent computing nodes may likewise generate topology  202 . Like link state routing protocols, OFP  14  operates to form a local copy of topology  202  at each of the computing nodes (or, in some instances, only at the primary computing nodes). OFP  14  may flood changes to topology  202  detected by computing node  200  (such as a computing node or a link going down) via announcements  204 , thereby allowing topology  202  to remain synchronized at each computing node  200  supporting execution of distributed operating system  10 . OFP may expose topology  202  (via an application programming interface—API) to SysEpochMan  16 . 
     During initialization, SysEpochMan  16  may first subscribe to an EpochManagement scope within OFP domain 0, and assuming SysEpochMan  16  is configured as being epoch manager (EM or Em) capable, subscribes to an EmCapableNodes scope within OFP domain 0. SysEpochMan  16  may initially publish an epoch manager object  210  into OFP domain 0 (which as noted above is formed by OFP  14  for use by the underlying infrastructure of distributed operating system  10 , such as OFP  14 , SysEpochMan  16 , IPCB  18 , etc.). The epoch manager object  210  indicates whether computing node  200  has been configured as capable of acting as an epoch manager, and an epoch manager priority configured for computing node  200  to act as an epoch manager. A higher epoch manager priority indicates that computing node  200  is more likely to be chosen as an epoch manager compared to a lower epoch manager priority. As such, the epoch manager priority allows network administrators to bias epoch manager functionality toward or away from particular computing nodes. 
     Epoch manager object  210  may also include a hardware master indication, which indicates whether computing node  200  owns hardware mastership, where such information may be used in two epoch manager-capable node systems to determine whether a quorum is present. Epoch manager object  210  may also include a master identifier (ID) indicating a nomination for computing node  200  for acting as epoch manager master. Epoch manager object  210  may also indicate a master priority, which may indicate a priority of computing node  200  for epoch manager master election. Like the epoch manager priority, a higher epoch manager master priority indicates that computing node  200  is more likely to be chosen as an epoch manager master compared to a lower epoch manager master priority. As such, the epoch manager master priority allows network administrators to bias epoch manager master functionality toward or away from particular computing nodes. 
     Epoch manager object  210  may also specify an epoch number, which may indicate an epoch of distributed operating system  10  in which computing node  200  previously participated. An epoch may refer to a version of distributed operating system  10  that was operational for some period of time. The epoch number allows for computing nodes  200  to coalesce on the most recently operational version of distributed operating system  10 . Epochs are discussed in more detail below. 
     Epoch manager object  210  may further include an indication of whether computing node  200  has been selected as an epoch manager, and an indication of whether computing node  200  has been elected as the epoch manager master. Additionally, epoch manager object  210  may include an indication of whether computing node  200  has successfully joined the epoch (which is qualified by successfully writing data to one of the IPCB servers), and an indication of whether computing node  200  is successfully functioning as an epoch manager (which is qualified by successfully writing data to one of the IPCB servers). 
     Furthermore, epoch manager object  210  may include a restart request that requests restart of distributed operating system  10 , either preserving the current epoch manager set or resetting the epoch manager set. Epoch manager object  210  may also include an indication that sets a maximum number of epoch manager capable computing nodes expected in the system, with a value of zero indicating that there is no set maximum. 
     To restate the above, epoch manager object  210  may include the following: 
     
       
         
           
               
             
               
                   
               
             
            
               
                 object EmNode { 
               
               
                  NodeID id;      // Node ID of publisher  
               
               
                  Int nonce;     // Random nonce  
               
               
                  Boolean emCapable;  // True if EM-capable  
               
               
                  Int emPriority;   // Priority for EM selection  
               
               
                  Boolean hwMaster; // True if hardware master  
               
               
                  NodeID masterId;   // ID of nominated master, or 0  
               
               
                  Int masterPriority;  // Priority for EM Master election  
               
               
                  SysEpoch epoch;   // Local system epoch, or 0  
               
               
                  Boolean epochManager;   // True if node is epoch manager  
               
               
                  Boolean emMaster; // True if node is epoch manager master  
               
               
                  Boolean epochUp;  // True if epoch is up  
               
               
                  Boolean managerUp;  // True if epoch manager is up  
               
               
                  Enum restartRequest; // Restart request  
               
               
                  Int maxEmCapableNodes; // Maximum # of EM-capable nodes, or 0  
               
               
                 } 
               
               
                   
               
            
           
         
       
     
     Each node updates its object in OFP whenever its contents change. All EM-capable computing nodes subscribe to these objects. 
     The fields have the following semantics: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 id 
                 The node ID of the publishing node. This value is provided by OFP. 
               
               
                 nonce 
                 A random number generated by OFP when the node restarts. This 
               
               
                   
                 value is compared to the value sent in the OFP reachability protocol. 
               
               
                   
                 If it differs, it means that the node has been restarted and the object 
               
               
                   
                 should be ignored. This effectively makes the object invisible if the 
               
               
                   
                 publishing node restarts. 
               
               
                 emCapable 
                 Set to True if the node is Epoch Manager-capable. 
               
               
                 emPriority 
                 The publishing node&#39;s priority for Epoch Manager selection, or 0 if the 
               
               
                   
                 node is not EM-capable. Higher priority nodes will be favored in 
               
               
                   
                 choosing the set of Epoch Managers, providing for a way to bias the 
               
               
                   
                 Epoch Manager function to particular nodes. 
               
               
                 hwMaster 
                 Set to True if the node owns hardware mastership (if it exists for this 
               
               
                   
                 node type), or False if not. This is used in two-EM-capable-node 
               
               
                   
                 systems to determine whether a quorum is present. 
               
               
                 masterId 
                 The node ID of the publishing node&#39;s nomination for Epoch Manager 
               
               
                   
                 Master, or 0 if the node hasn&#39;t decided or is not EM-capable. 
               
               
                 masterPriority 
                 The publishing node&#39;s priority for Epoch Manager Master election. 
               
               
                   
                 Higher priority nodes will be favored in the EM Manager election, 
               
               
                   
                 providing a way to bias the EM Manager function to particular nodes. 
               
               
                 epoch 
                 The publishing node&#39;s understanding of the System Epoch, or 0 if the 
               
               
                   
                 node hasn&#39;t joined an epoch. 
               
               
                 epochManager 
                 True if the publishing node has been selected as an Epoch Manager. 
               
               
                 emMaster 
                 True if the publishing node has been elected Epoch Manager Master. 
               
               
                 epochUp 
                 True if the publishing node has successfully joined the epoch (by 
               
               
                   
                 virtue of having successfully written data into Zookeeper). 
               
               
                 managerUp 
                 True if the publishing node is functioning as an Epoch Manager (by 
               
               
                   
                 virtue of having successfully written data into Zookeeper through the 
               
               
                   
                 publishing node&#39;s server). 
               
               
                 restartRequest 
                 The node&#39;s restart request. The possible values are None, Restart, and 
               
               
                   
                 ResetManagers. This is used for user-requested restarts (as opposed to 
               
               
                   
                 forced restarts due to loss of quorum). A value of Restart preserves 
               
               
                   
                 the previous EM manager set across the restart, and ResetManagers 
               
               
                   
                 resets it. The latter is used to allow restarting after a failure causes the 
               
               
                   
                 unrecoverable loss of EM-capable nodes such that a quorum of the 
               
               
                   
                 previous manager set cannot be met (otherwise the system will never 
               
               
                   
                 come back up). 
               
               
                 maxEmCapableNodes 
                 Set to the maximum number of EM-capable nodes expected in the 
               
               
                   
                 system. This is set to 1 in single-EM-node systems, to 2 in dual-EM- 
               
               
                   
                 node systems, and to 0 otherwise. 
               
               
                   
               
            
           
         
       
     
     Each node, including computing node  200 , then sets its epoch manager object  210  to as follows: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 local.id 
                 The node ID (provided by OFP). 
               
               
                 local.nonce 
                 The nonce (provided by OFP). 
               
               
                 local.emCapable 
                 Set to True if the node is Epoch Manager- 
               
               
                   
                 capable. 
               
               
                 local.emPriority 
                 The publishing node&#39;s priority for Epoch 
               
               
                   
                 Manager selection, or 0 if the node is not 
               
               
                   
                 EM-capable. 
               
               
                 local.masterId 
                 0. 
               
               
                 local.masterPriority 
                 The publishing node&#39;s priority for Epoch 
               
               
                   
                 Manager Master election, or 0 if the node 
               
               
                   
                 is not EM-capable or does not wish to be EM 
               
               
                   
                 Master. 
               
               
                 local.epoch 
                 0. 
               
               
                 local.epochManager 
                 False. 
               
               
                 local.emMaster 
                 False. 
               
               
                 local.epochUp 
                 False. 
               
               
                 local.managerUp 
                 False. 
               
               
                 local.restartRequest 
                 None. 
               
               
                 local.maxEmCapableNodes 
                 0, 1, or 2 depending on the hardware 
               
               
                   
                 configuration (all nodes in one- and two- 
               
               
                   
                 node systems are expected to know that 
               
               
                   
                 fact). 
               
               
                   
               
            
           
         
       
     
     Assuming computing node  200  has been configured as capable of operating as an epoch manager, SysEpochMan  16  receives each published epoch manager objects  210 . SysEpochMan  16  may determine from epoch manager objects  210 , which of the computing nodes capable of acting as epoch managers is to act as epoch manager master. SysEpochMan  16  may determine which of the computing nodes is to act as epoch manager master based on the epoch manager master priority of each epoch manager objects  210  after waiting some period of time (denoted as “EmWaitTime”) to allow for the arrival of objects (and to avoid recently restarted computing nodes from immediately electing themselves as epoch manager master). SysEpochMan  16  may also delete any IPCB state information, before proceeding to execute an object event process. 
     All computing nodes, including SysEpochMan  16  of computing node  200 , may execute the object event process at startup or whenever any EmNode object  210  or EmMaster object  212  (which is another way of referring to epoch manager master object  212 ) it subscribes to changes, including its own. Non-EM-capable computing nodes do not execute the object event process when updating the local copy of the EmNode object, since they do not subscribe to them. 
     When a computing node updates an object in the object event process, the computing node executes the object event process again (since its own object has changed) as long as it is subscribed to the scope into which that object is published. This repeats until no object is updated. When the computing node restarts in the procedure below, the computing node exits the object event process. 
     Early in the object event process, SysEpochMan  16  selects a single EmMaster object  210  (if such an object  210  exists). The object even process may reference an epochState field, which may be set to any of the following: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 EpochStart 
                 The initial state. In some sense this isn&#39;t the Epoch state, but rather the 
               
               
                   
                 state of the elected EM Master, which is trying to decide how to proceed. 
               
               
                   
                 The Epoch Manager Master has been elected and is waiting for a quorum 
               
               
                   
                 of EM-capable nodes to arrive. The EM Master does not publish an 
               
               
                   
                 EmMaster object in this state so that any old EmMaster object continues to 
               
               
                   
                 persist. Transitions to state EpochInit when a quorum of EM-capable 
               
               
                   
                 nodes forms. Transitions to state EpochFail if any node requests a system 
               
               
                   
                 restart. 
               
               
                 EpochInit 
                 The Epoch is initializing. A quorum of EM-capable nodes is reachable in 
               
               
                   
                 OFP but not all selected Epoch Managers are ready. Transitions to state 
               
               
                   
                 EpochRunning when a quorum of nodes selected as Epoch Managers are 
               
               
                   
                 ready. Transitions to state EpochFail if the quorum of EM-capable nodes 
               
               
                   
                 fails or any node requests a system restart. 
               
               
                 EpochRunning 
                 The epoch is up. Higher layers have started and the system is running. 
               
               
                   
                 Transitions to state EpochReconfig when the EM Master decides to 
               
               
                   
                 change the set of EM nodes. Transitions to state EpochFail if the EM 
               
               
                   
                 quorum fails or any node requests a system restart. 
               
               
                 EpochReconfig 
                 The set of EM nodes is being reconfigured by the EM Master but has not 
               
               
                   
                 yet completed. The system continues to run, although Zookeeper state 
               
               
                   
                 changes stall. Transitions to state EpochRunning when a quorum of the 
               
               
                   
                 selected Epoch Managers are ready. Transitions to state EpochFail if the 
               
               
                   
                 quorum of the selected Epoch Managers fails or any node requests a 
               
               
                   
                 system restart. 
               
               
                 EpochFail 
                 The epoch has failed due to lost quorum or full-system restart. A new 
               
               
                   
                 epoch will be created and this one will be destroyed. 
               
               
                   
               
            
           
         
       
     
     The object event process may operate with respect to the selected object as follows: 
     
       
         
           
               
             
               
                   
               
             
            
               
                   (Perform local housekeeping:)  
               
               
                 | If there are one or more existing EmNode objects for which local.id == remote.id and  
               
               
                   local.nonce != remote.nonce, delete the objects (the local node has restarted).  
               
               
                 | If there are one or more existing EmNode objects (other than the local object) for which  
               
               
                   local.id == remote.id and local.nonce == remote.nonce, restart the node (as an error has  
               
               
                   occurred necessitating restart).  
               
               
                   (Choose the EmMaster object:)  
               
               
                 | If there is at least one EmMaster object present:  
               
               
                 | | Select the best EmMaster object. As there may be more than one (due to the  
               
               
                    asynchronous nature of OFP), prefer objects where master.epochState != EpochFail,  
               
               
                    then prefer objects with the highest value of master.epochPreference, and then prefer  
               
               
                    objects with the highest value of master.masterId. This may cause all nodes to  
               
               
                    converge to a single EmMaster object and chooses the “best” epoch if more than one  
               
               
                    are present. and ignores failed epochs if there are any epochs that have not failed.  
               
               
                  (Set/validate the system epoch:)  
               
               
                 | | If local.epoch == 0: and master.epochState != EpochFail 
               
               
                    |  |  |   Set local.epoch = master.masterEpoch.  
               
               
                    |  |  Else if local.epoch != 0: (already part of an epoch)  
               
               
                 | | | If local.epoch != master.masterEpoch, restart the node. This means that the epoch  
               
               
                     has changed.  
               
               
                 | | | If master.epochState == EpochFail, restart the node. This means that the epoch has  
               
               
                     failed and the system is being restarted.  
               
               
                 | | | If master.epochState == EpochRunning and local.epochUp == True and the upper  
               
               
                     layers are not yet running, start the upper layers with the OFP Domain and  
               
               
                     Zookeeper parameters in the EmMaster object. This means that the system has  
               
               
                     come up.  
               
               
                  (Update the set of EM managers used for detecting Epoch Up:)  
               
               
                 | | If local.epochUp == False and local.epoch != 0:  
               
               
                 | | | Reset any previous Zookeeper Client session to the set of nodes in  
               
               
                     master.managers (the set of managers may have changed).  
               
               
                 | | | Open a Zookeeper Client session to the set of nodes in master.managers as servers  
               
               
                      via the master.zkClientPort port.  
               
               
                 | | | Post a Zookeeper write to “/SysEpochMan/EpochRunning/&lt;id&gt;”, where &lt;id&gt; is a  
               
               
                      textual representation of the publisher&#39;s node ID. If and when this write completes,  
               
               
                      it will result in an Epoch Up event.  
               
               
                 | | | Post a Zookeeper getChildren watch to “/SysEpochMan/SystemEpoch/”. If and 
               
               
                      when this read completes, it will result in a Zookeeper Epoch event.  
               
               
                     (All nodes see if quorum has been lost:)  
               
               
                 |  If local.epochUp == True and the quorum of EM nodes has failed (see section 8.4.11  
               
               
                   below), restart the node. If local.emMaster == True (this node is EM Master), set  
               
               
                   master.epochState = EpochFail and publish the updated EmMaster object before  
               
               
                   restarting. This means that the network has partitioned or too many EM nodes have  
               
               
                   failed and the epoch must be abandoned, and we need the EM Master to signal that fact.  
               
               
                     (Non-EM-capable nodes exit here, the exception being a node that wants to  
               
               
                 gracefully shut down but is currently an Epoch Manager. Such nodes continue in their EM  
               
               
                 role until they are dismissed by the EM Master:)  
               
               
                 | If local.emCapable == False and local.epochManager == False, exit the Object Event  
               
               
                   Process (the node is not EM-capable or has been relieved of its duties as Epoch  
               
               
                   Manager because it is shutting down).  
               
               
                     (All EM-capable nodes perform EM mastership election:)  
               
               
                 | Set local.masterId to the ID of the node that reports its own ID in remote.masterId with  
               
               
                   the highest value of remote.masterPriority, then the lowest node ID. If there is no such  
               
               
                   node, choose the ID of the node for which remote.emCapable == True with the highest  
               
               
                   value of remote.masterPriority, then the lowest node ID. Use 0 if there is no such node.  
               
               
                   (Note that if a node has become unreachable, its EmNode object is hidden, so only  
               
               
                   reachable nodes will be considered.)  
               
               
                     (All EM-capable nodes see if their EM status has changed:)  
               
               
                 | If local.epochManager == False and master.managers contains (local.id, local.nonce):  
               
               
                   (becoming Epoch Manager)  
               
               
                    | |  Set local.managerUp = False.  
               
               
                    | |  Set local.epochManager = True.  
               
               
                 | | Write the set of reachable servers in master.managers to the IPCB server  
               
               
                    configuration file.  
               
               
                 | | Erase any local persistent IPCB Server state.  
               
               
                 | | Launch a local IPCB Server on the ports specified in master.zkServerPort and  
               
               
                    master.zkElectionPort. If the size of master.managers is 1, start IPCB in Standalone  
               
               
                    mode; otherwise, start it in Replicated mode.  
               
               
                 | | Open a IPCB Client session to the node local.id. as server via the master.zkClientPort  
               
               
                    port and post a IPCB write to “/SysEpochMan/ManagerUp/&lt;id&gt;”, where &lt;id&gt; is a  
               
               
                    textual representation of the publisher&#39;s node ID. If and when this write completes, it  
               
               
                    will result in a Manager Up event. 
               
               
                 | Else if local.epochManager == True and master.epochState != EpochReconfig and  
               
               
                   master.managers does not contain (local.id, local.nonce): (no longer Epoch Manager)  
               
               
                    | |  Set local.managerUp = False.  
               
               
                    | |  Set local.epochManager = False.  
               
               
                 | | Shut down any local IPCB server.  
               
               
                 | | Close any client session for Manager Up events.  
               
               
                    (Switch IPCB between Standalone and Replicated modes if appropriate:)  
               
               
                 | Else If local.epochManager == True and master.managers contains (local.id local.nonce):  
               
               
                  (already Epoch Manager)  
               
               
                 | | If the size of master.managers is 1 and IPCB 18 is running in Replicated mode: 
               
               
                 | | | Write the server in master.managers to the IPCB server configuration file.  
               
               
                 | | | Relaunch the local IPCB Server in Standalone mode on the ports specified in  
               
               
                     master.zkServerPort and master.zkElectionPort. 
               
               
                 | | Else If the size of master.managers is greater than 1 and IPCB is running in  
               
               
                    Standalone mode:  
               
               
                 | | |  Write the set of reachable servers in master.managers to the IPCB server  
               
               
                     configuration file. 
               
               
                 | | |  Relaunch the local IPCB Server in Replicated mode on the ports specified in  
               
               
                     master.zkServerPort and master.zkElectionPort. 
               
               
                    (Perform EM Master duties if appropriate)  
               
               
                 |   If local.masterId == local.id: (this node is or just became master)  
               
               
                 | | If any local.emMaster == False: (becoming master)  
               
               
                 | | | If any remote.masterId != local.id, exit the Object Event Process. This means that  
               
               
                      the election of the local node is not yet unanimous.  
               
               
                 | | |  If master.epochState == EpochFail and master.managers is not empty and a  
               
               
                     quorum (see section 8.4.10) of the nodes in master.managers are not reachable  
               
               
                     (ignoring the nonce values), exit the Object Event Process. This means that we may  
               
               
                     have been partitioned and so do not want to advance the Epoch, lest we cause split  
               
               
                     brain. 
               
               
                 | | | If any EmMaster object exists with master.masterId == local.id, delete it (clean up  
               
               
                    old EmMaster objects from this node).  
               
               
                 | | | Set local.emMaster = True.  
               
               
                 | | | Initialize the local EmMaster object according to section 8.4.12.  
               
               
                 | | Update the EmMaster state according to section 8.4.13.  
               
               
                 | | If master.epochState != EpochStart:  
               
               
                 | | | If it has changed, update the EmMaster object in OFP. 
               
               
                 | | | If any EmMaster object exists with master.masterId != local.id, delete it (clean up  
               
               
                     old EmMaster objects from other nodes). 
               
               
                   
               
            
           
         
       
     
     The elected epoch manager master (assuming for the sake of explanation that this is computing node  200 ) may, upon being elected, publishes an epoch manager master object  212  into OFP domain 0. The epoch manager master object  212  may include the following information. 
     
       
         
           
               
             
               
                   
               
             
            
               
                 Object EmMaster { 
               
               
                  NodeID masterId;   // Node ID of publisher (the EM Master)  
               
               
                  SysEpoch masterEpoch; // Global system epoch  
               
               
                  Int epochPreference;   // Epoch preference  
               
               
                  Int zkClientPort;    // Zookeeper client port  
               
               
                  Int zkServerPort;     // Zookeeper server port  
               
               
                  Int zkElectionPort;   // Zookeeper leader election port  
               
               
                  Int ofpDomain;     // OFP domain ID  
               
               
                  Enum epochState;   // Epoch state  
               
               
                  (NodeID, Int) managers[ ];  // Selected EMs and their nonces  
               
               
                  (NodeID, Int) oldManagers[ ]; // Previous EMs and their nonces  
               
               
                  Int maxManagerCount; // Max number of Epoch Managers expected  
               
               
                  Int epochQuorum;  // EM Quorum size  
               
               
                  Int oldQuorum;   // Previous EM Quorum size  
               
               
                 } 
               
               
                   
               
            
           
         
       
     
     The EM Master updates this object in OFP whenever its contents change and epochState !=EpochStart. All computing nodes subscribe to this object. 
     The fields have the following semantics: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 masterId 
                 The node ID of the publishing node. This value is provided by OFP. 
               
               
                 masterEpoch 
                 The current global system epoch value. 
               
               
                 epochPreference 
                 The preference value for this epoch. If multiple EmMaster objects are 
               
               
                   
                 present, all nodes choose the one with the highest preference value. 
               
               
                   
                 This is used to preserve the “best” partition epoch when healing split 
               
               
                   
                 brain situations. 
               
               
                 zkClientPort 
                 The TCP port number used for client access to Zookeeper. 
               
               
                 zkServerPort 
                 The TCP port number used between Zookeeper servers for data 
               
               
                   
                 transfer. 
               
               
                 zkElectionPort 
                 The TCP port number used between Zookeeper servers for leader 
               
               
                   
                 election. 
               
               
                 ofpDomain 
                 The OFP domain ID to be used. 
               
               
                 epochState 
                 The state of the epoch. The possible states are EpochStart, EpochInit, 
               
               
                   
                 EpochRunning, EpochReconfig, and EpochFail. 
               
               
                 managers 
                 The set of (node ID, nonce) tuples of the nodes chosen to be Epoch 
               
               
                   
                 Managers. A particular node is considered to be in the managers list 
               
               
                   
                 only if it is reachable and both its id and nonce values match the values 
               
               
                   
                 in its EmNode object. 
               
               
                 oldManagers 
                 The set of (node ID, nonce) tuples of the EM nodes that were running 
               
               
                   
                 and reachable at the time of the last reconfig event. A quorum of these 
               
               
                   
                 nodes (as defined by oldQuorum) must remain reachable during the 
               
               
                   
                 reconfiguration to avoid failure. 
               
               
                 maxManagerCount 
                 The maximum number of Epoch Managers expected. 
               
               
                 epochQuorum 
                 The size of the Epoch Manager quorum. 
               
               
                 oldQuorum 
                 The size of the Epoch Manager quorum at the time of the last reconfig 
               
               
                   
                 event. 
               
               
                   
               
            
           
         
       
     
     All computing node state information (which may also be referred to as “state”) may be reflected in the objects published, and computing node  200  stores little local state. In other words, the internal state for computing node  200  is reflected in the computing node&#39;s EmNode object (which is another way to refer to epoch manager object  210 ), and the internal state for the Epoch Manager Master is reflected in the EmMaster object (which is another way to refer to epoch manager master object  212 ). In some instances, SysEpochMan  16  may only store an internal copy of the last version of each EmNode object  210  SysEpochMan  16  most recently publishes. The EM Master may use the contents of the published EmMaster object, since it is transferred between computing nodes when mastership changes. 
     In describing the elements of procedure, updating a named object field should be understood to be updating the internal copy, where SysEpochMan then publishes as an updated object at the end of the process in the event any changes are made to the local copy. Moreover, fields in the locally-produced EmNode object  210  are referred to as local.X, where X is the field name. Fields in EmNode objects from other computing nodes are referred to as remote.X, where X is the field name. Fields in the EmMaster object  212  are referred to as master.X, where X is the field name. 
     The procedure is defined for individual computing nodes. However, acting in concert, the collection of computing nodes may indirectly define the global behavior. Furthermore, the procedure is defined as a set of possible events, each of which triggers a process, and each of which may result in updating of published objects. The possible events are: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 Object Event 
                 A change has occurred in the set of EmNode and 
               
               
                   
                 EmMaster objects or node reachability has changed. 
               
               
                 Epoch Up 
                 The epoch is up on the node (IPCB 18 is functional). 
               
               
                 Manager Up 
                 The node has become a fully functional as an 
               
               
                   
                 Epoch Manager (the local IPCB Server is functional). 
               
               
                 System Restart 
                 An agent within the node has requested that the 
               
               
                   
                 entire system restart, abandoning the current 
               
               
                   
                 System Epoch. 
               
               
                 Node Shutdown 
                 An agent within the node has requested that the 
               
               
                   
                 node shut down gracefully. 
               
               
                 IPCB Epoch 
                 A watch on the System Epoch value within IPCB 
               
               
                   
                 has fired. This provides a means to ensure that 
               
               
                   
                 the same IPCB plane is not bound to two System 
               
               
                   
                 Epoch values. 
               
               
                 Mastership 
                 The hardware mastership status of the node changed 
               
               
                 Change 
                 (for nodes that have such hardware). 
               
               
                   
               
            
           
         
       
     
     After computing node  200  publishes epoch manager object  210  and determines that computing node  200  is epoch manager master (under the above assumption), SysEpochMan  16  may wait for at least a quorum of epoch manager capable computing nodes to publish epoch manager objects  210 . SysEpochMan  16  may determine the size of the epoch manager quorum in, as one example, the following way: 
     If master.maxManagerCount &gt;=3, or master.maxManagerCount==1, a quorum is master.epochQuorum nodes. 
     If master.maxManagerCount==2, a quorum is master.epochQuorum nodes with one node reporting remote.hwMaster==True (or, conversely, a single node without remote.hwMaster True is not a quorum). 
     Once the quorum is reached, SysEpochMan  16  may publish epoch manager master object  212  with an epochState field set to “EpochInit,” which initiates a new epoch. SysEpochMan  16  may perform the epoch manager master initialization process to initialize state of epoch manager master object  212  as follows. 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 masterId 
                 The node ID (provided by OFP). 
               
               
                 masterEpoch 
                 The old value of master.masterEpoch, or a random number if there is no 
               
               
                   
                 old EmMaster object. 
               
               
                 epochPreference 
                 The old value of master.epochPreference, if an old EmMaster object 
               
               
                   
                 exists. In general this value should represent the “goodness” of a 
               
               
                   
                 partition if more than one exists, so that the “best” partition survives; 
               
               
                   
                 this should probably be based on the number of objects in OFP Domain 
               
               
                   
                 X, or perhaps the number of reachable OFP nodes, or some combination 
               
               
                   
                 thereof. 
               
               
                 zkClientPort 
                 The old value of master.zkClientPort, or a value derived as described 
               
               
                   
                 below if there is no old EmMaster object. 
               
               
                 zkServerPort 
                 The old value of master.zkServerPort, or a value derived as described 
               
               
                   
                 below if there is no old EmMaster object. 
               
               
                 zkElectionPort 
                 The old value of master.zkElectionPort, or a value derived as described 
               
               
                   
                 below if there is no old EmMaster object. 
               
               
                 ofpDomain 
                 The old value of master.ofpDomain, or a value derived as described 
               
               
                   
                 below if there is no old EmMaster object. 
               
               
                 epochState 
                 The old value of master.epochState, or EpochStart if there is no old 
               
               
                   
                 EmMaster object. 
               
               
                 managers 
                 The old value of master.managers, or the empty set if there is no old 
               
               
                   
                 EmMaster object. 
               
               
                 oldManagers 
                 The old value of master.oldManagers, or the empty set if there is no old 
               
               
                   
                 EmMaster object. 
               
               
                 maxManagerCount 
                 The old value of master.maxManagerCount. If there is no old EmMaster 
               
               
                   
                 object, use the largest value of any remote.maxEmCapableNodes. If that 
               
               
                   
                 value is 0, use 3. 
               
               
                 epochQuorum 
                 The old value of master.epochQuorum. If there is no old EmMaster 
               
               
                   
                 object, use the value (master.maxManagerCount/2) + 1. (XXX should 
               
               
                   
                 be 1 for hw mastership nodes) 
               
               
                 oldQuorum 
                 The old value of master.oldQuorum, or 0 if there is no old EmMaster 
               
               
                   
                 object. 
               
               
                   
               
            
           
         
       
     
     The epoch manager master computing node may next update the epoch manager master state. 
     The local copy of the state is updated, but that state is written back into OFP as an updated object only when explicitly mentioned. The EM Master computing node updates the EmMaster state as follows: 
     
       
         
           
               
             
               
                   
               
             
            
               
                 |   Update the value of master.epochPreference. Note that this value  
               
               
                 should not change often, and in particular cannot change on every cycle of  
               
               
                 the Object Event Process or it will never converge (each update will  
               
               
                 trigger a new Object Event). See section 8.4.12 for a discussion of how  
               
               
                 this might be set. 
               
               
                 (See if any computing node is requesting a restart.) 
               
               
                 |   If any remote.restartRequest != None and remote.epoch ==  
               
               
                 master.masterEpoch or remote.epoch == 0: (some node is requesting  
               
               
                 restart)  
               
               
                 |  |  Set master.epochState = EpochFail.  
               
               
                 |  |  If remote.restartRequest == ResetManagers set  
               
               
                 master.managers = &lt;empty set&gt;. 
               
               
                 |  |  Update the EmMaster object and exit the Object Event Process. 
               
               
                 (Manage the Epoch Manager set.) 
               
               
                 |  Switch on master.epochState: 
               
               
                 |  |  Case EpochStart: 
               
               
                 |  |  |  If a quorum (see section 8.4.10) of nodes for which  
               
               
                 remote.emCapable == True are reachable via OFP: (enough nodes to  
               
               
                 form a quorum) 
               
               
                 |  |  |  |  Set master.epochState = EpochInit.  
               
               
                 |  |  Case EpochInit: 
               
               
                 |  |  |  Update the Epoch Manager set according to section  
               
               
                 8.4.14. 
               
               
                 |  |  |  If less than a quorum of nodes (see section 8.4.10) for  
               
               
                 which remote.emCapable == True are reachable via OFP, set  
               
               
                 master.epochState = EpochFail.  
               
               
                 |  |  |  If a quorum of nodes (see section 8.4.10) in  
               
               
                 master.managers are reachable via OFP and each is reporting  
               
               
                 remote.managerUp and remote.epoch == master.masterEpoch, set  
               
               
                 master.epochState = EpochRunning. 
               
               
                 |  |  Case EpochRunning: 
               
               
                 |  |  |  If less than a quorum (see section 8.4.10) of nodes in  
               
               
                 master.managers are reachable via OFP: 
               
               
                 |  |  |  |  Set master.epochState = EpochFail.  
               
               
                 |  |  |  Else: (a quorum is reachable) 
               
               
                 |  |  |  |  Update the Epoch Manager set according to section  
               
               
                 8.4.14. If master.managers changes, set master.epochState =  
               
               
                 EpochReconfig. 
               
               
                 |  |  Case EpochReconfig: 
               
               
                 |  |  |  If less than a quorum (see section 8.4.10) of nodes in  
               
               
                 master.managers are reachable via OFP: (the new EM set has lost  
               
               
                 quorum) 
               
               
                 |  |  |  |  Set master.epochState = EpochFail.  
               
               
                 |  |  |  Else if less than a quorum as defined by  
               
               
                 master.oldQuorum (see section 8.4.10) of nodes in  
               
               
                 master.oldManagers are reachable via OFP: (the old EM set has  
               
               
                 lost quorum) 
               
               
                 |  |  |  |  Set master.epochState = EpochFail.  
               
               
                 |  |  |  Else: (a quorum is reachable) 
               
               
                 |  |  |  |  If a quorum of nodes (see section 8.4.10) in  
               
               
                 master.managers are reachable via OFP and each is reporting  
               
               
                 remote.managerUp and remote.epoch == master.masterEpoch,  
               
               
                 set master.epochState = EpochRunning. 
               
               
                   
               
            
           
         
       
     
     If there is no old EmMaster object, SysEpochMan  16  may generate new values for the System Epoch, the IPCB ports, and the OFP Domain ID. For the System Epoch, SysEpochMan  16  may select a random number out of a number space large enough (64 bits) to make the probability of collision unlikely (less than 0.1%), and set that value for the OFP Domain ID. 
     However, the port number space is much smaller, and as such SysEpochMan  16  may select a random number divisible by three that lies in the port range, assigning that value to the client port, the value+1 to the server port, and the value+2 to the election port. The System Epoch is written into IPCB itself, and if more than one epoch is ever bound to the IPCB plane, it will be detected and the system restarted. The newly-elected epoch manager master posts an IPCB write “/SysEpochMan/SystemEpoch/&lt;epoch&gt;”, where &lt;epoch&gt; is a character representation of the new System Epoch. Each computing node may listen for changes on this path and requests a system restart if a conflict is detected. 
     SysEpochMan  16  may next wait for the quorum of epoch manager computing nodes to configure IPCB  18 , forming the IPCB ensemble and then successfully executing IPCB  18  (as indicated by a quorum of epoch manager computing nodes announcing a remote.managerUp with a Boolean value set to true. Upon successfully executing IPCB  18 , SysEpochMan  16  may bind the IPCB plane to a system epoch, initiating an IPCB epoch event. 
     Each computing node executes an IPCB epoch process in response to this IPCB epoch event, which may result in updated objects being published to OFP domain 0 (as is, in some examples, always the case). The process is as follows: 
     
       
         
           
               
             
               
                   
               
             
            
               
                 | Call getChildren on the “/SysEpochMan/SystemEpoch/” path,  
               
               
                 requesting a new watch.  
               
               
                 | If local.epoch != 0 and any child exists with a different value than  
               
               
                 local.epoch:  
               
               
                    |  |   Set local.restartRequest = Restart. 
               
               
                   
               
            
           
         
       
     
     After successfully executing IPCB  18 , SysEpochMan  16  may update the local copy of epoch manager master  212  to update epochState to a value of “EpochRunning,” and publish the updated local copy of epoch manager master  212 . At this point, distributed operating system  10  has coalesced (or, in other words, is operational) and may support execution of application specific functions, like SysMan  20 , distributor  22 , application specific OFP domains, and applications. SysEpochMan  16  may change the set of epoch manager computing nodes at any time by specifying a new set of epoch manager computing nodes via the managers field of epoch manager master object  212  with the epochState field of the epoch manager master object  212  set to “EpochReconfig.” 
     SysEpochMan  16 , acting as epoch manager master, may also maintain the set of epoch manager computing nodes. Epoch manager master computing node may ensure that IPCB  18  state maintains coherence, which means potentially ensuring that there is always at least one computing node in common between epoch manager sets. Epoch manager master may also maintain the number of epoch managers in the system, increasing and decreasing the count as the set of computing nodes changes. 
     The inputs to the epoch manager management process are the set of reachable epoch manager-capable computing nodes and the previous epoch manager master state. The epoch manager master may ensure the existing epoch manager set is preserved while honoring any computing nodes with higher epoch manager priority (which will remove computing node of lower epoch manager priority from the set of epoch managers). In order to satisfy the coherence requirements of IPCB dynamic reconfiguration, SysEpochMan  16  may ensure that there is at least one computing node in common in the old and new epoch manager sets. SysEpochMan  16  may iterate, when necessary, forming intermediate epoch manager sets under the at least one common computing node rule, until the new epoch manger set is formed. 
     The process is as follows: 
     
       
         
           
               
             
               
                   
               
             
            
               
                 First, calculate the new value of master.maxManagerCount: 
               
               
                 |  Set roughCount = (number of reachable EM-capable nodes/3) | 1  
               
               
                 |  Set minCount = Min(master.maxManagerCount, 3)  
               
               
                 |  Set master.maxManagerCount = Min(Max(roughCount, minCount), 
               
               
                    7) 
               
               
                   
               
            
           
         
       
     
     In the above calculation, roughCount may represent a rough approximation of the desired number of epoch manager (EM) computing nodes based on the total number of EM-capable nodes. The low order bit is set to guarantee that it is odd (and nonzero). Next, minCount is the lowest possible target number of EM nodes, which is the lesser of 3 and the current number (so as to accommodate one- and two-EM-node systems). Finally, SysEpochMan  16  may select the larger of the rough count and the minimum count, but limit the larger of the two to 7 as the value of additional EM nodes adds little value and may result in processing delays. In some examples, one and two-EM-node systems without mastership hardware may end up with master.maxManagerCount==1, two-EM-node systems with mastership hardware will always end up with master.maxmanagerCount==2, and all other systems will end up with an odd number in the range of three to seven. 
     Next, SysEpochMan  16 , acting as epoch manager master, may select a new value of master.managers as follows: 
     
       
         
           
               
             
               
                   
               
             
            
               
                 |   Set the prospective manager set to all OFP-reachable members of master.managers 
               
               
                 reporting remote.managerUp == True. If there are more than master.maxManagerCount 
               
               
                 members, drop enough members with the lowest values of remote.emPriority to trim the 
               
               
                 size of the set. 
               
               
                 |   Add all reachable EM-capable nodes with remote.epoch == master.masterEpoch 
               
               
                 whose value of remote.emPriority is greater than any members of the prospective manager 
               
               
                 set, if necessary replacing the existing members with the lowest values of remote.emPriority 
               
               
                 in order to keep the size of the set less than or equal to master.maxManagerCount. 
               
               
                 |   If the prospective manager set does not include any node in master.managers 
               
               
                 reporting remote.managerUp == True, replace the prospective member that has the lowest 
               
               
                 value of remote.emPriority with the reachable current member that has 
               
               
                 remote.managerUp == True and the highest value of remote.emPriority. 
               
               
                 |   Set master.oldManagers to master.managers. 
               
               
                 |   Set master.managers to the prospective manager set. 
               
               
                 |   Set master.oldQuorum to master.epochQuorum. 
               
               
                 |   Set master.epochQuorum to (master.maxManagerCount/2) + 1. 
               
               
                   
               
            
           
         
       
     
     One effect of the foregoing process is to keep the manager set stable, while potentially favoring higher priority EM-capable nodes when they arrive. If the resulting set does not overlap the current set, SysEpochMan  16  may select one out of the current set (because the IPCB Server sets must, in some instances, always overlap to preserve the shared file system). The value of master.epochQuorum may be set to 1 for one- and two-EM systems, and will be at least 2 for larger systems. 
     Each node, including computing node  200 , supporting execution of distributed operating system  10  may also individually monitor the health of epoch manager nodes participating in the quorum. Detection of failure of the quarum does not occur when the node has not yet determined that the epoch is operational or when the epoch is in state “EpochReconfig” as the quorum has not yet stabilized. The process by which to detect the failure of the quorum is as follows: 
     
       
         
           
               
             
               
                   
               
             
            
               
                 |   If there is no EmMaster object, exit with No Failure. 
               
               
                 |   If local.epochUp == False, exit with No Failure.  
               
               
                 |   If master.epochState == EpochReconfig, exit with No Failure. 
               
               
                 |   Determine the number of reachable in-epoch EM nodes by  
               
               
                 counting all reachable EmNode objects in which remote.epoch ==  
               
               
                 master.masterEpoch and remote.managerUp == True. 
               
               
                 |   If the node count is less than master.epochQuorum, exit with  
               
               
                 Failure. 
               
               
                 |   If the node count is 1 and mastermaxManagerCount == 2 and  
               
               
                 local.hwMaster == False, exit with Failure. (The network is  
               
               
                 partitioned and the local node is not hardware master.) 
               
               
                 |   Otherwise, exit with No Failure. 
               
               
                   
               
            
           
         
       
     
     When the quorum fails, the monitoring computing nodes will detect the failure and restart. Quorum validation may depend on the epoch state published via the epoch manager master object  212 . For example, in state “EpochInit,” the set of epoch manager is converging, so a failure is detected by a loss of quorum among the set of OFP-reachable epoch manager computing nodes. In state “EpochRunning,” the computing nodes may detect failure by a loss of quorum among the set of epoch manager computing nodes reporting remote.managerUp. In state “EpochReconfig,” the new set of epoch managers is still converging, and the computing nodes detect failure when a loss of quorum occurs among the set of epoch manager nodes. 
     In view of the above, which assumes that an epoch existed prior to startup, there are some considerations for the initial startup. During the initial startup, none of the nodes maintain either an epoch manager object  210  or an epoch manager master object  212 . As such, all nodes start up asynchronously and publish initial epoch manager objects  212 , where the first epoch manager capable node elects itself as the epoch manager master. The other nodes then follow the self-elected epoch manager master node. At this point, each node has local.epoch equal to zero since no epoch has yet been created. 
     The self-elected epoch manager master stays in the “EpochStart” state, and does not publish any updates to the EmMaster field of the epoch manager master object  212  until a quorum of epoch manager capable nodes announce themselves via epoch manager objects  210  and unanimously agree on the identity of the epoch manager master node. In some instances (such as race conditions), more than one node may elect itself as epoch manager master. The process may not advance until mastership converges. 
     Assuming mastership converges with computing node  200  electing itself as epoch manager master, SysEpochMan  16  of computing node  200  publishes epoch manager master object  212  with the new system epoch (e.g., a value of 1), the new set of epoch managers, and an EpochInit state. All nodes may then update their respective epoch manager objects  210  with the new epoch, publishing the updated epoch manager objects  210  to confirm initialization of the new epoch. The newly elected epoch manager nodes execute IPCB  18 . 
     All nodes next post a write to IPCB  18  (with the full set of epoch managers acting as IPCB servers) in order to detect successful execution of IPCB  18 . All epoch manager nodes may also write to IPCB  18  (with themselves as the only server) to detect success at joining the IPCB ensemble. As the IPCB ensemble converges, the pending IPCB writes will complete, resulting in all nodes publishing via epoch manager objects  210  an epochStatus of “epochUp,” and the epoch manager nodes publishing a “managerUp” status. 
     Every node executes the epoch up process when an epochUp event occurs (meaning when epochState indicates a value of “epochUp”). The epochUp event is triggered by the asynchronous completion of a write into IPCB  18  in the object event process described above with all EM nodes as servers, indicating that an IPCB quorum for the epoch has successfully formed. As usual, if this process modifies any object, it is updated in OFP. The process is as follows: 
     Set local.epochUp=True. 
     Once a quorum of epoch managers reaches the managerUp state, all nodes perform the following manager up process. Every node selected as an Epoch Manager may execute the Manager Up process when a managerUp event occurs. This event is triggered by the asynchronous completion of a write into IPCB  18  in the object event process with only the local node as a server, indicating that the local node has successfully joined the IPCB quorum. If this process modifies any object, it is updated in OFP. The process is as follows: 
     Set local.managerUp=True. 
     After performing the manager up process, the epoch manager master node publishes an updated epoch manager master object  212  with epochState set to a value of “EpochRunning.” In response to the updated epochState of “EpochRunning,” each node then executes the upper layers of software stack  11 . 
     Furthermore, OFP  14  (executed by one or more processors implemented in digital logic circuitry, not shown in  FIG. 3 ) includes state  208 . State  208  includes a data structure, such as a tree data structure (e.g., a radix trie) storing a plurality of objects, each of the objects defining a portion of state information for at least one of distributed operating system  10  or an application executed in an application space provided by distributed operating system  10 . 
     In general, computing node  200  (more particularly, the one or more processors) forms the tree data structure for state  208  to include a plurality of hierarchically arranged tree nodes, each of the tree nodes storing data for a message fragment including, e.g., a digest and a fragment ID. The tree data structure for state  208  may be arranged according to the fragment IDs of the tree nodes. 
     Computing node  200  may be configured to synchronize state  208  with corresponding data structures of other nodes executing other instances of distributed operating system  10 . In general, state  208  may be considered synchronized with data structures of the other nodes executing the other instances of distributed operating system  10  when tree data structures of each of the nodes executing instances of distributed operating system  10  have a common arrangement and interconnection of tree nodes within each of the tree data structures, and when corresponding tree nodes of the tree data structures have the same digest values. 
     Computing node  200  may further determine whether or not state  208  is synchronized with state data structures of other nodes executing other instances of distributed operating system  10 . Computing node  200  may recursively walk the tree nodes of a tree data structure representing state  208 . If local digests of tree nodes of the tree data structure representing state  208  match digests of corresponding tree nodes of tree data structures of nodes executing other instances of distributed operating system  10 , computing node  200  may determine that the tree nodes are synchronized. Otherwise, computing node  200  may determine that an update to state  208  is necessary. 
     If an update is necessary, computing node  200  may determine whether its version of state  208  is most current, or if another version of a tree data structure storing state information of a different node executing another instance of distributed operating system  10  is most current. If the version of a tree node of a tree data structure representing state  208  of computing node  200  is most current (e.g., has a most current logical clock value), computing node  200  may send message fragment data for the tree node of the tree data structure representing state  208  to one or more of the other nodes executing other instances of distributed operating system  10 . Otherwise, if the version of the tree node of the tree data structure representing state  208  of computing node  200  is not most current, computing node  200  may receive message fragment data for the tree node of the tree data structure representing state  208  from a different one of the other nodes executing another instance of distributed operating system  10  that is most current, and update the tree node of the tree data structure representing state  208  with the received message fragment data. 
       FIGS. 4A-8B  are block diagrams illustrating various aspects of node operation within the multi-chassis router shown in  FIG. 2  in addressing various scenarios that may impact execution of the distributed operating system maintained in accordance with the techniques described in this disclosure.  FIGS. 4A-4D  are block diagrams illustrating operation of nodes  302 A- 302 F (“nodes  302 ”) in addressing epoch manager failures during execution of the distributed operating system in accordance with various aspects of the techniques described in this disclosure. Each of nodes  302  may be substantially similar to computing node  200  shown in  FIG. 3 . 
     In the example of  FIG. 4A , nodes  302  may collectively execute distributed operating system  300 A with node  302 A operating as the epoch manager master (“EM master”), node  302 D and  302 F operating as elected epoch managers (“EMs”), node  302 C operating as an unelected epoch manager, but epoch manager-capable (“EM-capable”), and nodes  302 B and  302 E operating as non-EM-capable. The heavier weighted lines interconnecting nodes  302  may represent multicast (or, in some instances, broadcast) links, while the less heavier weighted lines interconnecting nodes  302  may represent unicast links. 
     The state of distributed operating system  300 A is “epochRunning,” with a quorum of three nodes (i.e., nodes  302 A,  302 D, and  302 F in the example of  FIG. 4A ) executing as IPCB servers. As such, the quorum may be maintained so long as two of the three nodes (given that (3/2)+1=2) forming the quorum remain operational. In other words, distributed operating system  300 A may remain operational despite failure of one of nodes  302 A,  302 D, and  302 F forming the quorum. 
     In the example of  FIG. 4B , nodes  302 A and  302 B fail resulting in distributed operating system  300 B. However, because two epoch managers (i.e., nodes  302 D and  302 F in the example of  FIG. 4B ) remain operational, distributed operating system  300 B may maintain state information coherence and continue to execute. Given that epoch manager master node  302 A failed, distributed operating system  300 B elects node  302 D as epoch manager master. 
     Given that the quorum only includes two nodes, the quorum cannot lose another epoch manager node while still maintaining operation of distributed operating system  300 B. As such, epoch manager master node  302 D may reconfigure the epoch manager set to include node  302 C using the above described process. In electing EM-capable node  302 C to operate as an epoch manager, node  302 C may execute as an IPCB server and copy all IPCB state to a local IPCB server executed by node  302 C, resulting in distributed operating system  300 C. 
     In the example of  FIG. 4D , it is assumed that nodes  302 A,  302 B, and  302 D all fail at the same time (i.e., transitioning from distributed operating system  300 A of  FIG. 4A  to distributed operating system  300 D shown in  FIG. 4D  in this example). Losing nodes  302 A,  302 B, and  302 D, distributed operating system  300 D loses the quorum of epoch managers (and IPCB servers). 
     As such, it is possible that an IPCB client could have written to IPCB servers, and received an acknowledgement that such state was written to IPCB, but that the state was only present at nodes  302 A and  302 D at the time of failure. In this scenario, the state is unrecoverable (or, in other words, lost), and as such, distributed operating system  300 D fails, restarting one or more layers in software stack  11  of distributed operating system  300 D. As nodes  302  reboot, nodes  302  converge on a new value of the system epoch, resulting in distributed operating system  300 D with node  302 F acting as epoch manager master and node  302 C acting as epoch manager. 
       FIGS. 5A-5C  are block diagrams illustrating operation of nodes  302  in addressing partition of nodes  302  as a result of failures during execution of the distributed operating system in accordance with various aspects of the techniques described in this disclosure. Each of nodes  302  may be substantially similar to computing node  200  shown in  FIG. 3 . 
     In the example of  FIG. 5A , distributed operating system  310 A has partitioned due to failure of links  312 A- 312 C, resulting in a first partition consisting of nodes  302 A and  302 B and a second partition of nodes  302 C- 302 F. Each of the first partition and the second partition exist because there is no communication between the first partition and the second partition. 
     From the perspective of nodes  302 C- 302 F in the second partition, nodes  302 A and  302 B have failed. At the time of failure the system epoch value was 42, and both the first partition and the second partition continue to execute distributed operating system  310 A with a system epoch value of 42. Upon failure, nodes  302 A and  302 B of the first partition determines that the quorum of epoch managers has been lost, resulting in restarting one or more layers of software stack  11  of nodes  302 A and  302 B. Nodes  302 C- 302 F of the second partition lose only a single epoch manager (i.e., epoch manager master node  302 A in this example) and the quorum of epoch managers is not lost, thereby allowing distributed operating system  310 A to continue operation by nodes  302 C- 302 F of the second partition. 
       FIG. 5B  illustrates the result of the restart of the first partition, which cannot reboot and organize because the quorum still does not yet exist due to the failure of links  312 A- 312 C. Node  302 A elects itself as epoch manager master, but detects the loss of quorum and therefore cannot functionally participate in execution of distributed operating system  310 B (resulting in an inability to assign a system epoch value as the first partition is not functional, where the lack of system epoch value is expressed as “??” in the example of  FIG. 5B ). 
     Node  302 A stores the epoch manager set (e.g., as a list of node IDs assigned by OFP  14 ) and the number of nodes in the quorum of epoch managers, maintaining both the epoch manager set and the number of nodes in the quorum through the reset process. As such, node  302 A may determine that a single node (i.e., node  302 A in the example of  FIG. 5B ) is insufficient to satisfy the quorum threshold of (N/2)+1, where N is the total number of nodes in the previous quorum. Node  302 A may also, even when there are a sufficient number of EM-capable nodes in the first partition, determine that the quorum threshold is not satisfied because the set of epoch managers from the previous quorum would not match the set of EM-capable nodes. 
     In the second partition, node  302 D is elected as the epoch manager master, with node  302 F remaining as an epoch manager. Node  302 D may reconfigure the epoch manger set using the above described processes to include node  302 C, thereby allowing for distributed operating system  310 B to remain operational even when one of nodes  302 C,  302 D, and  302 F fails. 
     In the example of  FIG. 5C , link  312 A becomes operational, allowing the partitions to merge (or, in other words, “heal”). Once link  312 A becomes operational, node  302 D remains as the epoch manager master given that node  302 A was never an operational epoch manager master given that the quorum threshold was not satisfied in the first partition. Nodes  302 C and  302 F remain as epoch managers, while node  302 A is demoted to EM-capable. Nodes  302  thereby execute distributed operating system  310 C, having a system epoch value of 42. 
       FIGS. 6A and 6B  are block diagrams illustrating operation of nodes  302  in addressing controlled shutdown of nodes  302  during execution of the distributed operating system in accordance with various aspects of the techniques described in this disclosure. Again, each of nodes  302  may be substantially similar to computing node  200  shown in  FIG. 3 . 
     In some instances, a system administrator may require that one or more of nodes  302  are removed from supporting execution of distributed operating system  310 A shown in  FIG. 5C . In the example of  FIG. 6A , nodes  302 D and  302 F are withdrawn (or, in other words, removed) from supporting execution of distributed operating system  310 A shown in  FIG. 5C  resulting in distributed operating system  320 A. Withdrawing node  302 D results in the loss of the epoch manager master, while the withdrawing node  302 F results in the loss of an epoch manager. Furthermore, withdrawing two of the three epoch managers (i.e., nodes  302 D and  302 F in the example of  FIG. 6A ) would result in loss of the quorum. 
     To avoid losing the quorum, nodes  302 D and  302 F issue a request to withdraw the respective EM-capable status prior to being withdrawn. Node  302 C may receive the requests, and elect itself as epoch manager master, promoting node  302 A as an epoch manager to maintain the quorum (as two nodes meet the quorum threshold of (N/2)+1). In promoting node  302 A as an epoch manager, node  302 C may reconfigure the epoch manager set to remove node  302 D and  302 F, thereby allowing nodes  302 D and  302 F to withdrawn and thereby no longer support operation of distributed operating system  320 A, resulting distributed operating system  320 B shown in  FIG. 6B . 
     Although shown as taking a single iteration to withdraw one or more of nodes  302 , there may be instances where multiple iterations are required to withdraw one or more of nodes  302 . Any number of nodes can be withdrawn so long as at least one epoch manager remains between the new set of epoch managers and the old set of epoch managers. The requirement for one epoch manager to remain between the old and new set of epoch mangers is to preserve the IPCB state. Thus, in the instance where all of the epoch managers in the set are to be withdrawn, one epoch manager may remain, forming an intermediate set of epoch managers. Once the intermediate set of epoch managers is formed, the old epoch manager m managing the transition between the old and intermediate set of epoch managers may withdraw to form the new set of epoch managers. 
     In some examples of single chassis routers includes only one or two routing engines and one or more of forwarding units, which may include, as one example, flexible PIC concentrators (FPCs). The forwarding units may not be EM-capable because the forwarding units may not be capable of running IPCB  18 . As such, the systems that have only one or two EM-capable nodes in the routing engines (and thus only one or two IPCB Servers). 
     The requirement of the IPCB dynamic reconfiguration mechanism that the old and new ensemble memberships overlap by at least one node essentially means that there must be at least three IPCB nodes for the process to be useful (the guy going away, the guy sticking around, and the new guy). Furthermore, IPCB may require at least two nodes to function at all, as that is the smallest possible quorum. 
     In order to execute in one or two EM-capable node systems, IPCB may operate in a different way on these one or two EM-capable node systems. There may be two issues to modifying IPCB—how to keep IPCB running and consistent, and how to avoid Split Brain when a two-EM-capable node system partitions. 
     IPCB may execute in two modes, Replicated and Standalone. In Replicated mode, multiple IPCB servers are present and they coordinate with each other (and there must be at least two of them). In Standalone mode, there is only a single IPCB server. IPCB may be restarted in order to switch between the modes. 
     On single-node systems, IPCB may operate in Standalone mode. There is no redundancy in this system, so it is not particularly problematic in the scheme described in this document—if the sole epoch manager fails, the system also fails. 
     On two-EM-capable node systems, IPCB may be switched back and forth between Replicated and Standalone modes when one of the EM-capable nodes fails and then recovers. When going from Standalone to Replicated mode, consistency may be guaranteed because there is only one copy of the IPCB state, which is reloaded from the IPCB transaction log on the local file system on the first node, and the second IPCB server receives all state from the first IPCB server. When going from Replicated to Standalone mode, consistency may be guaranteed because a two-EM-capable node system has a IPCB quorum size of two, which may result in the nodes having the latest transaction written to their transaction logs before the transaction is committed. The single IPCB server left after the restart may, in this way, have all transactions. 
     When a two-EM-capable node system becomes partitioned, the split brain situation described in more detail below could occur (where both sides would come up in Standalone mode). However, two-EM-capable systems have mastership hardware, an FPGA that designates one node or the other as master. The mastership hardware may be leveraged, and the quorum rules may be adjusted in the two-EM-capable node system to define when a quorum is present to be only when one node is reachable and that node has hardware mastership. This quorum rule adjustment solves the split brain situation because only one of the two nodes will be master, and the other will not have quorum and will thus restart and stay down until the partitions merge. 
       FIGS. 7A-7C  are block diagrams illustrating operation of nodes  302  in addressing multiple partitions of nodes  302  as a result of failures during execution of the distributed operating system in accordance with various aspects of the techniques described in this disclosure. Each of nodes  302  may be substantially similar to computing node  200  shown in  FIG. 3 . 
     In the example of  FIG. 7A , links  312 A- 312 E all fail, resulting in three partitions. The first partition includes node  302 A and  302 B. Node  302 A elects itself as the epoch manager master, but cannot reestablish the quorum as there are insufficient epoch managers of the old epoch manager set in the first partition to satisfy the quorum threshold of (N/2)+1. 
     The second partition includes nodes  302 C and  302 D. Prior to the failure of links  312 A- 312 E, node  302 D was an epoch manager. Node  302 D may elect itself as epoch manager master, but is unable to maintain the quorum as two of the three epoch manager nodes from the previous quorum (i.e., nodes  302 A and  302 F in the example of  FIG. 7A ) are unavailable. 
     The third partition includes nodes  302 E and  302 F. Prior to the failure of links  312 A- 312 E, node  302 F was an epoch manager. Node  302 F may elect itself as epoch manager master, but is unable to maintain the quorum as two of the three epoch manager nodes from the previous quorum (i.e., nodes  302 A and  302 D in the example of  FIG. 7A ) are unavailable. 
     As such, distributed operating system  330 B shown in  FIG. 7B  results in which none of the three partitions are able to execute distributed operating system  330 B. Because none of nodes  302  are able to execute distributed operating system  330 B, the system epoch value for each of the partitions is unknown (as denoted by the “??” in  FIG. 7B ). The epoch manager masters of each partition (i.e., nodes  302 A,  302 D, and  302 F in the example of  FIG. 7B ) wait until one or more of links  312 A- 312 E become operational to reform the quorum and continue execution of the distributed operating system. 
     In the example of  FIG. 7C , the partitions have merged as a result of links  312 A and  312 E becoming operational. Nodes  302  negotiate which of the previous epoch manager masters will remain master (e.g., by way of the EM master priority discussed above). Node  302 F remains as epoch manager master in the example of  FIG. 7C  with node  302 D and  302 A executing as epoch managers. As such, nodes  302  exchange state information to regain coherency, and update to a system epoch value of 43 (from 42 as shown in the example of  FIG. 7A ). Nodes  302  may collectively execute distributed operating system  330 C with a system epoch value of 43 (to distinguish from the version identified by the system epoch value of 42). 
       FIGS. 8A and 8B  are block diagrams illustrating operation of nodes  302  in addressing “split brain” situations as a result of failures during execution of the distributed operating system in accordance with various aspects of the techniques described in this disclosure. Each of nodes  302  may be substantially similar to computing node  200  shown in  FIG. 3 . 
     Split brain situations refer to situations in which a system divides into two or more partitions where at least two of the partitions remain operational as a result of not being aware that the other partition is still operational resulting in a divided or split execution environment (or, in other words, “brain”). In normal operation where distributed operating system has previously executed, split brain situations are avoided by the quorum system regulated by the quorum threshold and the previous set of epoch managers as discussed above. 
     Split brain situations may occur when a system/device is started with no previous state (e.g., in particular, no set quorum size and/or threshold) and the nodes of the device are partitioned. In the example of  FIG. 8A , nodes  302  are partitioned into two partitions due to link failures, where the first partition includes nodes  302 A and  302 B, and the second partition includes nodes  302 C- 302 F. Node  302 A is elected as the epoch manager master of the first partition, and designates node  302 B as an epoch manager for the first partition. Node  302 D is elected as the epoch manager master for the second partition, and designates each of nodes  302 C and  302 F as epoch managers. 
     In this split brain situation, the first partition of distributed operating system  340 A may select a system epoch value of 1234, while the second partition of distributed operating system  340 A selects a system epoch value of 42. Considering that the system epoch value denotes a version of distributed operating system  340 A and allows for proper synchronization between different versions of distributed operating system  340 A, the selection of system epoch values during initial boot of distributed operating system  340 A is random so as to avoid two partitions selecting the same system epoch value as that would impact synchronization between different nodes. 
     Assuming that one of the links becomes operations as shown in the example of  FIG. 8B , the two partitions of distributed operating system  340 B merge. SysEpochMan  16  utilizes a preference mechanism (described above) to determine which epoch to preserve and which to discard, in order to avoid restarting the most recent version (or “best”) version after the partitions merge. 
       FIG. 9  is a flowchart illustrating exemplary operation of the node of the multi-chassis router shown in  FIG. 3  in performing various aspects of the distributed operating system techniques described in this disclosure. As described above, computing node  200  initially executes OFP  14  to determine a topology of nodes that allows for coalescence and execution by the nodes of the single instance of distributed operating system  10  ( 400 ). OFP physical topology discovery may occur in a manner similar to that of link state protocols. OFP  14  constructs the graph data structure representative of the topology of primary and secondary nodes interconnected with one another by the links based on the announcements. 
     Next, computing node  200  may execute SysEpochMan  16 , which may, based on the graph data structure representative of the topology of primary and secondary nodes, elect an epoch manager master from among those nodes configured to execute as epoch managers ( 402 ). The elected epoch manager master may elect one or more of the epoch managers (including the elected epoch manager master) to act as epoch managers ( 404 ). Each of the epoch managers may then execute IPCB  18  ( 404 ). 
     IPCB  18  forms a network of servers and clients. The servers may be referred to as an IPCB ensemble. IPCB  18  may establish a quorum of epoch managers in which a majority of servers (e.g., more than (N/2)+1, where N represents the number of servers/epoch managers) are connected and functioning for IPCB  18  to continue successful operation of distributed operating system  10  ( 408 ). In this way, the techniques may allow for separate (or in other words individual) compute nodes to coalesce for purposes of executing distributed operating system  10 . 
     After forming the quorum and establishing IPCB  18  by which the clients may interface with the shared file system, IPCB  18  may monitor the IPCB servers (which is another way to refer to epoch managers) to detect epoch manager failures (e.g., as measured by whether connectivity between one or more of the plurality of nodes has failed) ( 410 ). When no connectivity failures occurs (“NO”  412 ), IPCB  18  continues to monitor the quorum to detect epoch manager failures ( 410 ). 
     When an IPCB epoch manager fails or a link fails (which may be generally referred to as a “connectivity failure”) (“YES”  412 ), the remaining IPCB epoch managers may determine whether the quorum of epoch managers exists. The remaining IPCB epoch manager may determine whether the quorum of epoch managers exists by comparing the number of operational epoch managers (denoted by the variable “N”) is less than a quorum threshold (e.g., (N/2)+1) ( 414 ). 
     When the number of operational epoch managers is less than the quorum threshold (“YES”  414 ), the remaining epoch managers may restart distributed operating system  10  (which may not require restarting multi-chassis router  4  or kernel  12 , but only restarting one or more of those layers above kernel  12  in software stack  11 , such as OFP  14 , SysEpochMan  16 , IPCB  18 , SysMan  20 , and/or distributor  22 ) ( 416 ). Upon restarting, the process starts again with execution of the protocol to determine the topology of nodes, etc. ( 400 - 410 ). When the number of operational epoch managers is greater than or equal to the quorum threshold (“NO”  414 ), the remaining epoch managers may maintain the quorum and continue operating (monitoring the quorum to detect epoch manager failures — 410 ), potentially adding to the quorum new epoch managers that were not elected as epoch managers during the formation of the quorum. 
       FIG. 10  is a conceptual diagram illustrating an example tree data structure  470  for storing state information in accordance with techniques of this disclosure. In this example, tree data structure  470  includes root tree node  450  and tree nodes  452 - 464 . Each of tree nodes  452 - 464  includes a prefix value and a digest value. In this example, the prefix value of tree node  452  may be XX/104, the prefix value of tree node  454  may be XXX/108, the prefix value of tree node  456  may be XXY/108, the prefix value of tree node  458  may be XXXX/112, the prefix value of tree node  460  may be XXXY/112, the prefix value of tree node  462  may be XXYX/112, and the prefix value of tree node  464  may be XXYY/112. In this example, tree nodes  458 - 464  are leaf tree nodes of tree data structure  470 , because tree nodes  458 - 464  do not have any child tree nodes. 
     Each of the tree nodes of tree data structure  470  also includes a digest value. In general, each digest value represents all fragments in the blocks they represent. Thus, root  450  includes digests that represent all messages in tree data structure  470 . The digest for tree node  454  covers message fragments of tree nodes  454 ,  458 , and  460 , while the digest for tree node  456  covers message fragments of tree nodes  464 ,  464 . To determine whether two tree data structures, such as tree data structure  470 , are the same, the digests of tree node  452  and a corresponding tree node of a different tree data structure being compared to tree data structure  470  can be compared, and if each of these digests matches between the tree data structures, the tree data structures can be said to be the same, and therefore, are synchronized. 
     If two such tree data structures are not synchronized, a node, such as computing node  200  of  FIG. 3 , may recursively walk tree data structure  470  to determine which of tree nodes  452 - 464  is to be updated. Computing node  200  may start at tree node  452  and walk tree data structure  470  down to the leaf tree nodes, i.e., tree nodes  458 - 464 . Computing node  200  may then compare each of the leaf tree nodes  458 - 464  to the corresponding leaf tree nodes of the other tree data structure. For each leaf tree node that does not match, computing node  200  may exchange messages with another node of distributed operating system  10  that is storing the other tree data structure to synchronize the corresponding tree nodes of the tree data structure, as discussed above. 
       FIG. 11  is a flowchart illustrating an example method for synchronizing state information between different instances of a distributed operating system executed by respective computing nodes of a network device in accordance with the techniques of this disclosure. In this example, two nodes are described, although it should be understood that additional nodes may perform a substantially similar method. The nodes may each include components similar to those discussed with respect to computing node  200  of  FIG. 3 . 
     In this example, a first node initially constructs a data structure including a plurality of objects, each of the objects storing state information ( 500 ). The data structure may be a tree data structure as discussed above. Thus, construction of the tree data structure may further involve calculating digests for leaf tree nodes of the tree data structure, as well as digests for non-leaf tree nodes of the tree data structure. The digests for the non-leaf tree nodes may represent data for the corresponding tree nodes and tree nodes accessible by the corresponding tree nodes (e.g., child tree nodes down to the leaf tree nodes). The state information may be, for example, state information for the distributed operating system itself, and/or for one or more applications executed in an application space provided by the distributed operating system. The objects may represent messages or message fragments, as discussed above. Furthermore, the objects may be distributable according to the object flooding protocol (OFP), as also discussed above. Accordingly, the first node floods the objects to other computing nodes of the network device ( 502 ), e.g., in accordance with OFP. Accordingly, a second node in this example receives the objects ( 504 ) and stores a data structure including the objects ( 506 ). 
     Subsequently, the first node receives updated state information ( 508 ). For example, the first node may receive updated state information for one of the applications or for the distributed operating system. In response, the first node updates relevant objects of the data structure ( 510 ), i.e., the objects corresponding to the updated state information, to store the updated state information. When updating the objects of the data structure, the first node may also update a logical clock value associated with the objects of the data structure, to represent a time at which the objects of the data structure were updated. As discussed above, assuming the data structure is a tree data structure, the first node may update a tree node of the tree data structure corresponding to the updated state information, as well as digests for each tree node of the tree data structure between the root tree node and the hierarchically lowest tree node impacted by the updated state information. 
     Moreover, after updating the data structure, the first node floods the updated objects (messages or message fragments) to other computing nodes of the network device ( 512 ), e.g., according to OFP. The first node also updates its configuration using the updated data structure. For example, assuming the data structures are tree data structures, the first and second computing nodes of the network device may compare digests of corresponding tree nodes of the tree data structures to determine whether the corresponding tree nodes of the tree data structures match (that is, have equal digests). For each tree node of the tree data structures that do not have matching digests, the first node (which is assumed to have a more up to date version of the state information in this example) floods object data (i.e., the message fragment of the tree node) to the second node. More generally, the first and second computing nodes of the network device (and any other computing nodes of the network device) may compare logical clock values for the corresponding tree nodes of the tree data structures to determine which tree data structure has a most current version of the tree node of the tree data structures, and then the computing node of the network device having the most up to date tree node of the tree data structures floods the object for the tree node of the tree data structure to the other computing nodes of the network device. 
     In response to receiving the flooded objects from the first node ( 516 ), the second node also updates objects of its data structure ( 518 ) in a manner similar to the first node, and updates its configuration using the updated data structure as well ( 520 ). 
     In this manner, the method of  FIG. 11  represents an example of a method including receiving, by a first computing node of a network device that executes a first instance of a distributed operating system, updated state information for at least one of the distributed operating system or an application executed in an application space provided by the distributed operating system, updating, by the first computing node of the network device, a local data structure of the first computing node of the network device to include the updated state information, the local data structure storing a plurality of objects, each of the objects defining a portion of state information for at least one of the distributed operating system or the application, and synchronizing, by the first computing node of the network device, the updated local data structure with a remote data structure of a second instance of the distributed operating system executed by a second computing node of the network device. 
     Although in  FIG. 11  only the first computing node of the network device is shown as receiving updated state information, it should be understood that in other examples, other computing nodes of the network device may receive updated state information and flood corresponding objects to the first node. For example, the second node discussed with respect to  FIG. 11  may receive updated state information, update its data structure to include the updated state information, and then flood objects representing the updated state information to the first computing node of the network device. As noted above, in general, each computing node of the network device compares digests of tree nodes of respective tree data structures to determine whether the tree nodes of the tree data structures match. When digests of corresponding tree nodes of the tree data structures do not match, the computing nodes of the network device may compare logical clock values associated with the tree nodes of the tree data structures to determine which tree data structure includes the most up to date tree node of the tree data structure. The computing node of the network device having the most up to date tree node of the tree data structure floods data for the tree node of the tree data structure to the other computing nodes of the network device. 
     One or more of the techniques described herein may be partially or wholly executed in software. For example, a computer-readable medium may store or otherwise comprise computer-readable instruction, i.e., program code that can be executed by a processor to carry out one or more of the techniques described above. For example, the computer-readable medium may comprise random access memory (RAM), read-only memory (ROM), non-volatile random access memory (NVRAM), electrically erasable programmable read-only memory (EEPROM), flash memory, magnetic or optical media, or the like. 
     Various embodiments of the invention have been described. Although described in reference to a multi-chassis router, which each chassis including a plurality of routing engines, the techniques may be applied to any multi-chassis device having a plurality of control nodes in at least one chassis. Examples of other devices include switches, gateways, intelligent hubs, firewalls, workstations, file servers, database servers, and computing devices generally. Furthermore, the described embodiments refer to hierarchically-ordered and temporally-linked data structures, but other embodiments may use different data structures. These and other embodiments are within the scope of the following claims.