Patent Publication Number: US-7225356-B2

Title: System for managing operational failure occurrences in processing devices

Description:
This is a non-provisional application of provisional application Ser. No. 60/517,776 by A. Monitzer filed Nov. 6, 2003. 
    
    
     FIELD OF THE INVENTION 
     This invention concerns a system for managing operational failure occurrences in processing devices of a group of networked processing devices. 
     BACKGROUND OF THE INVENTION 
     Computing platforms are used in various industries (telecommunication, healthcare, finance, etc.) to provide high availability online network accessed services to customers. The operational time (uptime) of these services is important and affects customer acceptance, customer satisfaction, and ongoing customer relationships. Typically a service level agreements (SLA) which is a contract between a network service provider and a service customer, defines a guaranteed percentage of time the service is available (availability). The service is considered to be unavailable if the end-user is not able to perform defined functionality at a provided user interface. Existing computing network implementations employ failover cluster architectures that designate a back-up processing device to assume functions of a first processing device in the event of an operational failure of the first processing device in a cluster (group) of devices. Known failover cluster architectures typically employ a static list (protected peer nodes list) of processing devices (nodes of a network) designating back-up processing devices for assuming functions of processing devices that experience operational failure. A list is pre-configured to determine a priority of back-up nodes for individual active nodes in a cluster. In the event of a failure of an active node, a cluster typically attempts to fail over to a first available node with highest priority on the list. 
     One problem of such known systems is that multiple nodes may fail to the same back-up node causing further failure because of over-burdened computer resources. Further, for a multiple node cluster, existing methods require a substantial configuration effort to manually configure a back-up processing device. In the event that two active nodes fail in a multiple node cluster configured with the same available back-up node as highest priority in their failover list, both nodes failover to this sarne back-up node. This requires higher computer resource capacity for the back-up node and increases the cost of the failover configuration. In existing systems, this multiple node failure situation may possibly be prevented by user manual reconfiguration of failover configuration priority lists following a single node failure. However, such manual reconfiguration of an operational node cluster is not straightforward and involves a risk of causing failure of another active node leading to further service disruption. Further, in existing systems a node is typically dedicated as a master server and other nodes are slave servers. A cluster may be further separated into smaller cluster groups. Consequently, if a disk or memory shared by master and slave or separate groups in a cluster fails, the cluster may no longer be operational. Also, load balancing operations are commonly employed in existing systems to share operational burden in devices in a cluster and this comprises a dynamic and complex application that increases risk. A system according to invention principles provides a processing device failure management system addressing the identified problems and deficiencies. 
     SUMMARY OF THE INVENTION 
     A system automatically adaptively modifies a fail-over configuration priority list of back-up devices of a group (cluster) of processing devices based on factors including, for example, a current load state of the group, memory usage of devices in the group, and availability of passive back-up processing devices in the group to improve availability and reduce risks and costs associated with manual configuration. A system is used by individual processing devices of a group of networked processing devices, for managing operational failure occurrences in devices of the group. The system includes an interface processor for maintaining transition information identifying a second processing device for taking over execution of tasks of a first processing device in response to an operational failure of the first processing device and for updating the transition information in response to a change in transition information occurring in another processing device of the group. An operation detector detects an operational failure of the first processing device. Also, a failure controller initiates execution, by the second processing device, of tasks designated to be performed by the first processing device in response to detection of an operational failure of the first processing device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows a block diagram of a system used by a group of networked processing devices, for managing operational failure occurrences in devices of the group, according to invention principles. 
         FIG. 2  shows a flowchart of a process used by the system of  FIG. 1  for managing operational failure occurrences in devices of a group of networked processing devices, according to invention principles. 
         FIG. 3  shows a network diagram of a group of networked processing devices managed by the system of  FIG. 1 , according to invention principles. 
         FIG. 4  shows an exemplary configuration of a group of networked processing devices managed by the system of  FIG. 1 , according to invention principles. 
         FIGS. 5-9  show prioritized lists illustrating automatic failure management of back-up processing devices assuming functions of processing devices in the event of device operational failure, according to invention principles. 
         FIG. 10  shows a flowchart of a process used by AFC  10  of the system of  FIG. 1  for managing operational failure occurrences in devices of a group of networked processing devices, according to invention principles. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION  
       FIG. 1  shows a block diagram of a system including automatic failover controller (AFC)  10  for managing operational failure occurrences in processing devices (nodes) of a group of networked processing devices (not shown) accessed via communication network  20 . The system enables grouping (clustering) of multiple nodes and improves overall cluster availability. An individual node in a cluster in the system has a priority list identifying back-up nodes for each active node that is protected. The list contains a priority list of active nodes and may be termed a protected peer nodes list. In a known existing failure system implementation, a protected peer node list is static, therefore in case of a failure of one active node, a failure management system searches for a first available back-up node in the priority list independent of the current resource utilization of the backup node found. In contrast, the system of  FIG. 1  automatically adapts and optimizes a protected peer nodes list for a current state of processing devices (nodes) in a cluster. The  FIG. 1  system facilitates failure management of multiple nodes operating in a clustered configuration. A node is a single processing device or topological entity connected to other nodes via a communication network (e.g., network  20 , a LAN, intra-net or Internet)). A processing device as used herein, includes a server, PC, PDA, notebook, laptop PC, mobile phone, set-top box, TV, or any other device providing functions in response to stored coded machine readable instruction. It is to be noted that, the terms node and processing device as well as the terms cluster and group are used interchangeably herein. 
     A cluster is a group of nodes that are connected to a cluster network and that share certain functions. Functions provided by a cluster are implemented in software or hardware. Individual nodes that participate in a cluster incorporate failure processing functions and provide failure management (fail-over) capability to back-up nodes. In the  FIG. 1  system an individual node also provides the processor implemented function supporting cluster management including adding nodes to a cluster and removing nodes from the cluster. A processor as used herein is a device and/or set of machine-readable instructions for performing tasks. As used herein, a processor comprises any one or combination of, hardware, firmware, and/or software. A processor acts upon information by manipulating, analyzing, modifying, converting or transmitting information for use by an executable procedure or an information device, and/or by routing the information to an output device. A processor may use or comprise the capabilities of a controller or microprocessor, for example. 
     The  FIG. 1  system reconfigures a cluster configuration (and updates a back-up priority list) based on detection of a change of state (e.g., from available to unavailable) of a node in the cluster. This reconfiguration function is implemented, for example, using Failover Engine  14  which uses network controller  12  and network  20  to notify primary Automatic Failover Controller (AFC) configuration repository  40  and other AFC cluster processing devices  19  about state changes. Failover Engine  14  also responds to configuration changes and synchronization messages forwarded by the network controller  12  and responds to notifications communicated by heartbeat engine  18 . In response to the received messages Failover Engine  14  initiates modification of a local failover configuration stored in repository  16  and communicates data indicating the modified configuration via network controller  12  and network  20  to primary configuration repository  40  and other AFC units  19 . 
     The  FIG. 1  system architecture provides a robust configuration capable of managing multiple failures of operational devices in a group. The system dynamically optimizes a configuration indicated in a predetermined processing device back-up list for a group including multiple processing devices. The system readily scales to accommodate an increase in number of nodes and reduces data traffic required between AFC  10  and other AFCs  19  in the event of multiple node failures. Further, upon the occurrence of two node failures in a group of nodes, both nodes do not failover to the same back-up node if different backup nodes are available as indicated by the priority lists. The system reduces or eliminates the need for manual intervention and extensive testing to ensure that, after a particular node assumes operation of tasks of a failed node of a group, other active nodes failover to a backup node different to the particular node. This also reduces risk associated with repair and manual reconfiguration and maintenance cost of a cluster configuration. 
     Individual nodes of a group include an adaptive failover controller (e.g., AFC  10 ) which includes various modules providing the functions and connections described below. Failover Engine  14  of AFC  10  controls and configures other modules of AFC  10  including Heartbeat Engine  18 , Cluster Network Controller  12  as well as Local AFC Configuration Data repository  16  configured via Configuration Data Access Controller  45 . Failover Engine  14  also initializes, maintains and updates a state machine used by AFC  10  and employs and maintains other relevant data including utilization parameters. These utilization parameters identify resources (e.g., processing devices, memory, CPU resources, IO resources) that are used for performing particular computer operation tasks and are used by Failover Engine  14  in managing processing device back-up priority lists. The utilization parameters are stored in the Local AFC Configuration Data repository  16 . Failover Engine  14  advantageously uses state and utilization parameter information to optimize a protected peer nodes list for a group of processing devices (e.g., including a device incorporating AFC  10  and other devices individually containing an AFC such as other AFCs  19 ). Failover Engine  14  derives utilization parameter information from Local AFC Configuration Data repository  16  in a synchronized manner. Further, Engine  14  employs Cluster Network Controller  12  to update state and utilization parameter information for the processing devices in a group retained in Primary AFC Configuration Repository  40  and to update state and utilization parameter information retained in local AFC configuration data repositories of the other AFCs  19 . 
     Failover Engine  14  communicates messages including data identifying updates of Local AFC Configuration Data repository  16  to Heartbeat Engine  18  via Failover-Heartbeat Interface  31 . Heartbeat engine uses Configuration Data Access Controller  45  to read configuration information from Local AFC Configuration Data repository  16  via communication interface  22 . Heartbeat Engine  18  also uses Cluster Network Controller  12  to establish a communication channel with Heartbeat Engines of other AFCs  19  using the configuration data acquired from repository  16 . Configuration Data Access Controller  45  supports read and write access to repository  16  via interface  22  and supports data communication via interface  24  with Failover engine  14  and via interface  35  with Heartbeat Engine  18 : For this purpose, Configuration Data Access Controller  45  employs a communication arbitration protocol that protects data from corruption during repository  16  data modification. 
     Cluster Network Controller  12  provides communication Interfaces  27  and  38  supporting access by Failover Engine  14  and Heartbeat Engine  18  respectively to network  20 . Controller  12  provides bidirectional network connectivity services over Cluster Communication Network  20  and supports delivery of information from a connection source to a connection destination. Specifically, Controller  12  provides the following connectivity services over a dedicated network connection (or via a dynamically assigned connection via the Internet, for example) from AFC  10  to Other AFCs  19 , or to Primary AFC Configuration Repository  40 . Controller  12  supports bidirectional communication between AFC  10  and network controllers of other nodes, e.g., controllers of other AFCs  19 , Cluster Network Controller  12  is Internet Protocol (IP) compatible, but may also employ other protocols including a protocol compatible with Open Systems Interconnect (OSI) standard, e.g. X.25 or compatible with an intra-net standard. In addition, Cluster Network Controller  12  advantageously provides network wide synchronization and a data content auto-discovery mechanism to enable automatic identification and update of priority back-up list and other information in repositories of processing devices in a cluster. Primary AFC Configuration Repository  40  is a central repository that provides non-volatile data storage for processing devices networked via Communication Network  20 . 
       FIG. 2  shows a flowchart of a process used by AFC  10  of  FIG. 1  for managing operational failure occurrences in devices of a group of networked processing devices. After the Start at step  200  Failover Engine  14  of AFC  10  initializes and commands Cluster Network Controller  12  to connect to Cluster Communication Network  20 . In response to Cluster Network  20  being accessible, Failover Engine  14  acquires available configuration information from Primary AFC Configuration Repository  40  in step  205 . Failover Engine  14  stores the acquired configuration information in Local AFC Configuration Data repository  16 . If Primary AFC Configuration Repository  40  is not accessible, Failure Engine  14  uses configuration information derived from Local AFC Configuration Data repository  16  for the subsequent steps of the process of  FIG. 2 . 
     In step  210 , Failover Engine  14  configures an auto-discovery function of network controller  12  to automatically detect state and utilization information of other AFCs  19  in processing devices comprising the cluster associated with AFC  10  that is connected via Cluster Communication Network  20 . Failover Engine  14  also registers as a listener for acquiring information, identifying changes in device state and utilization parameter information for the processing devices in a group, from Primary AFC Configuration Repository  40 . After set-up of Cluster Network Controller  12  in step  210 , Failover Engine  14  initiates operation of Heartbeat Engine  18  in step  215 . Heartbeat Engine  18  acquires configuration information including a protected peer nodes list from Local AFC Configuration Data repository  16  and uses Cluster Network Controller  12  to establish heart beat communication between the AFC  10  and other AFCs  19 . Specifically, Heartbeat Engine  18  uses Cluster Network Controller  12  to establish heart beat communication between the AFC  10  and other AFCs  19 .that indicate AFC  10  as a back-up node in the individual protected peer nodes lists of other AFCs  19 . Heart beat communication comprises a periodic exchange of information to verify that an individual peer node is still operational. Failover Engine  14  also registers with other AFCs  19  and Primary AFC Configuration Repository  40  to be notified in the event of a failure in a node identified in the protected peer node list of AFC  10 . The  FIG. 1  system advantageously uses cluster-wide configuration, synchronization and discovery in step  215  to notify Heartbeat Engine  18  of state changes and updates to Local AFC Configuration Data repositories  16  of other AFCs  19 . Heartbeat Engine  18  also optimizes the fail over strategy for nodes in the associated cluster of processing devices. 
     In step  220 , Failover Engine  14  advantageously updates Local AFC Configuration Data repository  16  with acquired processing device state and utilization parameter information and uses Cluster Network Controller  12  to synchronize these updates with updates to Primary AFC Configuration Repository  40  and other AFCs  19 . Specifically, cluster Network Controller  12  notifies Failover Engine  14  about auto-discovered updates to Primary AFC Configuration Repository  40  and other AFCs  19  and Failover Engine  14  updates Local repository  16  with this acquired information. Similarly, heartbeat Engine  18  notifies Failover Engine  14  of changes in availability of protected peer nodes and Failover Engine  14  updates Local repository  16  with this acquired information. Failover Engine  14  correlates acquired information and notifications and optimizes cluster wide the protected peer nodes list stored in the Local AFC Configuration Data  16 . The process of  FIG. 2  terminates at step  230 . 
     Load balancing operations are commonly employed in existing systems to share operational burden in devices in a cluster. For this purpose, measured CPU (Central Processing Unit) usage and total number of IOPS (Interface Operations Per Second) are used (individually or in combination) to balance load from a heavily utilized server to another machine, for example. Further, a cluster of processing devices in existing systems typically operate in a configuration where nodes are active and incoming load requests to the cluster are distributed and balanced across available servers in the cluster. A master server controls distribution and balancing of load across the servers. Load distributed to active nodes is measured and reported to the master node. 
     In contrast, the architecture of AFC  10  is used as an Active/Passive configuration where several active nodes receive an inbound load and share passive fail-over nodes (without active node load balancing). Load balancing is a complex application that adds additional risk and reduces device availability. Requests are forwarded from client devices to a virtual IP address that can be moved from one physical port to another physical port via communication network  20 . In contrast to known systems, a dedicated master unit does not control a cluster and decisions are based on a distributed priority list of backup nodes. Therefore, failover management in AFC  10  is based on a prioritized back-up device priority list. In another embodiment, the architecture of AFC  10  employs active load balancing using parameters such as CPU load utilization, memory utilization and total number of IOPS, for example, to balance the load across active servers in a cluster. 
       FIG. 3  shows a network diagram of a group of networked processing devices managed by the system of  FIG. 1 . Specifically,  FIG. 3  comprises a network diagram of an active-passive cluster. Active nodes  300  and  302  and passive nodes  304  and  306  are connected to client communication network  60  to provide services to processing devices  307  and  309  connected to this network and for cluster internal communication. Nodes  300 - 306  are also connected to storage systems and an associated storage area network  311  to provide shared drives used by the cluster. Further, nodes may have identical software installed (operating system, application programs etc.). Active nodes ( 300 ,  302 ) have one or more virtual IP addresses associated with a physical port connected to client communication network  60 . Passive nodes ( 304 ,  306 ) do not have a virtual IP address associated with their physical port to client communication network  60 . Client devices ( 307 ,  309 ) communicate message requests and data to a virtual IP address that is associated with one of the active nodes  300  and  302 . In the event of fail-over (a failure of one or more of nodes  300 - 302 , for example) a passive node (e.g., node  304  or  306 ) takes ownership of the virtual IP address of the active node and assigns it to its own physical port. Virtual resources fail-over to a back-up resource. In response to assignment of a virtual IP address, a passive node becomes active and changes to the group of active nodes. 
     In the event of a fail-over, those operations or transactions being performed by the processing device that fails are lost. The operations and transactions recorded in an operations log as being performed or to be performed by the failed device are executed (or re-executed) by the back-up device that assumes the operations of the failed device.  FIG. 4  shows an exemplary configuration of a group of networked processing devices managed by the system of  FIG. 1 . Specifically, the  FIG. 4  configuration shows three active nodes (nodes  1 ,  2  and  3 ) and two backup nodes (nodes  4  and  5 .) but this configuration may readily be extended to more backup nodes. Either back-up node  4  or back-up node  5  may act as a primary or secondary back-up node to individual active nodes  1 , 2  and  3 . Active nodes  1 ,  2  and  3  execute copies of the same application programs and the respective virtual IP addresses of these nodes are assigned to their corresponding node physical ports. Passive nodes  4  and  5  are in standby mode and have no virtual IP addresses assigned to their respective physical ports. 
       FIGS. 5-9  show prioritized lists illustrating automatic failure management of the back-up processing devices that assume functions of processing devices of the  FIG. 4  configuration in the event of device operational failure. The backup priority list of  FIG. 5  is stored in each AFC (AFCs  1 - 5  of  FIG. 4 ).  FIG. 5  indicates primary backup node  4  monitors a protected node using a heartbeat engine (e.g., unit  18   FIG. 1 ). Specifically, back-up node  4  is the primary backup node for node  1 , node  2 , and node  5 . If one of the monitored nodes  1 ,  2  or  5  fails, node  4  takes ownership of the particular virtual IP address and virtual server of the failed node and changes to an unavailable state. In the back-up list of  FIG. 5 , node state: A=Available, N=Not available. 
     In exemplary operation, passive node  4  experiences an operational problem. Specifically, a reduction in memory capacity that reduces its capability to assume operational load in the event of node  4  being required to assume tasks being performed by a failure of one of nodes  1 ,  2  or  5 , for example. Subsequently,-active node  1  fails before the problem on node  4  is fixed. 
     In an existing known (non-load balancing) system, nodes  1  may disadvantageously repetitively and unsuccessfully attempt to fail-over to node  4  which is indicated as being in the Available state by the list of  FIG. 5 . This causes substantial operational interruption. In contrast, in the system of  FIG. 1  node  4  detects its own reduction in memory capacity and updates its node state entry in its back-up priority list as indicated in  FIG. 6 . Specifically, the back-up node list stored in node  4  (shown in  FIG. 6 ) illustrates the node state entry for node  4  (item  600 ) has become Not available. However, initially the back-up node lists of other nodes  1 - 3  and  5 , illustrated in  FIG. 7 , have not received the information updating the availability status of node  4 . 
     Other nodes  1 - 3  and  5  acquire updated node  4  availability information from the AFC unit of node  4  using an auto-discovery method. The AGCs of nodes  1 - 3  and  5  employ a network controller  12  ( FIG. 1 ) in interrogating back-up list information of other nodes in the cluster connected to network  20 . In another embodiment, the AFC unit of node  4  detects the back-up list information change and communicates the updated information via network  20  to nodes  1 - 3  and  5  and primary AFC repository  40 . In acquiring and distributing updated back-up list information, network controller  12  employs communication and routing protocols for communicating node  4  back-up list availability information to nodes  1 - 3  and  5 . For this purpose, network controller  12  employs IP compatible communication protocols including OSPF (Open Shortest Path First) routing protocol and protocols compatible with IETF (Internet Engineering Task Force): RFC1131, RFC1247, RFC1583, RFC1584, RFC2178, RFC2328 and RFC2370, for example, to distribute data representing state information of node  4  to nodes  1 - 3  and  5  and primary AFC repository  40 . The RFC (Request For Comment) documents are available via the Internet and are prepared by Internet standards working groups. 
       FIG. 8  shows back-up priority lists of nodes  1 - 5  following processing of the received data representing state information of node  4  by respective AFCs of nodes  1 - 5  and the update of their respective back-up priority lists in local repositories (such as repository  16 ). The back-up priority lists of nodes  1 - 5  show that wherever node  4  was designated as a primary or secondary back-up node (items  800 - 808  in  FIG. 8 ), it is now marked as Not available in response to the state change update.  FIG. 8  shows node  4  is illustrated in  FIG. 8  as being Not available in the 5 columns corresponding to the back-up arrangements of the 5 nodes of the system of  FIG. 4 . Consequently, now node  5  is the primary backup for node  1  (primary not available: secondary becomes primary), node  2  (primary not available: secondary becomes primary), and node  3 . 
     Node  5  detects the failure in node  1  (using a heartbeat engine such as unit  18  of  FIG. 1 ), validates the detected failure has occurred, takes over the tasks to be executed by node  1  and updates its back-up list record in its local repository. Network controller  12  in node  5  communicates data representing the change in state of node  5  (identifying a change to Not available state) to nodes  14  in a manner as previously described. The state change information is communicated to nodes  14  to ensure consistent back-up list information using the routing and communication protocols previously described. This makes sure that the information is consistently updated in nodes  1 - 5 .  FIG. 9  shows back-up priority lists of nodes  1 - 5  following processing of received data representing state information of node S by respective AFCs of nodes  1 - 5  and update of their respective back-up priority lists in local repositories (e.g., repository  16 ). The system failover strategy employs cluster parameters (e.g. state and resource utilization information) in determining available back-up nodes. This advantageously reduces downtime during a fail-over condition and reduces manual system reconfiguration. 
     In an alternative embodiment, back-up priority list information is communicated by individual nodes to primary AFC repository  40  and individual nodes  1 - 5  acquire back-up node list information from repository  40 . An individual node of nodes  1 - 5  store back-up list information in repository  40  in response to detection of a state change or change in back-up list information stored in the individual node local repository (e.g., repository  16 ). An update made to back-up list information stored in repository  40  is communicated by repository system  40  to nodes  1 - 5  in response to detection of a change in stored back-up list information in repository  40 . In another embodiment, individual nodes of nodes  1 - 5  intermittently interrogate repository  40  to acquire updated back-up list information. 
       FIG. 10  shows a flowchart of a process used by AFC  10  of the system of  FIG. 1  for managing operational failure occurrences in devices of a group (cluster) of networked processing devices employing similar executable software. In step  702 , after the start at step  701 , AFC  10  maintains transition information in an internal repository identifying a second currently non-operational passive processing device for taking over execution of tasks designated to be performed by a first processing device, in response to an operational failure of the first processing device. An operational failure of a processing device comprises, a software execution failure or a hardware failure, for example. The transition information comprises a prioritized back-up list of processing devices for assuming execution of tasks of a first processing device in response to an operational failure of the first processing device. In step  704 , AFC  10  updates the transition information in response to, (a) detection of an operational failure of another processing device in the group or (b) detection of available memory of another processing device of the group being below a predetermined threshold. AFC  10  in step  706  detects an operational failure of the first processing device. In step  708  AFC  10  initiates execution, by the second processing device, of tasks designated to be preformed by the first processing device in response to detection of an operational failure of the first processing device. 
     AFC  10  dynamically updates internally stored back-up device priority list information in step  712  in response to communication from another processing device of the group in order to maintain consistent transition information in the individual processing devices of the group. Specifically, the internally stored prioritized list is dynamically updated in response to factors including, detection of an operational failure of another processing device in the group or detection of available memory of another processing device of the group being below a predetermined threshold. The factors also include, (a) detection of operational load of another processing device in the group exceeding a predetermined threshold, (b) detection of use of CPU (Central Processing Unit) resources of another processing device of the group exceeding a predetermined threshold or (c) detection of a number of I/O (input-output) operations, in a predetermined time period, of another processing device of the group exceeding a predetermined threshold. The prioritized list is also dynamically updated in response to state information provided by a different processing device of the group indicating, a detected change of state of another processing device of the group from available to unavailable or a detected change of state of another processing device of the group from unavailable to available. For this purpose, AFC  10  interrogates other processing devices of the group to identify a change in transition information occurring in another processing device of the group. The process of  FIG. 10  terminates at step  718 . 
     The system of  FIG. 1  advantageously adapts cluster fail-over back-up list configuration based on parameters (e.g. failover state, resource utilization) of nodes participating in the cluster. The system further optimizes a back-up node list based on the parameters and adapts heart beat operation based on the updated back-up nodes list. The system provides cluster-wide automatic synchronization of parameters maintained in nodes  1 - 5  using auto-discovery functions to detect changes in back-up list information stored in local repositories of nodes  1 - 5 . 
     The systems and processes presented in  FIGS. 1-10  are not exclusive. Other systems and processes may be derived in accordance with the principles of the invention to accomplish the same objectives. Although this invention has been described with reference to particular embodiments, it is to be understood that the embodiments and variations shown and described herein are for illustration purposes only. Modifications to the current design may be implemented by those skilled in the art, without departing from the scope of the invention. A system according to invention principles provides high availability application and operating system software. Further, any of the functions provided by system  10  ( FIG. 1 ) may be implemented in hardware, software or a combination of both and may reside on one or more processing devices located at any location of a network linking the  FIG. 1  elements or another linked network including another intra-net or the Internet.