Abstract:
A method and apparatus for providing a recent set of replicas for a cluster data resource within a cluster having a plurality of nodes. Each of the nodes having a group services client with membership and voting services. The method of the present invention concerns broadcasting a data resource open request to the nodes of the cluster, determining a recent replica of the cluster data resource among the nodes, and distributing the recent replica to the nodes of the cluster. The apparatus of the present invention is for providing a recent set of replicas for a cluster data resource. The apparatus has a cluster having a plurality of nodes in a peer relationship, each node has an electronic memory for storing a local replica of the cluster data resource. A group services client, which is executable by each node of the cluster, has cluster broadcasting and cluster voting capability. A database conflict resolution protocol (“DCRP”), which is executable by each node of the cluster, interacts with the group services clients such that the DCRP broadcasts to the nodes a data resource modification request having a data resource identifier and a timestamp. The DCRP determines a recent replica of the cluster data resource among the nodes with respect to the timestamp of the broadcast data resource modification request relative to a local timestamp associated with the data resource identifier, and distributes the recent replica of the cluster data resource to each node of the plurality of nodes.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present invention is related to the following co-pending patent applications: 
     U.S. patent application Ser. No. 09/282,225 entitled “Apparatus and Method for Tracking Access to Data Resources in a Cluster Environment”; and 
     U.S. patent application Ser. No. 09/282,908 entitled “Apparatus and Method for Maintaining Consistency of Shared Data Resources in a Cluster Environment”; 
     which are hereby incorporated by reference herein. 
     TECHNICAL FIELD 
     The present invention relates generally to distributed networks, and in particular to core cluster functions for maintaining consistency of shared data resources in a cluster environment. 
     BACKGROUND INFORMATION 
     As computer systems and networks become increasingly complex, the need to have high availability of these systems is becoming correspondingly important. Data networks, and especially the Internet, are uniting the world into a single global marketplace that never closes. Employees, sales representatives, and suppliers in far-flung regions need access to enterprise network systems every hour of the day. Furthermore, increasingly sophisticated customers expect twenty-four hour sales and service from a Web site. 
     As a result, tremendous competitive pressure is placed on companies to keep their systems running continuously, and to be continuously available. With inordinate amounts of downtime, customers would likely take their business elsewhere, costing a company their goodwill and a revenue loss. Furthermore, there are costs associated with lost employee productivity, diverted, canceled, and deferred customer orders, and lost market share. In sum, network server outages can potentially cost big money. 
     In the past, companies have operated with a handful of computers executing relatively simple software. This made it easier to manage the systems and isolate problems. 
     But in the present networked computing environment, information systems can contain hundreds of interdependent servers and applications. Any failure in one of these components can cause a cascade of failures that could bring down a server and leave a user susceptible to monetary losses. 
     Generally, there are several levels of availability. The particular use of a software application typically dictates the level of availability needed. There are four general levels of systems availability: base-availability systems, high-availability systems, continuous-operations environments, and continuous-availability environments. 
     Base-availability systems are ready for immediate use, but will experience both planned and unplanned outages. Such systems are used for application development. 
     High-availability systems include technologies that significantly reduce the number and duration of unplanned outages. Planned outages still occur, but the servers also includes facilities that reduce their impact. As an example, high-availability systems are used by stock trading applications. 
     Continuous-operations environments use special technologies to ensure that there are no planned outages for upgrades, backups, or other maintenance activities. Frequently, companies also use high-availability servers in these environments to reduce unplanned outages. Continuous-operations environments are used for Internet applications, such as Internet servers and e-mail applications. 
     Continuous-availability environments seek to ensure that there are no planned or unplanned outages. To achieve this level of availability, companies must use dual servers or clusters of redundant servers in which one server automatically takes over if another server goes down. Continuous-availability environments are used in commerce and mission critical applications. 
     As network computing is being integrated more into the present commercial environment, the importance of having high availability for distributed systems on clusters of computer processors has been realized, especially for enterprises that run mission-critical applications. Networks with high availability characteristics have procedures within the cluster to deal with failures in the service groups, and make provisions for the failures. High availability means a computing configuration that recovers from failures and provides a better level of protection against system downtime than standard hardware and software alone. 
     Conventionally, the strategy for handling failures is through a failfast or failstop function. A computer module executed on a computer cluster is said to be failfast if it stops execution as soon as it detects a severe enough failure and if it has a small error latency. Such a strategy reduces the possibility of cascaded failures due to a single failure occurrence. 
     Another strategy for handling system failures is through fault containment. Fault containment endeavors to place barriers between components so that an error or fault in one component would not cause a failure in another. 
     With respect to clusters, an increased need for high availability of ever increasing clusters is required. But growth in the size of these clusters increases the risk of failure within the cluster from many sources, such as hardware failures, program failures, resource exhaustion, operator or end-user errors, or any combination of these. 
     Up to now, high availability has been limited to hardware recovery in a cluster having only a handful of nodes. But hardware techniques are not enough to ensure that high availability hardware recovery can compensate only for hardware failures, which accounts for only a fraction of the availability risk factors. 
     An example for providing high availability has been with software applications clustering support. This technique has implemented software techniques for shared system resources such as a shared disk and a communication protocol. 
     Another example for providing high availability has been with network systems clustering support. With systems clustering support, failover is initiated in the case of hardware failures such as the failure of a node or a network adapter. 
     Generally, a need exists for simplified and local management of shared resources such as databases, in which local copies of the resource is maintained at each member node of the cluster. Such efficient administrative functions aids the availability of the cluster and allows processor resources to be used for the execution and operation of software applications for a user. 
     SUMMARY OF THE INVENTION 
     Thus, provided herein is a method and apparatus for providing a recent set of replicas for a cluster data resource within a cluster having a plurality of nodes. Each of the nodes having a group services client with membership and voting services. The method of the present invention concerns broadcasting a data resource open request to the nodes of the cluster, determining the most recent replica of the cluster data resource among the nodes, and distributing the recent replica to the nodes of the cluster. 
     The apparatus of the present invention is for providing a recent set of replicas for a cluster data resource. The apparatus has a cluster having a plurality of nodes in a peer relationship, each node has an electronic memory for storing a local replica of the cluster data resource. A group services client, which is executable by each node of the cluster, has cluster broadcasting and cluster voting capability. A database conflict resolution protocol (“DCRP”), which is executable by each node of the cluster, interacts with the group services clients such that the DCRP broadcasts to the plurality of nodes a data resource modification request having a data resource identifier and a timestamp. The DCRP determines a recent replica of the cluster data resource among the nodes with respect to the timestamp of the broadcast data resource modification request relative to a local timestamp associated with the data resource identifier, and distributes the recent replica of the cluster data resource to each required node of the plurality of nodes. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram representation of a computer used for providing a node in the cluster of the present invention; 
         FIG. 2  is a block diagram representing a cluster having a plurality of nodes; 
         FIG. 3  is a flow chart of error detection protocol executed by the cluster; 
         FIG. 4  is an example of no error occurring; 
         FIG. 5  is an example of an error occurring on a non-requesting node; and 
         FIG. 6  is another example of an error occurring. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. It should be noted, however, that those skilled in the art are capable of practicing the present invention without such specific details. In other instances, well-known circuits have been shown in block diagram form in order not to obscure the present invention in unnecessary detail. 
     Although the present invention is described with reference to a specific embodiment for a technique to provide an aspect of high-availability to a cluster, it should be understood that the present invention can be adapted for use with other high-availability techniques. All such variations are intended to be included within the scope of the present invention. It will be recognized that, in the drawings, only those signal lines and processor blocks necessary for the operation of the present invention are shown. 
     Referring to the drawings, depicted elements are not necessarily shown to scale, and like or similar elements are designated by the same reference numeral through the several views. 
     An aspect of high availability concerns error detection. The most common error is the single error, which is defined as a cluster node having a computational result that is different from the computational result of other cluster nodes. 
     Referring to  FIG. 1 , shown is a block diagram representation of a computer  100  used for providing a cluster of the present invention. The computer  100  has suitable hardware and operating system capabilities for providing networking capabilities for communication between different computers, or nodes, in a cluster  200  (see  FIG. 2 ). Each computer  100  used in the cluster has an executable core cluster software services component  102 . The core cluster services software component  102  is a middle-ware layer having a set of executables and libraries that run on the resident operating system  104 . The core cluster services is 32-bit and SMP (symmetric multiprocessor) ready. The core cluster services software component  102  has sub-components that include a portability layer  106 , a cluster coordinator  108 , topology services  110 , group services  112 , and a Cluster Search Query Language (“CSQL”) services  114 . 
     The portability layer  106  provides a set of common functions used by the other components to access the resident operating system  104  while also masking operating system-dependent implementations, and functions relating to Reliability-Availability-Serviceability (“RAS”) facilities such as tracing and logging of computer operations. The portability layer  106  in effect encapsulates operating-system dependent interfaces. Accordingly, the remaining sub-components of the core cluster services software component  102  may interact with the operating system  104  without having to be structured to interact with the particulars of that operating system  104 . 
     The cluster coordinator sub-component  108  provides software facilities for start-up, stop, and restart of the core cluster services  102 . Each computer in the cluster  200  has a cluster coordinator, but the individual cluster coordinators do not communicate with each other; the scope of each cluster coordinator sub-component  108  is restricted to the computer  100  on which it runs. The cluster coordinator sub-component  108  is executed first, and then it brings up the other core cluster services sub-components. Also, the cluster coordinator sub-component  108  monitors each of the other services, and restarts the core cluster services component  102  in the event of a failure. 
     The topology services sub-component  110  exchanges heartbeat messages with topology services in other computers. Heartbeat messages are used to determine which nodes of a cluster  200  are active and running. Each node of a cluster  200  checks the heartbeat of its neighbor node. Through knowledge of the configuration of the cluster  200  and alternate paths, the topology services sub-component  110  can determine if the loss of a heartbeat represents an adapter failure or a node failure. The topology services sub-component  110  maintains information about which nodes are reachable from other nodes, and this information is used to build a reliable messaging facility. 
     The group services sub-component, or client,  112  allows the formation of process groups containing processes on the same or different machines in the cluster  200 . A process can join a group as a provider or a subscriber. Providers participate in protocol action on the group while subscribers are notified on changes to the state of the group or membership in the group. The group services client  112  supports notification on joins and departures of processes to a process group. The group services client  112  also supports a host group that can be subscribed to in order to obtain the status of all the nodes in the cluster. This status is a consistent view of the node status information maintained by the topology services sub-component  110 . 
     With respect to the present invention, the group services client  112  provides cluster-aware functions to handle failure and reintegration of members in a process group. 
     These functions are built on top of the reliable messaging facility being either atomic broadcast, or n-phase commit protocols. 
     The CSQL services sub-component  114  provides support for databases, which may contain configuration and status information. The CSQL services sub-component  114  can operate in stand-alone or cluster mode. The database of the CSQL services sub-component  114  is a distributed resource which, through the use of the group services client  112 , is guaranteed to be coherent and highly available. Each database is replicated across all nodes and check pointed to disk so that changes are retained across reboots of the core cluster services  102 . The CSQL services sub-component  114  serves or provides each cluster node with an identical copy of data. 
     Referring to  FIG. 2 , shown is a block diagram representing a cluster  200 . As an example, the cluster  200  represents an application with components operating on several nodes within the cluster  200 . As shown, the cluster  200  has cluster nodes  202 ,  204 ,  206 ,  208 , and  210  each executing a component of a software application. Each of the nodes is understood to be provided by a computer  100  as described in detail with respect to  FIG. 1 . Furthermore, each of the nodes  202 ,  204 ,  206 ,  208 , and  210 , are members of the cluster  200  because each have a group services client application  112 , which collectively provide the group services  212  for the cluster  200 . 
     The members are coordinated by the group services  212 . Each of the cluster nodes  202 ,  204 ,  206 ,  208 , and  210  have a core cluster services software component  102  with a group services client  112  (see  FIG. 1 ), and each of these nodes are peers with respect to each other. 
     The group services  212  is formed by the combination of the group services sub-component  112  of the cluster nodes  202 ,  204 ,  206 ,  208 , and  210 . The term “client” as used herein means, on a network, a computer that accesses shared network resources provided by another computer. 
     The group services  212  can also support entities known as subscribers. These are cluster nodes that do not directly participate with the group members in planning and executing recovery actions, but are interested in recovery actions taken by the group members. 
     Accordingly, the group services  212  of the present invention provides updates that are real-time representations that are stored as a replica or copy on each of the cluster nodes  202 ,  204 ,  206 ,  208 ,  210 . The group services  212  also provides cooperative processes to coordinate the maintenance and recovery activities across the cluster  200 . An example of an addition of a member or subscriber is shown in  FIG. 2 , where an application component on node  214  seeks to become a member of the cluster node  200 . 
     The inclusion of a node with respect to the present invention is a function of the shared resources of the cluster  200 . For example, if the node  214  either lacks a data resource, such as a database, common to the other nodes of the cluster  200 , or has an outdated database, the group services  212  coordinates the installation of a copy of the shared database. 
     Cluster functions are provided under an n-phase protocol. The n-phase protocol has a set of available votes, which for the present invention is the voting set of {CONTINUE, APPROVE, REJECT}. Each of the nodes participating in the cluster broadcasts a message having a header containing a VOTE field to convey the respective votes of the cluster nodes  202 ,  204 ,  206 ,  208 , and  210 , and membership seeking node  214 . Such messaging formats are known to those skilled in the art. An n-phase refers to the n-series of broadcast/vote sequences generated by the members, or providers, of the cluster  200  to arrive at a consensus with respect to a proposed request. 
       FIG. 3  illustrates a flow chart of error detection protocol (“EDP”)  300  executed by the cluster  200 . As necessary, reference is made to  FIGS. 1 and 2 . 
     The occurrence of the EDP  300  concerns the error of a cluster node in performing a data operation. The flow chart is an example of such an error detection with respect to the modification of a data resource, such as a database, data array, or other form of data structure. 
     Error detection contributes to the high availability of network resources across the cluster  200  in that detection allows an appropriate error handling routine to be executed to remedy the error, and to maintain data resource integrity for the network as a whole. It should be noted, however, that the aspect of the present invention discussed in detail is with reference to error detection and possible recovery action. For clarity and simplicity, a node having an error is removed from membership in the cluster  200  (see  FIG. 2 ). 
     Entering the EDP  300  at step  302 , a data resource modification request is issued to the group services client  212  by a cluster node  202 ,  204 ,  206 ,  208 ,  210 , or  214 , accordingly, at step  304 . An example of a data resource modification request is an update request. The modification request is broadcast by the group services client  212  to the cluster nodes  202 ,  204 ,  206 ,  208 ,  210 , and  214 . 
     In the next step  306 , the data resource is modified as requested at each of the local copies of the resource for each of the cluster nodes  202 ,  204 ,  206 ,  208 ,  210 , and  214 , accordingly. After the local copies of the data resource are modified, each cluster node issues a modification-result code, which is initially stored locally. A modification-result code indicates either a successful completion occurred or an error occurred. It also contains a detailed error message, which is used by all nodes to compare the result of the operation at different nodes. 
     After each client node executes the request data source modification, then at step  307 , each node checks if the result was a fatal error (for example: out of memory or out of paging space). If such an error occurred, then at step  309  the node starts its recovery actions. A typical recovery action in this case would be to shut down and restart the node. 
     If no fatal error has occurred, then at step  308 , the requesting node issues the modification-result code, which is sent to the group services client  212  that broadcasts the result to the nodes of the cluster  200 . 
     Each node of the cluster  200  then compares the broadcast result with a local modification-result code and votes accordingly. The vote set available with respect to the comparison is {APPROVE, CONTINUE}. An APPROVE vote indicates that the comparison result is TRUE, or that there is no difference between the broadcast result and the local modification-result code. A CONTINUE vote indicates that there is a difference between the broadcast result and the local modification-result code. 
     In step  312 , a query is made to determine whether any of the nodes of the cluster  200  issued a CONTINUE vote. If no CONTINUE votes were issued, then at step  329 , all the nodes of the cluster  200  issued an APPROVE vote. It should be noted that the phase count is typically incremented in the ordinary course of duties of the group services  212 , and is discussed to illustrate the function of the EDP  300 . Because an APPROVE vote was reached by all the nodes of the cluster  200  (see  FIG. 2 ) in one phase, the Group Services client  112  then broadcasts APPROVE to the cluster nodes in step  318 , indicating successful completion across the cluster  200 . 
     No error occurred across the cluster  200  with respect to execution of the modification request made at step  304 . An example of this conclusion is discussed in detail later with respect to  FIG. 4 . The exit is executed at step  338 . 
     The term “phase” as used herein means the a broadcast/vote sequence where the group services client  212  issues a broadcast action, and the nodes of the cluster  200  issue a vote action. 
     Referring again to  FIG. 3 , if there is a CONTINUE vote at step  312 , it means that one or more nodes in the cluster  200  had a modification result code that did not match that of the requested node. The purpose of the remaining steps in  FIG. 3  is that such results are visible to all nodes in the cluster  200  depending on the number of nodes that have different results, and a recovery action is taken. 
     In step  324 , the cluster nodes that voted “CONTINUE” in the previous phase will broadcast their modification_result code to all the cluster nodes. If there is more than one such message, group services will choose one of them and broadcast. At step  326 , each node that voted CONTINUE in the previous phase will compare its modification_result code with that it received. If the same, at step  328  it will vote “APPROVE” and listen for the next phase. If false, it will rebroadcast its modification_result code and vote “CONTINUE” at step  330 . This loop will terminate when all the results are visible to all nodes. 
     Group services  112  automatically increments the phase number each time there is a broadcast/vote cycle. The phase number is checked in step  320 . If the phase number is 2, this indicates there was no error and the process exits at step  338 . If false, the recovery actions are executed in step  332 . At step  334 , a check is made if there is consensus. If not, the modifications are backed out in step  336 . If there is consensus, some actions may still be needed. The algorithm for achieving consensus adopted here is that a majority of the nodes (n/2+1) have the same results. At step  337 , the nodes that have different results from the majority will back out its updates and closes the data resource. A re-open of the data resource will result in an up-to-date version of the data resource to be transmitted to these nodes. 
     The EDP  300 , whether a sole error has or has not occurred, exits at step  338 . After the exit step  338 , additional cluster activities can be conducted. It should be noted, however, that the EDP  300  can be a multitask protocol or can be a devoted protocol. That is, the EDP  300  can be executed while other processes are executing on the cluster  200  or on individual nodes. 
     To summarize, three error conditions may exist within the cluster  200 : no error occurs (see  FIG. 4 ); an error occurs on a non-requesting node (see  FIG. 5 ); and an error occurs on the requesting node (see  FIG. 6 ). 
       FIG. 4  is an example of the error detection protocol  300  (see  FIG. 3 ) applied in a situation where there is no error after executing a modification request by a node of the cluster  200 . 
     In this example, node_alpha  202  issues a data resource modification request to the group services client  212  (see  FIG. 2 ), which broadcasts the modification request to the cluster nodes node_beta  204 , node_gamma  206 , node_zeta  208 , node_epsilon  210 , and node_delta  214 . Each of the nodes of the cluster  200  apply the modification request to the local copy of the resource of that respective node. The requesting node, which in this example is node_alpha  202 , sends its result of the modification request to the non-requesting nodes beta  204 , gamma  206 , zeta  208 , epsilon  210 , and delta  214 . 
     The non-requesting nodes compare their result with the broadcast result and vote, as shown in phase 1. If there are no CONTINUE votes, the phase counter is incremented by one, and the group services client  212  (see  FIG. 2 ) broadcasts the designation APPROVED in phase 2 to the nodes of the cluster  200 , and the phase counter is again incremented, indicating that the EDP  300  completed in less than or equal to two phases. The EDP  300  then concludes (see  FIG. 3 ). 
       FIG. 5  is an example of error detection protocol  300  (see  FIG. 3 ) applied in a situation where there is an error among the nodes of the cluster  200 . 
     In this example, node_alpha  202  issues a data resource modification request to the group services client  212  (see  FIG. 2 ), which broadcasts the modification request to the cluster nodes node_beta  204 , node_gamma  206 , node_zeta  208 , node_epsilon  210 , and node_delta  214 . Each of the nodes of the cluster  200  apply the modification request to the local copy of the resource of that respective node. The requesting node, which in this example is node_alpha  202 , sends its result of the modification request to the non-requesting nodes beta  204 , gamma  206 , zeta  208 , epsilon  210 , and delta  214 . 
     The non-requesting nodes compare their results with the broadcast result of the requesting node, and vote, as shown in phase 1. If there are no CONTINUE votes, the phase counter is incremented by one, and the group services client  212  (see  FIG. 2 ) broadcasts the designation APPROVED in phase 2 to the nodes of the cluster  200 , and the phase counter is again incremented, indicating that the EDP  300  completed in less than or equal to two phases. The EDP  300  then concludes (see  FIG. 3 ). 
     In the example of  FIG. 5 , however, an error has occurred with respect to node_gamma  206 . The comparison conducted in step  312  of the EDP  300  (see  FIG. 3 ) continues with each of the nodes indicating the result of the modification request with a vote, as shown in phase 1. 
     The term “error” as used herein, is understood to be the condition where the result of a modification or a computation request is either (1) dissimilar from the result of the requesting node if the dissimilarity occurs in a non-requesting node, or (2) the requesting node has an error code other than a SUCCESS code. For example, errors can be a syntax error, or an out-of-resources error. Nevertheless, if every node of the cluster  200  has the same result, such as a syntax error, then no error is considered to have occurred that would require further error handling in the form of isolating the error of the present invention. Other forms of error handling and detection, which are known to those skilled in the art, would be used. 
     In  FIG. 5 , node_alpha  202  vote APPROVE with an error code of value SUCCESS, node_beta  204 , node_epsilon  208 , and node_zeta  210  vote APPROVE but do not send out their error code of SUCCESS. Node_gamma  206  votes CONTINUE, indicating a dissimilar result with the requesting node_alpha  202 . The phase counter is incremented by one to indicate that phase 1 of the EDP  300  has been completed. 
     In phase 2, the group services client has received the votes of all the nodes, and the error code of requesting node node_alpha  202  is broadcast, and in step  328  (see  FIG. 3 ), is compared with the result codes at each of the nodes. Each of the nodes then vote again. 
     Each of the nodes that previously voted APPROVE, again vote APPROVE because the comparison is made with respect to the result of the requesting node. The requesting node in this example is node_alpha  202 . Node_gamma  206  votes CONTINUE and sends with the CONTINUE vote its error code and error information, as shown in step  330  (see  FIG. 3 ). The phase number is incremented, and the EDP  300  passes to phase 3. 
     In phase 3, the group services client  212  broadcasts the error message to the nodes of cluster  200 . Node_gamma  206  votes APPROVE with no message because it has already broadcast its error result to all other nodes. All other nodes vote APPROVE because they agree with the SUCCESS result from the requesting node node_alpha  202 . The phase number is again incremented. 
     In phase 4, the protocol has been approved by all nodes and the group services client broadcast no new message to all the nodes. All nodes deduce that node_gamma  206  was the only node that has a dissimilar result with other nodes. 
     Further error handling can be used accordingly in view of the specific error that occurred. For simplicity, an example of handling the error is to simply revoke the node membership in the cluster  200 , until the error can be corrected. Accordingly, node_gamma  206  is removed from the cluster  200 . 
       FIG. 6  is an example of the error detection protocol  300  (see  FIG. 3 ) applied in a situation where a single error is detected for the requesting node. 
     In this example, node_alpha  202  issues a data resource modification request to the group services client  212  ( FIG. 2 ), which broadcasts the modification request to the cluster nodes node_beta  204 , node_gamma  206 , node_zeta  208 , node_epsilon  210 , and node_delta  214 . Each of the nodes of the cluster  200  apply the modification request to the local copy of the resource of that respective node. The requesting node, which in this example is node_alpha  202 , sends its result of the modification request to the non-requesting nodes_beta  204 , gamma  206 , zeta  208 , epsilon  210 , and delta  214 . 
     The non-requesting nodes compare their results with the broadcast result of the requesting node, and vote, as shown in phase 1. If there are no CONTINUE votes, the phase counter is incremented by one, and the group services client  212  (see  FIG. 2 ) broadcasts the designation APPROVED in phase 2 to the nodes of the cluster  200 , and the phase counter is again incremented, indicating that the EDP  300  completed in less than or equal to two phases. The EDP  300  then concludes (see  FIG. 3 ). 
     In the example of  FIG. 6 , however, an error has occurred with respect to requesting node_alpha  202 . In the example provided, the requesting node, node_alpha  202 , has an error code other than a SUCCESS value. The non-requesting nodes have an error code with a SUCCESS value. Accordingly, the EDP  300  (see  FIG. 3 ) serves to detect the node with a dissimilar result. 
     Accordingly, the comparison conducted in step  312  of the EDP  300  (see  FIG. 3 ) continues with each of the nodes indicating the result of the modification request with a vote, as shown in phase 1. 
     In phase 1, the request node node_alpha  202  vote CONTINUE with its error code. All other nodes have an error code of SUCCESS and hence vote APPROVE with no message. Those non-requesting nodes should not send any message because only the requesting node is allowed to broadcast its error code in phase 2. 
     In phase 2, the group services client has received the ERROR message from the requesting node node_alpha  202  votes APPROVE for its own error result. All other nodes have dissimilar results with the requesting node and hence must broadcast their results. All other nodes vote CONTINUE with its SUCCESS return code. The phase number is then incremented to phase 3. 
     In phase 3, the group services client received the SUCCESS message from node_gamma  202 . Node_gamma  202  votes APPROVE from this point on. All other nodes again vote CONTINUE with their SUCCESS result. This process repeats until all remaining nodes have broadcast their result. The protocol will be approved at the 6th phase for a cluster of 5 nodes given that no node fails during the processing of this protocol. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.