Patent Publication Number: US-7590898-B2

Title: Heartbeat mechanism for cluster systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. patent application Ser. No. 10/305,483 entitled “Heartbeat mechanism for cluster systems,” filed Nov. 27, 2002 now U.S. Pat. No. 7,451,359, by Wim Coekaerts, and assigned to the present assignee. 
     This application is related to U.S. patent application entitled “Clustering System and Method Having Interconnect”, Ser. No. 10/305,567 filed Nov. 27, 2002, inventor: Wim Coekaerts, which is also assigned to the present assignee. 
    
    
     BACKGROUND 
     A cluster is a group of independent servers that collaborate as a single system. The primary cluster components are processor nodes, a cluster interconnect (private network), and a disk subsystem. The clusters share disk access and resources that manage the data, but each distinct hardware cluster nodes do not share memory. Each node has its own dedicated system memory as well as its own operating system, database instance, and application software. Clusters can provide improved fault resilience and modular incremental system growth over single symmetric multi-processors systems. In the event of subsystem failures, clustering ensures high availability. Redundant hardware components, such as additional nodes, interconnects, and shared disks, provide higher availability. Such redundant hardware architectures avoid single points-of-failure and provide fault resilience. 
     In a database cluster, CPU and memory requirements for each node may vary depending on the database application. Performance and cost requirements also vary between database applications. One factor that contributes to performance is that each node in a cluster needs to keep other nodes in that cluster informed of its health and configuration. This has been done by periodically broadcasting a network message, called a heartbeat, across a network. The heartbeat signal is usually sent over a private network, a cluster interconnect, which is used for internode communications. However, lost or delayed heartbeat messages may cause false reports that a node is not functioning. 
     In prior systems, the cluster interconnect has been built by installing network cards in each node and connecting them by an appropriate network cable and configuring a software protocol to run across the wire. The interconnect was typically a low-cost/slow-speed Ethernet card running TCP/IP or UDP, or a high-cost/high-speed proprietary interconnect like Compaq&#39;s Memory Channel running Reliable DataGram (RDG) or Hewlett-Packard&#39;s Hyperfabric/2 with Hyper Messaging Protocol (HMP). A low-cost/high-speed interconnect would reduce clustering costs for users and reduce latency during run-time. 
     SUMMARY 
     In one embodiment, a heartbeat mechanism includes a quorum file for receiving heartbeat messages from the plurality of nodes. A network controller connects the quorum file to the plurality of nodes with a serial bus that establishes peer-to-peer and point-to-point device communication. A node map maintained by the network controller identifies active nodes based on signals from the serial bus. A status logic for determining a status of a node from the plurality of nodes by comparing heartbeat messages in the quorum file written by the node and the node map. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings which are incorporated in and constitute a part of the specification, embodiments of a system and method are illustrated, which, together with the detailed description given below, serve to describe the example embodiments of the system and method. It will be appreciated that the illustrated boundaries of elements (e.g. boxes or groups of boxes) in the figures represent one example of the boundaries. One of ordinary skill in the art will appreciate that one element may be designed as multiple elements or that multiple elements may be designed as one element. An element shown as an internal component of another element may be implemented as an external component and vise versa. 
         FIG. 1  is an example system diagram of one embodiment of a cluster node; 
         FIG. 2  is one embodiment an example diagram of the interconnect bus controller of  FIG. 1 ; 
         FIG. 3  is an example of a shared disk cluster architecture; 
         FIG. 4  is an example of an share-nothing cluster architecture; 
         FIG. 5  is one embodiment of a methodology of communicating data using the interconnect bus; 
         FIG. 6  is an example embodiment of a methodology of detecting a topology change; 
         FIG. 7  is another example methodology of detecting a topology change; 
         FIG. 8  is another embodiment of a cluster including a heartbeat system; 
         FIG. 9  is another embodiment of a heartbeat system; 
         FIG. 10  is an example methodology of maintaining a quorum file; and 
         FIG. 11  is an example methodology of determining the status of a node using the quorum file. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS 
     The following includes definitions of selected terms used throughout the disclosure. Both singular and plural forms of all terms fall within each meaning: 
     “Computer-readable medium” as used herein refers to any medium that stores signals, instructions and/or data. Such a medium may take many forms, including but not limited to, non-volatile media, and volatile media. Non-volatile media may include, for example, optical or magnetic disks. Volatile media may include dynamic memory. Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, any other physical medium with patterns of holes, a RAM, a PROM, an EPROM, a FLASH-EPROM, any other memory chip or cartridge, or any other medium from which a device can read. 
     “Logic”, as used herein, includes but is not limited to hardware, firmware, software and/or combinations of each to perform a function(s) or an action(s), and/or to cause a function or action from another component. For example, based on a desired application or needs, logic may include a software controlled microprocessor, discrete logic such as an application specific integrated circuit (ASIC), or other programmed logic device. Logic may also be fully embodied as software stored on a computer-readable medium. 
     “Signal”, as used herein, includes but is not limited to one or more electrical signals, analog or digital signals, a change in a signal&#39;s state (e.g. a voltage increase/drop), one or more computer instructions, messages, a bit or bit stream, or other means that can be received, transmitted, and/or detected. 
     “Software”, as used herein, includes but is not limited to one or more computer readable and/or executable instructions that cause a computer or other electronic device to perform functions, actions, and/or behave in a desired manner. The instructions may be embodied in various forms such as routines, algorithms, modules or programs including separate applications or code from dynamically linked libraries. Software may also be implemented in various forms such as a stand-alone program, a function call, a servlet, an applet, instructions stored in a memory, part of an operating system or other type of executable instructions. It will be appreciated by one of ordinary skill in the art that the form of software is dependent on, for example, requirements of a desired application, the environment it runs on, and/or the desires of a designer/programmer or the like. 
     Illustrated in  FIG. 1  is one embodiment of a simplified clustered database system  100  in accordance with one embodiment of the present invention. Although two nodes are shown in the example, node  105  and node  110 , different numbers of nodes may be used and clustered in different configurations. Although a database cluster is used as an example, the system can also be applied to other types of clustered systems. Each node is a computer system that executes software and processes information. The computer system may be a personal computer, a server, or other computing device. Each node may include a variety of components and devices such as one or more processors  115 , an operating system  120 , memories, data storage devices, data communication buses, and network communication devices. Each node may have a different configuration from other nodes. An example of one type of clustering system is described in U.S. Pat. No. 6,353,836, entitled “METHOD AND APPARATUS FOR TRANSFERRING DATA FROM THE CACHE OF ONE NODE TO THE CACHE OF ANOTHER NODE,” assigned to the present assignee, and which is incorporated herein by reference in its entirety for all purposes. 
     With further reference to  FIG. 1 , node  105  will be used to describe an example configuration of a node in the clustered database system  100 . In this embodiment, nodes are networked in a data sharing arrangement where each node has access to one or more data storage devices  125 . The data storage devices  125  can maintain a variety of files such as database files that may be shared by the nodes connected in the cluster. A network controller  130  connects the node  105  to a network  135 . The operating system  120  includes a communication interface between software applications running on the node  105  and the network controller  130 . For example, the interface may be a network device driver  140  that is programmed in accordance with the selected communications protocol of the network  135 . 
     Examples of communication protocols that may be used for network controller  130  and network  135  include the Fibre Channel ANSI Standard X3.230 and/or the SCSI-3 ANSI Standard X3.270. The Fibre Channel architecture provides high speed interface links to both serial communications and storage I/O. Other embodiments of the network controller  130  may support other methods of connecting the storage device  125  and nodes  105 ,  110  such as embodiments utilizing Fast-40 (Ultra-SCSI), Serial Storage Architecture (SSA), IEEE Standard 1394, Asynchronous Transfer Mode (ATM), Scalable Coherent Interface (SCI) IEEE Standard 1596-1992, or some combination of the above, among other possibilities. 
     The node  105  further includes a database instance  145  that manages and controls access to data maintained in the one or more storage devices  125 . Since each node in the clustered database system  100  executes a database instance that allows that particular node to access and manipulate data on the shared database in the storage device  125 , a lock manager  150  is provided. The lock manager  150  is an entity that is responsible for granting, queuing, and keeping track of locks on one or more resources, such as the shared database stored on the storage device  125 . Before a process can perform an operation on the shared database, the process is required to obtain a lock that grants to the process a right to perform a desired operation on the database. To obtain a lock, a process transmits a request for the lock to a lock manager. To manage the use of resources in a network system, lock managers are executed on one or more nodes in the network. 
     A lock is a data structure that indicates that a particular process has been granted certain rights with respect to the resource. There are many types of locks. Some types of locks may be shared by many processes while other types of locks prevent any other locks to be granted on the same resource. A more detailed description of one example of a lock management system is found in U.S. Pat. No, 6,405,274 B1 entitled “ANTICIPATORY LOCK MODE CONVERSIONS IN A LOCK MANAGEMENT SYSTEM,” assigned to the present assignee, and which is incorporated herein by reference in its entirety for all purposes. 
     To keep track of and manage the nodes on the network that may have access to the storage device  125 , a cluster configuration file  155  is maintained. The cluster configuration file  155  contains a current list of active nodes in the cluster including identification information such as node address, node ID, and connectivity structure (e.g. neighbor nodes, parent-child nodes). Of course, other types of information may be included in such a configuration file and may vary based on the type of network system. When a topology change occurs in the cluster, the node is identified and the cluster configuration file  155  is updated to reflect the current state of the cluster node. Examples of topology changes include when a node is added, removed, or stops operating. 
     With further reference to  FIG. 1 , the database cluster system  100  further includes an interconnect network  160  that provides node-to-node communication between the nodes  105  and  110 . The interconnect network  160  provides a bus that allows all nodes on the network to have two-way communication with each other. The interconnect  160  provides an active communication protocol for sending messages and data to and from each node over the same bus. To be connected to the interconnect network  160 , each node includes an interconnect bus controller  165  which may be a peripheral card plugged into a PCI slot of the node. The controller  165  includes one or more connection ports  170  for connecting cables between nodes. Three connection ports are illustrated in port  170  although different numbers of ports may be used. 
     In one embodiment, the interconnect bus controller  165  operates in accordance with IEEE 1394 protocol, also known as firewire or i.LINK. In order for the database instance  145 , or other application running on node  105 , to communicate with the interconnect bus  160 , a bus device driver  175  is provided. The bus device driver  175  works with the operating system  120  to interface applications with the interconnect bus controller  165 . For example, database commands from the database instance  145  are translated by the bus device driver  165  to IEEE 1394 commands or open host controller interface (OHCI) commands. The IEEE 1394 OHCI specification defines standard hardware and software for connections to the IEEE 1394 bus. OHCI defines standard register addresses and functions, data structures, and direct memory access (DMA) models. 
     IEEE 1394 is a bus protocol that provides easy to use, low cost, high speed communications. The protocol is very scaleable, provides for both asynchronous and isochronous applications, allows for access to large amounts of memory mapped address space, and allows peer-to-peer communication. It will be appreciated by one of ordinary skill in the art that the interconnect bus controller  165  may be modified to accommodate other versions of the IEEE 1394 protocol such as IEEE 1394a, 1394b, or other future modifications and enhancements. 
     The IEEE 1394 protocol is a peer-to-peer network with a point-to-point signaling environment. Nodes on the bus  160  may have several ports on them, for example ports  170 . Each of these ports acts as a repeater, retransmitting any data packets received by other ports within the node. Each node maintains a node map  180  that keeps track of the current state of the network topology/configuration. In its current form, the IEEE 1394 protocol supports up to 63 devices on a single bus, and connecting to a device is as easy as plugging in a telephone jack. Nodes, and other devices, can be instantly connected without first powering down the node and re-booting the network. Management of the database cluster topology will be described in greater detail below. 
     With the interconnect network  160 , the database  145  in node  105  may directly request data, transmit/receive data, or send messages to a running database application on node  110  or other node in the cluster. This avoids having to send messages or data packets to the storage device  125  which would involve one or more intermediate steps, additional disk I/O, and would increase latency. 
     Illustrated in  FIG. 2  is an example of the interconnect bus controller  165  based on the IEEE 1394 standard. It includes three ISO protocol layers: a transaction layer  200 , a link layer  205  and a physical layer  210 . The layers may be implemented in logic as defined above including hardware, software, or both. The transaction layer  200  defines a complete request-response protocol to perform bus transactions with three basic operations: read, write, and lock. The link layer  205  is the midlevel layer and it interacts with both the transaction layer  200  and the physical layer  210 , providing asynchronous and isochronous delivery service for data packets. Components to control data delivery include a data packet transmitter, data packet receiver, and a clock cycle controller. 
     The physical layer  210  provides the electrical and mechanical interface between the controller  165  and a cable(s) that forms part the interconnect bus  160 . This includes the physical ports  170 . The physical layer  210  also ensures that all nodes have fair access to the bus using an arbitration mechanism. For example, when a node needs to access the bus, it sends a request to its parent node(s), which forwards the request to a root node. The first request received by the root is accepted; all others are rejected and withdrawn. The closer the node is to the root, the better its chance of acceptance. To solve consequent arbitration unfairness, periods of bus activity are split into intervals. During an interval, each node gets to transmit once and then it waits until the next interval. Of course, other schemes may be used for arbitration. 
     Other functions of the physical layer  210  include data resynchronization, encoding and decoding, bus initialization, and controlling signal levels. As mentioned previously, the physical layer of each node also acts as a repeater, translating the point-to-point connections into a virtual broadcast bus. A standard IEEE 1394 cable provides up to 1.5 amps of DC power to keep remote devices “aware,” even when they are powered down. Based on IEEE 1394, the physical layer also allows nodes to transmit data at different speeds on a single medium. Nodes, or other devices, with different data rate capabilities communicate at the slower device rate. 
     The interconnect bus controller  165 , operating based on IEEE 1394 protocol, is an active port and provides for a self-monitoring/self-configuring serial bus. This is known as hot plug-and-play that allows users to add or remove devices even if the bus is active. Thus, nodes and other devices may be connected and disconnected without interrupting network operation. A self-monitoring/self-configuring logic  215  automatically detects topology changes in the cluster system based on changes in the interconnect bus signal. The bus controller  165  of a node places a bias signal on the interconnect bus  160  once the node is connected to the bus. Neighboring nodes, through the self-monitoring logic  215 , automatically detect the bias signal which may appear as a change in voltage. Thus, the detected bias signal indicates that a node has been added and/or that the node is still active. Conversely, the absence of the bias signal indicates that a node has been removed or has stopped functioning. In this manner, topology changes can be detected without using polling messages that are transmitted between nodes. The self-configuring aspect of the logic  215  will be described in greater detail with reference to  FIGS. 6 and 7 . 
     An application program interface (API) layer  220  may be included in the bus controller  165  as an interface to the bus device driver  175 . It generally includes higher level system guidelines/interfaces that bring the data, the end system design, and the application together. The API layer  220  may be programmed with desired features to customize communication between the database instance  145  (and other applications) and the interconnect bus controller  165 . Optionally, the functions of the API layer  220  may be embodied in whole or in part within the transaction layer  200  or the bus device driver  175 . 
     With reference to  FIG. 3 , one embodiment of a database cluster architecture  300  is shown in which the present system and method may be implemented. The architecture  300  is generally known as a shared disk architecture and is similar to  FIG. 1  except that additional nodes are shown. Generally in a shared disk database architecture, files and/or data are logically shared among the nodes with each database instance having access to all data. The shared disk access is accomplished, for example, by direct hardware connectivity to one or more storage devices  305  that maintain the files. Optionally, the connections may be performed by using an operating system abstraction layer that provides a single view of all the storage devices  305  on all the nodes. The nodes A-D are also connected via the node interconnect  160  to provide node-to-node communication. In the shared disk architecture, transactions running on any database instance within a node can directly read or modify any part of the database on storage device  305 . Access is controlled by one or more lock managers as described previously. 
     With reference to  FIG. 4 , another embodiment of a cluster architecture is shown that may incorporate the present system and method. Cluster architecture  400  is typically referred to as a shared-nothing architecture. An example of a shared-nothing architecture is described in U.S. Pat. No. 6,321,218, entitled “HYBRID SHARED NOTHING/SHARED DISK DATABASE SYSTEM,” assigned to the present assignee, and which is incorporated herein by reference in its entirety for all purposes. In a pure shared-nothing architecture, database files, for example, are partitioned among the database instances running on nodes A-D. Each database instance or node has ownership of a distinct subset of the data and all access to this data is performed exclusively by this “owning” instance. The nodes are also connected with the interconnect  160 . 
     For example, if data files stored on storage devices A-D contained employee files, the data files may be partitioned such that node A controls employee files for employee names beginning with the letters A-G, node B controls employee files on storage device B for employee names H-N, node C controls employee files for names “O-U” on storage device C and node D controls employee file names “V-Z” on storage device D. To access data from other nodes, a message would be sent requesting such data. For example, if node D desired an employee file which was controlled by node A, a message would be sent to node A requesting the data file. Node A would then retrieve the data file from storage device A and transmit the data to node D. It will be appreciated that the present system and method may be implemented on other cluster architectures and configurations such as tree structures and with other data access rights and/or restrictions as desired for a particular application. 
     Illustrated in  FIG. 5  is one embodiment of a methodology associated with the cluster system of  FIG. 3  or  4 . The embodiment describes directly transmitting and receiving data between nodes using the interconnect bus  160 . The illustrated elements denote “processing blocks” and represent computer software instructions or groups of instructions that cause a computer to perform an action(s) and/or to make decisions. Alternatively, the processing blocks may represent functions and/or actions performed by functionally equivalent circuits such as a digital signal processor circuit or an application specific integrated circuit (ASIC). The diagram, as well as the other illustrated diagrams, does not depict syntax of any particular programming language. Rather, the diagram illustrates functional information one skilled in the art could use to fabricate circuits, to generate computer software, or a combination of hardware and software to perform the illustrated processing. It will be appreciated that electronic and software applications may involve dynamic and flexible processes such that the illustrated blocks can be performed in other sequences different than the one shown and/or blocks may be combined or, separated into additional components. They may also be implemented using various programming approaches such as machine language, procedural, object oriented and/or artificial intelligence techniques. The foregoing applies to all methodologies described herein. 
     With reference to  FIG. 5 , diagram  500  is one example of communicating data between nodes using the node-to-node interconnect network  160 . When a node (a requesting node) desires data from another node, a data request message is transmitted (block  505 ) to a destination node via the interconnect bus  160 . The data request may be sent directly to one or more selected destination nodes by attaching the node name and/or address to the request. If the location of the requested data is unknown, the data request may be broadcasted to each node in the interconnect network. 
     When the data request is received by the appropriate node, the database instance determines whether the data is available on that node (block  510 ). If the data is not available, a message is transmitted to the requesting node that the data is not available (block  515 ). If the data is available, the data is retrieved from local memory (block  520 ) by direct memory access, and it is transmitted to the requesting node over the interconnect bus (block  525 ). Remote direct memory access can also be implemented to perform a direct memory to memory transfer. In this manner, messages and data may be transmitted directly between nodes without having to transmit the messages or data to a shared storage device. The node-to-node communication reduces latency and reduces the number of disk inputs/outputs. 
     Illustrated in  FIG. 6  in an example methodology of reconfiguring the cluster architecture based on the IEEE 1394 bus protocol. When a node in the database cluster is added, removed, or stops functioning, the database cluster needs to detect the change, identify the node, and the cluster needs to be reconfigured appropriately. As described previously, the interconnect bus controller  165  ( FIG. 1 ), operating based on IEEE 1394 protocol, is an active port and provides for a self-configuring serial bus. Thus, nodes and other devices may be connected and disconnected without interrupting network operation. 
     For example, when a node is added to the bus, the bus is reset (block  605 ). The interconnect controller  165  of the added node automatically sends a bias signal on the bus and neighboring nodes can detect its bias signal (block  610 ). Similarly, the absence of a node&#39;s bias signal can be detected when a node is removed. In other words, the interconnect controller  165  of neighboring nodes can detect signal changes on the interconnect bus  160  such as a change in the bus signal strength caused by adding or removing a node. The topology change is then transmitted to all other nodes in the database cluster. The bus node map is rebuilt with the changes (block  615 ). In one embodiment, the node map can be updated with the changes. The database instance is notified and it updates the cluster configuration file (block  620 ) to keep track of the active nodes for the lock managers. Of course, the order of the illustrated sequence may be implemented in other ways. 
     Using the IEEE 1394 protocol, the interconnect controller  165  is an active port that includes a self-monitoring/self-configuration mechanism as described above. With this mechanism, the database cluster system can be reconfigured without the added latency involved with polling mechanisms since nodes can virtually instantly detect a change in the topology. The active port also allows reconfiguration of the cluster without having to power-down the network. 
     Illustrated in  FIG. 7  is another embodiment of detecting and reconfiguring the cluster. Each node monitors the interconnect bus (block  705 ) to detect a change in the bus signal such as the presence or absence of a bias signal. When a node detects a topology change (block  710 ), it sends a bus reset signal on the bus, starting a self-configuring mechanism. This mechanism, managed by the physical layer  210 , may include three phases: bus initialization, tree identification, and self identification. During bus initialization, active nodes are identified and a treelike logical topology is built (block  715 ). Each active node is assigned an address, a root node is dynamically assigned, and the node map is rebuilt or updated with the new topology (block  720 ). Once the bus is configured itself, the nodes can then access the bus. The database instances on each node are notified of the topology change (block  725 ) and the database lock manager(s) are reconfigured with the changes so that the shared database can be managed properly throughout the cluster (block  730 ). 
     It will be appreciated that the network connections, such as network  135  may be implemented in other ways. For example, it may include communications or networking software such as the software available from Novell, Microsoft, Artisoft, and other vendors, and may operate using TCP/IP, SPX, IPX, and other protocols over twisted pair, coaxial, or optical fiber cables, telephone lines, satellites, microwave relays, radio frequency signals, modulated AC power lines, and/or other data transmission wires known to those of skill in the art. The network  135  can be connectable to other networks through a gateway or similar mechanism. It will also be appreciated that the protocol of the interconnect bus  160  may include a wireless version. 
     With reference to  FIG. 8 , one embodiment of a heartbeat system is shown for a database cluster  800 . A heartbeat system is a mechanism where nodes periodically generate signals or messages indicating that they are active and functioning. The mechanism also allows nodes to determine the health or status of other nodes in the cluster based on the generated signals. As shown, the cluster  800  includes nodes  805  and  810  although any number of nodes may be connected to the cluster. The illustrated nodes may have a similar configuration as the nodes shown in  FIG. 1 . However, a simplified configuration is shown for illustrative purposes. 
     The nodes  805 ,  810  share access to a storage device  815  that maintains files such as database files. The nodes are connected to the storage device  815  by a shared storage network  820 . In one embodiment, the network  820  is based on IEEE 1394 communication protocol. To communicate with each other, nodes  805 ,  810  and the storage device  815  include an IEEE 1394 network controller  825 . The network controller  825  is similar to the interconnect bus controller  165  and in one embodiment, is a network card that is plugged into each device. Alternatively, the controller may be fixed within the node. The network controller  825  includes one or more ports so that cables can be connected between each device. Additionally, other types of network connections may be utilized, for example wireless connections, that are based on the IEEE 1394 protocol, or other similar protocol standards. 
     With further reference to  FIG. 8 , each node includes a database instance  830  that controls access to the files on the storage device  815 . Since resources are shared between nodes in the database cluster  800 , each node includes logic to inform other nodes of their health and includes logic to determine the health of other nodes on the network. For example, a heartbeat logic  835  is programmed to generate and transmit a heartbeat message within a predetermined time interval. A heartbeat message is also referred to as a status signal. The predetermined time interval may be any selected interval but is typically on the order of milliseconds to seconds, for example, 300 milliseconds to 5 seconds. So if the interval is one second, each node would transmit a heartbeat message every one second. 
     In one embodiment, the network load is used as a factor in determining the heartbeat time interval. For example, if heartbeat messages are transmitted on the same network as data, then a high frequency of heartbeat messages on the network may cause delays in data transmission processes.  FIG. 8  shows a network that may be impacted by this situation while  FIG. 9  shows a network that reduces the amount of network traffic by implementing the heartbeat system on a different network. It will be further appreciated that the networks of  FIGS. 8 and 9  may also be configured as a shared-nothing architecture. 
     With reference again to  FIG. 8 , heartbeat messages from each node are collected and stored in a quorum file  840 . In this embodiment, the quorum file  840  is one or more files or areas defined within the storage device  815  which also maintains the shared files. Each node in the cluster  800  is allocated address space within the quorum file  840  to which its heartbeat messages are stored. The space of the quorum file  840  is typically equally divided and allocated to each node although other configurations may be possible. Thus, the quorum file  840  can be implemented as a separate file for each node rather than one file for the entire cluster even though the file may be logically defined as one data structure. The quorum file may be implemented as a stack, an array, a table, a linked list, a text file or other type of data structure, stored in one or more memory locations, registers, or other type of storage area. Once a node&#39;s quorum space is full, the oldest messages in the space are pushed out or overwritten as new messages are received. 
     Illustrated in  FIG. 9  is another embodiment of a database cluster  900  and a heartbeat system. In this embodiment, nodes  905  and  910  communicate with a quorum device  915  over a quorum network  920 . The quorum network  920  is a separate network than a shared storage network  925 . Thus, the nodes access shared files on storage device  930  using a different network bus than the quorum network. The quorum network  920  may be part of a node-to-node interconnect network as previously described. The quorum device  915  includes data storage configured to maintain a quorum file for storing heartbeat messages received from the nodes in the cluster. 
     With further reference to  FIG. 9 , the nodes  905 ,  910  are connected to the quorum device  915  and communicate to each other in accordance with the IEEE 1394 communication protocol. Each node and the quorum device  915  includes an IEEE 1394 controller  935  similar to the controllers described previously. Since a separate network is configured for data communication to the files, each node includes a separate shared network controller  940  that communicates to the storage device  930 . The shared network controller  940  may be an IEEE 1394 controller or other network protocol such as fibre channel protocol. A database instance  945  within each node processes data requests over the shared network controller  940 . 
     A heartbeat logic  950  controls the heartbeat mechanism and uses the IEEE 1394 controller  935  to communicate with the quorum device  915 . With this architecture, adding or replacing a quorum devices  915  within an existing database cluster  900  can be easily performed with minimal impact on the existing network. Also, since the heartbeat mechanism is processed over a separate network, traffic on the shared storage network  925  is reduced allowing quicker responses for data processing requests. It will also be appreciated that the clusters of  FIGS. 8 and 9  may include a node-to-node interconnect network. 
     Illustrated in  FIG. 10  is an example methodology  1000  of a heartbeat system performed with the quorum file  840  or quorum device  915 , both of which will be referred to below as a quorum file. Once a quorum file is configured and activated within a database cluster, memory within the quorum file is allocated to each of the nodes in the cluster (block  1005 ). The quorum file may be equally divided and allocated to each node or other allocations may be defined. Once the quorum file is active, it receives heartbeat messages from each node in accordance with the IEEE 1394 protocol (block  1010 ). Each heartbeat message includes a node identifier that identifies the node sending the message and a time stamp indicating the time of the message. Each message received by the quorum file is then stored in its node&#39;s allocated location (block  1015 ) and the process repeats for each received heartbeat message. 
     For each node, heartbeat messages are stored in the quorum file in the order they are received. Thus, by comparing the most recently received time stamps to the current time, the system can determine which nodes are actively sending their heartbeat messages. This information can indicate whether a node is active or not. For example, if a node has missed a predetermined number of consecutive time stamps, a potential problem may be assumed. Any number of messages can be stored for each node including one message. As mentioned previously, the heartbeat logic of each node is programmed to generate and transmit a heartbeat message at a predetermined interval. Thus, by reading the data from the quorum file, the logic can determine if a number of missed intervals has occurred. This type of status check logic may be part of the heartbeat logic  835  or  950  and will be described in greater detail with reference to  FIG. 11 . 
       FIG. 11  illustrates an example methodology for determining the health or status of a node. As described previously, the heartbeat logic includes logic for generating each heartbeat message at the predetermined time interval and transmitting the message to the quorum file. At any desired time, the heartbeat logic of a node may update its cluster configuration file to determine the current set of active nodes and to determine if any nodes have stopped functioning or otherwise have been removed from the network. This determination may also be synchronized throughout the cluster. A status check logic (not illustrated) may be programmed as part of the heartbeat logic to perform this task as follows. 
     To begin a status check, the quorum file is read to review the time stamped information for each of the nodes (block  1105 ). Based on the time stamped data stored for each node, the logic can determine if a particular node is still functioning based on the time of the last messages written to the quorum file (block  1110 ). A threshold may be set to allow a predetermined number of time stamps to be missed before the determination indicates that a problem may exist. For example, a node may be allowed to miss two consecutive time stamps but if a third is missed, then the node may not be functioning properly. The threshold may also be set to other values, for example a value of 1. 
     If a node misses the designated amount of time stamp messages (block  1120 ), it may not necessarily mean that the node has stopped functioning. Since the nodes are connected to the quorum file in accordance with the IEEE 1394 standard, an additional status check can be performed. As explained previously, the IEEE 1394 bus is active and each device connected to the bus can detect if a neighboring node stops functioning or is removed from the network. This additional information may help to better determine the health of a node. The status logic can compare the time stamp information from the quorum file and the node map data maintained by the IEEE 1394 controller. 
     For example, if a node misses its time stamp (block  1120 ) and the node is not an active node in the node map (block  1125 ), then it is determined that the node is presumed down or has been removed from the network (block  1130 ). However, if a node misses its time stamp but the node is still active in the node map, then the node is possibly hung-up or some other delay may exist in the cluster (block  1135 ). If this is the case, the process may optionally re-check the quorum file for that node to determine if a new time stamp has been received, a message can be generated to indicated a possible delay, and/or the node can be removed from the active node list. 
     Referring to decision block  1120  again, if a node does not miss its time stamp, the node is presumably functioning properly. However, an additional determination may be made by checking if the node is active in the node map (block  1140 ). If the node is active (block  1145 ), then the node is functioning properly. If the node is not active (block  1150 ), then a possible network bus error may exist. Thus, with information from both the quorum file and the node map of the IEEE 1394 bus, a more detailed analysis of node health may be determined. Furthermore, in the cluster configuration shown in  FIG. 9  in the embodiment where the shared storage network  925  is also a IEEE 1394 bus, two separate network node maps are maintained. The additional node map may also be included in the above comparison process and status check. 
     With reference again to  FIG. 11 , a simplified embodiment may be implemented. At the decision block  1120 , if a node fails to write its time stamps, the logic can declare that node as non-functioning and remove it from the cluster configuration file of the database instances. In this process, the node maps are not reviewed. 
     It will be appreciated that the various storage devices described herein, including the quorum device for allocating a quorum file, may be implemented in numerous ways. For example, a storage device may include one or more dedicated storage devices such as magnetic or optical disk drives, tape drives, electronic memories or the like. A storage device may also include one or more processing devices such as a computer, a server, a hand-held processing device, or similar device that contains storage, memories, or combinations of these for maintaining data. The storage device may also be any computer-readable medium. 
     Suitable software for implementing the various components of the present system and method are readily provided by those of skill in the art using the teachings presented here and programming languages and tools such as Java, Pascal, C++, C, CGI, Perl, SQL, APIs, SDKs, assembly, firmware, microcode, and/or other languages and tools. The components embodied as software include computer readable/executable instructions that cause a computer to behave in a prescribed manner. The software may be as an article of manufacture and/or stored in a computer readable medium as defined previously. 
     While the present invention has been illustrated by the description of embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention, in its broader aspects, is not limited to the specific details, the representative apparatus, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the applicant&#39;s general inventive concept.