Abstract:
A method and apparatus for configuring a software application on a cluster is provided. A configuration coordinator executing on a configuration manager communicates with one or more configuration slaves executing on a set of nodes that are operating as a cluster. The configuration coordinator sends messages to the one or more configuration slaves to initiate a configuration operation for a software application. Each configuration slave automatically performs a series of actions to configure the node on which it resides. When all the nodes complete the configuration operation for the software, the process is complete. While performing the series of actions, the configuration slaves generate logs that reflect their progress in performing the series of actions. If a problem occurs during performance of the series of actions, the configuration slave that encounters the problem indicates to the configuration coordinator that an error occurred. The configuration coordinator responds to the error by causing the configuration slaves to roll back changes made during performance of the series of actions. The configuration slaves that have begun but not completed the series of actions inspect their logs to determine which changes to roll back. By automatically configuring software on a cluster, and automatically rolling back changes on all cluster nodes in the event of an error during the configuration process, the cluster configuration process is made atomic, automatic, and significantly faster and less error-prone than manual cluster-wide configuration operations.

Description:
FIELD OF THE INVENTION 
     The present invention relates to configuring a cluster, and more specifically, to a method and apparatus for configuring on a cluster a software application that is not necessarily designed for execution on a cluster. 
     BACKGROUND OF THE INVENTION 
     A computer network typically includes a set of devices connected in a way that allows the devices to communicate with each other. Such devices, which can include workstations with memory and one or more processors, are often referred to as nodes. A cluster is a group of nodes that work together as a single system. One software application that allows groups of nodes to operate as a single system is NT Enterprise, which is generally available from Microsoft Corporation. 
     Clusters can be either “shared data” or “shared nothing” clusters. In a shared data cluster, all nodes have access to one or more shared storage devices. In a shared nothing cluster, storage devices are “owned” by nodes, and nodes only have access to the storage devices that they own. 
     In general, clustering technology is designed to minimize downtime for client/server network computing applications. Downtime may be minimized, for example, by shifting the responsibilities of a first node in the cluster to a second node in the cluster if the first node in the cluster fails. Shifting responsibilities in this manner is referred to as fail over. A node that assumes the responsibilities of another node in response to a fail over is referred to herein as a fail over node. 
     The responsibilities that a node is able to handle is determined in part by the software that is executing on the node. For example, a node may be able to process database requests because it is executing a database server. If the node fails, the responsibility for processing database requests can only be shifted to a fail over node that is able to execute the database server. Since the fail over node is not currently executing the database server, the database server must be started on the fail over node in response to the fail over. Techniques for performing automatic fail over in a client/server system are described in U.S. patent application Ser. No. 08/866,842 entitled “Automatic Failover for Clients Accessing a Resource Through a Server”, filed on May 30, 1997, the contents of which are incorporated herein by reference. 
     Many software programs must be specifically configured for a node before they can be safely executed on the node. Configuring a software program may involve, for example, (1) configuring the network required to run the client/server based application, (2) configuring the application itself, and (3) configuring any other software that may be required for the application to run. The process of configuring a software program for a node can be complex and time consuming. It typically requires the user to manually perform a series of steps specified by the software provider. For sophisticated software programs, the steps can be both numerous and complex. Further, if one step in the configuration process fails, the entire configuration operation may have to be restarted. 
     Applications designed to run on a single node are generally referred to as stand alone applications. An application that runs in a cluster environment and is capable of fail over to another node in the cluster when the primary node fails is referred to as a fail safe application. 
     Before a stand alone application is configured for fail safe operation, the application can only run on one of the clustered nodes. This node is referred to as the owner node. Fail safe operation requires the application to be configured both on the owner node and on other nodes in the cluster so that the application can run on multiple nodes in the cluster to provide fail over capability. 
     In fail over systems, software programs must be configured on both (1) nodes that will initially execute the programs, and (2) nodes that may have to execute the programs if fail over occurs. Thus, depending on the fail over policies employed within a cluster, a given software program may have to be configured on all of the nodes in a cluster even though it is planned to be executed on only one of the nodes in the cluster at a time. 
     A configuration operation becomes exponentially more complex and time consuming the more nodes for which the program must be configured. Consequently, configuring applications for use on clusters that employ fail over can be prohibitively burdensome. For example, one software program has a forty-step configuration process. Configuring such a program on a relatively small cluster of nodes has taken an expert engineer approximately nineteen hours. 
     Based on the foregoing, it is clearly desirable to reduce the complexity of configuring software in clusters that employ fail over policies. 
     SUMMARY OF THE INVENTION 
     A method and apparatus for turning a stand alone application into a fail safe application automatically with minimum expertise required of the user of the application. According to one aspect of the invention, a configuration coordinator executing on a configuration manager communicates with one or more configuration slaves executing on a set of nodes that are operating as a cluster. The configuration coordinator sends messages to the one or more configuration slaves to initiate a configuration operation for a software application. The configuration coordinator generates log information to track which configuration slaves have initiated and completed configuration operations. 
     Each configuration slave automatically performs a series of actions to configure the node on which it resides. While performing the series of actions, the configuration slaves generate logs that reflect their progress in performing the series of actions. If a problem occurs during performance of the series of actions, the configuration slave that encounters the problem indicates to the configuration coordinator that an error occurred. The configuration coordinator responds to the error by causing the configuration slaves to roll back changes made during performance of the series of actions. The configuration slaves that have begun but not completed the series of actions inspect their logs to determine which changes to roll back. 
     By automatically configuring software on a cluster, and automatically rolling back changes on all cluster nodes in the event of an error during the configuration process, the cluster configuration process is made atomic, automatic, and significantly faster and less error-prone than manual cluster-wide configuration operations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
     FIG. 1 is a block diagram of a computer system on which an embodiment of the present invention can be implemented; 
     FIG. 2 is a block diagram of a computerized system that includes a cluster that may be configured to execute a software application using techniques provided by the present invention; 
     FIG. 3 is a flow chart illustrating steps for configuring a software program on a cluster according to an embodiment of the invention; and 
     FIG. 4 is a flow chart illustrating steps for performing a cluster-wide roll back according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A method and apparatus for automatically configuring software on a cluster is described. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention. 
     HARDWARE OVERVIEW 
     FIG. 1 is a block diagram that illustrates a computer system  100  that represents a node upon which an embodiment of the invention may be implemented. Computer system  100  includes a bus  102  or other communication mechanism for communicating information, and a processor  104  coupled with bus  102  for processing information. Computer system  100  also includes a main memory  106 , such as a random access memory (RAM) or other dynamic storage device, coupled to bus  102  for storing information and instructions to be executed by processor  104 . Main memory  106  also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor  104 . Computer system  100  further includes a read only memory (ROM)  108  or other static storage device coupled to bus  102  for storing static information and instructions for processor  104 . A storage device  110 , such as a magnetic disk or optical disk, is provided and coupled to bus  102  for storing information and instructions. 
     Computer system  100  may be coupled via bus  102  to a display  112 , such as a cathode ray tube (CRT), for displaying information to a computer user. An input device  114 , including alphanumeric and other keys, is coupled to bus  102  for communicating information and command selections to processor  104 . Another type of user input device is cursor control  116 , such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor  104  and for controlling cursor movement on display  112 . This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane. 
     The invention is related to the use of computer system  100  for configuring a set of nodes to execute an application. According to one embodiment of the invention, automatic multi-node configuration is coordinated by computer system  100  in response to processor  104  executing one or more sequences of one or more instructions contained in main memory  106 . Such instructions may be read into main memory  106  from another computer-readable medium, such as storage device  110 . Execution of the sequences of instructions contained in main memory  106  causes processor  104  to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the invention. Thus, embodiments of the invention are not limited to any specific combination of hardware circuitry and software. 
     The term “computer-readable medium” as used herein refers to any medium that participates in providing instructions to processor  104  for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device  110 . Volatile media includes dynamic memory, such as main memory  106 . Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus  102 . Transmission media can also take the form of acoustic or light waves, such as those generated during radio-wave and infra-red data communications. 
     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, punchcards, papertape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read. 
     Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to processor  104  for execution. For example, the instructions may initially be carried on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system  100  can receive the data on the telephone line and use an infra-red transmitter to convert the data to an infra-red signal. An infra-red detector coupled to bus  102  can receive the data carried in the infra-red signal and place the data on bus  102 . Bus  102  carries the data to main memory  106 , from which processor  104  retrieves and executes the instructions. The instructions received by main memory  106  may optionally be stored on storage device  110  either before or after execution by processor  104 . 
     Computer system  100  also includes a communication interface  118  coupled to bus  102 . Communication interface  118  provides a two-way data communication coupling to a network link  120  that is connected to a local network  122 . For example, communication interface  118  may be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface  118  may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface  118  sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. 
     Network link  120  typically provides data communication through one or more networks to other data devices. For example, network link  120  may provide a connection through local network  122  to a host computer  124  or to data equipment operated by an Internet Service Provider (ISP)  126 . ISP  126  in turn provides data communication services through the world wide packet data communication network now commonly referred to as the “Internet”  128 . Local network  122  and Internet  128  both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link  120  and through communication interface  118 , which carry the digital data to and from computer system  100 , are exemplary forms of carrier waves transporting the information. 
     Computer system  100  can send messages and receive data, including program code, through the network(s), network link  120  and communication interface  118 . In the Internet example, a server  130  might transmit a requested code for an application program through Internet  128 , ISP  126 , local network  122  and communication interface  118 . In accordance with the invention, one such downloaded application provides for automated configuration as described herein. 
     The received code may be executed by processor  104  as it is received, and/or stored in storage device  110 , or other non-volatile storage for later execution. In this manner, computer system  100  may obtain application code in the form of a carrier wave. 
     EXEMPLARY CLUSTER 
     Referring to FIG. 2, it is a block diagram of a system  200  that includes a cluster  224 . Cluster  224  includes nodes  202 ,  204 ,  206  and  208 . The nodes of cluster  224  are connected through and communicate over a local area network  220 . Local area network  220  also includes nodes  210  and  212  that are not part of cluster  224 . Local area network  220  is connected to a wide area network  222 , such as the Internet, thus allowing the nodes that belong to local area network  220  to communicate over long distances with other nodes (not shown). 
     In addition, the clustered nodes also connect to a set of common disks/storage systems  230 . In a shared nothing cluster, each disk can only be accessed by one cluster node at a time, while in a shared disk cluster, all of the cluster nodes can access the common disks simultaneously. 
     It should be noted that the illustrated system  200  is merely exemplary. The actual number and type of nodes in the cluster, and the mechanism that allows communication between the nodes, may vary from implementation to implementation. The present invention is not limited to any particular type of node, cluster, or inter-node communication mechanism. 
     AUTOMATED ATOMIC CONFIGURATION 
     According to one embodiment of the invention, configuring a cluster is performed both automatically and atomically. The automated nature of the configuration process is achieved by causing various software entities, including a configuration coordinator and one or more configuration slaves, to perform the configuration steps on the various nodes of the cluster. 
     A set of actions is said to be “atomic” if techniques are used to guarantee that the set of actions are treated as an indivisible unit. Specifically, a set of actions is atomic when all of the actions in the set are performed if any action in the set is performed. The configuration process described herein is atomic in that an application is configured on either all of the necessary nodes or on none of the nodes. The atomic nature of the configuration process is achieved through the combination of progress tracking and a rollback mechanism. The configuration process shall now be described in greater detail with reference to FIG.  3 . 
     Referring to FIG. 3, it is a flow chart illustrating the steps performed by a process (the “configuration coordinator”) to coordinate a configuration operation for an application according to an embodiment of the invention. Initially, a node is selected to be a configuration manager. The configuration manager executes the configuration coordinator. The configuration manager may be a node that belongs to the cluster being configured, or to a node that is able to communicate with the cluster. For the purposes of explanation, it shall be assumed that cluster  224  is being configured for an application called “APP1”, and that node  210  has been designated as the configuration manager. 
     At step  302 , the configuration coordinator polls each node in the cluster to find out which node has the given application configured as a stand alone application, and therefore is owner of the application. In response to being polled, the nodes in the cluster send the configuration coordinator information that indicates whether or not they are the owner of the given stand alone application. In the present example, a configuration coordinator process running on node  210  polls nodes  202 ,  204 ,  206  and  208  during step  302 . 
     Based on the information retrieved during the polling process, at step  304  the configuration coordinator determines the owner of the application to be configured. At step  306  the owner is recorded in the configuration manager. In the present example, it shall be assumed that APP1 is configured as a stand alone application on node  202 . Therefore, the configuration coordinator stores in node  210  data indicating that node  202  is the owner of APP1. 
     At step  308 , the configuration coordinator initiates the configuration of the owner so that the application will run in the cluster environment as a fail safe application. The configuration coordinator generates log information to indicate that the configuration process has been initiated on the owner. According to one embodiment, the configuration coordinator initiates the configuration of the owner by invoking a slave process (a “configuration slave”) on the owner. A configuration slave is software, executing as one or more processes, for automatically configuring a node for an application. A configuration slave may be designed for automatically configuring a node for one or more particular applications, or may be more generically designed for automatically configuring a node for any number and type of applications. In the latter case, the configuration slave receives input that specifies the particular configuration steps that must be performed for a given application. 
     The processes that implement a configuration slave may also perform other services. For example, according to one embodiment of the invention, the same processes that are used to implement fail over among nodes in the cluster also serve as configuration slaves to configure applications prior to fail over. 
     Referring again to FIG. 3, in response to step  308  the configuration slave automatically performs the steps required to configure the application on the owner. While the configuration slave is configuring the owner for the application, the configuration slave generates a log of its progress. 
     In the present example, it shall be assumed that steps S 1 , S 2 , S 3  and S 4  must be successfully performed to configure APP1 on a node. Therefore, the configuration coordinator initiates a configuration slave on node  202  to begin the configuration process on node  202 . The configuration slave performs steps S 1 , S 2 , S 3  and S 4  on node  202  while generating a log to record its progress. 
     The actual steps that must be performed by the configuration slave will depend on the application being configured. The present invention is not limited to any particular application or type of application, and therefore is not restricted to any particular type or sequence of configuration steps. 
     At step  310  it is determined whether the configuration was successfully completed. The configuration is not successfully completed if, for example, the configuration coordinator receives a message from a configuration slave that the configuration slave was unable to successfully perform one of the configuration steps, or if the configuration coordinator fails to receive a response from a configuration slave after a predetermined period of time. 
     If the configuration was successfully completed, the configuration slave sends a “configuration complete” message to the configuration coordinator and control proceeds to step  311 . At step  311 , the configuration coordinator persistently stores log information in the configuration manager to record that configuration has been successfully performed on the owner. As shall be described in greater detail hereafter, the log information on the configuration manager is used both to determine when the cluster-wide configuration of an application has completed successfully, and to determine which nodes have to be rolled back if the cluster-wide configuration cannot be completed successfully. 
     After the owner has been successfully configured, the configuration coordinator gathers all the necessary configuration information from the application on the owner node and stores the information in a cluster-wide repository provided by the cluster application programming interface (API). This configuration data is used by all of the other nodes in the cluster to configure the application. After the log information is recorded on the configuration manager and the configuration information is stored in a cluster-wide repository, control passes to step  312 . 
     According to one embodiment of the invention, a configuration slave deletes its configuration progress log when it completes its configuration steps. At that point the configuration slave simply keeps track of the configuration information that it will have to delete if it is asked to roll back the configuration operation. 
     In the present example, a configuration complete message is sent from node  202  to node  210  when the configuration slave successfully completes steps S 1 , S 2 , S 3  and S 4  on node  202 . The configuration coordinator then persistently stores on node  210  a record that configuration of node  202  is complete. 
     Steps  310 ,  311 ,  312  and  314  form a loop where a configuration operation is performed for each node of the cluster that must be able to execute the application. The nodes that must be able to execute an application depend on the fail over policy that applies to the cluster. For example, a fail over policy may order the nodes of a cluster in a circular list and specify that if any node fails, its responsibilities will be assumed by the next node in the order. In this example, all of the nodes of the cluster will have to be able to execute all applications, since it is possible that all nodes but one will fail. 
     An alternative policy may divide the nodes of the cluster into “fail safe groups”, where each group is ordered in a circular list. If any node fails, its responsibilities are assumed by the next node in the list for its group. Using this policy, all nodes within a fail safe group will have to be able to execute all programs that will run on any node in the group, but will not have to be able to execute programs that are executed on nodes that do not belong to the group. 
     According to one embodiment, the fail over policy is user-configurable. Consequently, a user can adopt a policy directed to specific needs and applications. The configuration coordinator receives input that indicates the applicable fail over policies and is thereby able to determine which nodes in the cluster have to be able to execute an application that is owned by a given node. 
     Step  314  is repeated for each node that must be configured for the application. During step  314 , the configuration coordinator instructs a configuration slave on a node to begin configuration of the node. The configuration coordinator also generates log information to indicate the new node on which the configuration operation has been initiated. The configuration slave configures the node upon which it is executing and generates a log of its progress. 
     Once the node is configured, the configuration slave sends a configuration complete message to the configuration manager, where a record of the successful configuration of the node is persistently stored. This process continues for each node in the cluster that must be configured for the application until all the nodes in the cluster that have to be configured have been configured. When all of the nodes that have to be configured have been successfully configured, the configuration process ends at step  318 . 
     For the purposes of explanation, it shall be assumed that cluster  224  implements a fail over policy that requires all nodes in the cluster to be able to execute APP1. Therefore, the configuration coordinator on node  210  invokes configuration slaves on nodes  204 ,  206  and  208 . As each configuration slave performs steps S 1 , S 2 , S 3  and S 4 , it records its progress. When each configuration slave completes, the configuration slave sends to the configuration coordinator a message indicating that it has completed the configuration of its node. The configuration coordinator stores log information on node  210  to record which nodes have completed the configuration process. 
     After node  210  has received messages indicating successful configuration from all of the nodes  204 ,  206  and  208 , the configuration coordinator updates the log information at node  202  to indicate that the cluster-wide configuration operation was successful. At this point, the configuration coordinator may optionally send messages to terminate each of the configuration slaves. 
     According to the flow chart in FIG. 3, configuration on one node is not initiated until configuration of the previous node is completed. However, in alternative embodiments, the configuration coordinator does not wait for nodes to be configured before initiating the configuration process on other nodes. Thus, multiple configuration slaves can configure multiple nodes in parallel, thus reducing the time required to complete the configuration process. 
     To the extent that configuration steps do not have to be performed sequentially, configuration slaves may be implemented by multiple processes executing in parallel to further reduce configuration time. For example, if S 1  and S 2  can be performed in any order, then a configuration slave performing S 1  and S 2  can spawn two processes to execute S 1  and S 2  in parallel. The benefit of distributing configuration tasks between multiple processes hinges on the availability of hardware that supports parallel processing. Therefore, configuration slaves may be configured to spawn multiple configuration processes if hardware on a node supports parallel processing, and to perform all configuration steps with a single process if hardware on the node does not support parallel processing. 
     FAILURE DURING CONFIGURATION 
     At any time during the configuration, the configuration or system may fail. When a failure occurs, control passes from step  310  to step  316 . During step  316 , configuration operations on one or more nodes are rolled back and the log information maintained by the configuration coordinator is updated to reflect the roll back. 
     A configuration operation is rolled back by removing changes made during the configuration operation. For example, assume that power goes out on node  206  after the configuration slave on node  206  has performed steps S 1  and S 2 . After power is returned to node  206 , the aborted configuration operation on node  206  is rolled back by removing the changes made during the performance of S 1  and S 2 . 
     At step  320  it is determined whether the reconfiguration process for the current node should be restarted. Whether the reconfiguration process should be restarted on the current node depends on the type of error encountered. If the error is unrecoverable, then control passes to step  322 . Otherwise, reconfiguration of the current node is restarted and control passes to step  310 . When restarted, log information is generated to indicate that the configuration operation has been restated. 
     Significantly, the entire configuration process does not need to start over after every failure. The configuration process can be restarted at the last step where a configuration complete message was received by and recorded at the configuration manager. For example, if nodes  202  and  204  have sent configuration completion messages to node  210  prior to a system failure, then the record that those nodes are configured will persist in the log information on node  210  after the failure. Upon reading the log information, the configuration coordinator will know that nodes  202  and  204  have been successfully configured. Therefore, configuration will only have to be restarted at nodes  206  and  208 . 
     CLUSTER-WIDE ROLL BACK 
     When configuration fails due to an unrecoverable error, control passes to step  322  where the configuration process is rolled back at all nodes of the cluster. This may occur, for example, if prerequisite software needed to run the application being configured is missing or if a node does not have enough memory to execute an application. After cluster wide rollback, the configuration operation is not automatically restarted. 
     For example, assume that node  206  does not have enough memory to execute APP1. Merely rolling back and restarting the configuration process on node  206  will not solve this problem. Therefore, a cluster-wide configuration roll back is performed and a diagnosis of the problem is sent to the configuration coordinator. A user at the configuration manager may inspect the diagnosis, correct the problem, and then restart the configuration process. 
     Upon detecting an error that requires cluster-wide roll back, the configuration coordinator transmits rollback messages to all nodes on which configuration has been initiated. In response to the rollback messages from the configuration coordinator, the changes caused by any previously executed configuration steps are removed from the various nodes. Specifically, each configuration slave responds to a rollback message by removing from its node the changes made to its node up to that point during the configuration operation. For configuration slaves that have already completed the configuration process on their nodes, rollback of a non-owner node may simply involve deleting configuration files that were generated during the configuration of the node. On the owner node, rollback involves configuring the application to run on the node as a standalone application again. 
     Upon finishing rollback, a configuration slave sends a “rollback complete” message to the configuration coordinator. When the configuration coordinator receives a rollback complete message from a configuration slave, the rollback coordinator updates the log information on the configuration manager to indicate that the node associated with the configuration slave has been rolled back. When the configuration coordinator has received a rollback complete message from all of the nodes that required roll back, the cluster-wide rollback operation is complete. 
     Referring to FIG. 4, it is a flow chart illustrating steps for cluster wide rollback according to an embodiment of the invention. FIG. 4 illustrates a cluster wide roll back in which the nodes are rolled back sequentially, rather than in parallel. However, the present invention is not limited to sequential roll back. 
     Cluster wide roll back begins at step  400  and proceeds to step  402  where it is determined whether any nodes in the cluster are still configured. If not, all of the nodes have already been rolled back and roll back is done (step  408 ). 
     Otherwise, a configured node is selected to be rolled back, and roll back of the selected node is initiated at step  404 . When roll back of the selected node is completed, the configuration data for the node is deleted at step  406 . Steps  402 ,  404  and  406  define a loop during which each node is rolled back and the configuration data for each node is deleted when the roll back of the node is complete. This loop continues until all nodes have been rolled back and all configuration data has been deleted. 
     APPLICATION VERIFICATION 
     According to one embodiment of the invention, the configuration slaves extend the configuration process beyond the provider-specified configuration steps to include application verification. During application verification, an application is executed on a node to determine whether it has been accurately configured for the node. 
     For example, after performing steps S 1 , S 2 , S 3  and S 4 , a configuration slave that is configuring APP1 on a node will perform the additional step (S 5 ) of verifying APP1 on the node. If APP1 executes correctly on the node, the configuration slave reports to the configuration coordinator that the configuration was successful. If APP1 does not execute as expected, then the configuration slave rolls back all of the configuration steps (S 1 , S 2 , S 3 , S 4  and S 5 ) and sends a message to the configuration coordinator to indicate that the configuration failed. 
     In embodiments that extend the configuration process to include application verification, parallelism during the cluster configuration process may be reduced. Specifically, some applications may not support concurrent execution on multiple nodes, particularly in shared nothing clusters. Therefore, under these conditions, application verification among the various nodes must be performed by each configuration slave serially relative to the other configuration slaves. 
     Significantly, the applications that are configured according to the techniques described herein need not be aware that they are executing on a cluster. The cluster-wide configuration is automatically performed by the configuration coordinator and one or more configuration slaves. Similarly, fail over may be automatically performed by separate fail over software. Consequently, applications designed for single node operation do not have to be modified to be used in a clustered, fail-safe environment. 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.