Abstract:
An asynchronous, peer-to-peer design is provided for synchronizing data on machines within a cluster using messaging. The data may be represented in memory as a document object model in secondary storage. Messaging may be handled by a messaging system with topic destinations. The design supports persistence management, locking, and transactions with rollback, and maintains a uniform state for the data across all nodes and secondary storage at all times. Furthermore, barring catastrophic failure of all machines in the cluster and secondary storage, there is no possibility of losing committed data.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates to distributed data processing and, in particular, to peer-to-peer data sharing and synchronization in a distributed data processing system.  
           [0003]    2. Description of Related Art  
           [0004]    Application Servers are software that is designed to serve multiple applications for multiple users that work on related tasks. Application Servers are an evolving concept that is more than just multiuser software which allows access to the same data. Application servers provide a mechanisms that help servers coordinate application and user data, and track on-going projects.  
           [0005]    The heart of an application server is a system for maintaining state of applications and data between the physical servers that make up the application server cluster of machines. Typically this system is a messaging system. As state changes on one machine in the cluster, the machine sends a message to all other machines with a notification of the state change, and data representing the new value of the state.  
           [0006]    Application groupware is one example of applications that are supported by application servers. Groupware applications allow users to collaborate on related work tasks and share data among the users involved in the tasks. Typically messages are used to notify team members, obtain responses and send alerts. Other applications include document sharing and document management, group calendaring and scheduling, group contact and task management, threaded discussions, text chat, data conferencing and audio and videoconferencing. Workflow, which allows messages and documents to be routed to the appropriate users, is often part of a groupware system.  
           [0007]    The use of the Internet and intranets has grown, because of the ease with which documents can be created and shared. However, as documents become widely used and distributed throughout an enterprise, security and synchronization problems surface. Document management, access control, and replication become issues. Thus, what starts out as a simple way to electronically publish information winds up presenting a new set of problems.  
           [0008]    Therefore, it would be advantageous to provide an improved mechanism for clustering application servers and the data and documents shared in the application server cluster.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention provides an asynchronous, peer-to-peer design for synchronizing data on machines within a cluster using messaging. In a preferred embodiment, the data is represented in active memory as a document object model; however, representation in secondary storage may or may not take the form of a DOM. Messaging may be handled by a messaging system with topic destinations. The present invention supports persistence management, locking, and transactions with rollback, and maintains a uniform state for committed data across all nodes and secondary storage at all times. Furthermore, barring catastrophic failure of all machines in the cluster and secondary storage, there is no possibility of losing committed data.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
         [0011]    [0011]FIG. 1 depicts a pictorial representation of a network of data processing systems in which the present invention may be implemented;  
         [0012]    [0012]FIG. 2 is a block diagram of a data processing system that may be implemented as a server in accordance with a preferred embodiment of the present invention;  
         [0013]    [0013]FIG. 3 is a block diagram illustrating a data processing system in which the present invention may be implemented;  
         [0014]    FIGS.  4 A- 4 D are block diagrams depicting data flow between a cluster node and a messaging service in accordance with a preferred embodiment of the present invention;  
         [0015]    [0015]FIG. 5 is a flowchart illustrating the operation of a user thread in accordance with a preferred embodiment of the present invention;  
         [0016]    [0016]FIG. 6 is a flowchart depicting the operation of a topic consumer thread in accordance with a preferred embodiment of the present invention;  
         [0017]    [0017]FIG. 7 is a flowchart illustrating the operation of an edit thread in accordance with a preferred embodiment of the present invention;  
         [0018]    [0018]FIG. 8 is a flowchart depicting the operation of a transaction thread in accordance with a preferred embodiment of the present invention; and  
         [0019]    [0019]FIG. 9 is a flowchart illustrating the operation of a result thread in accordance with a preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]    With reference now to the figures, FIG. 1 depicts a pictorial representation of a network of data processing systems in which the present invention may be implemented. Network data processing system  100  is a network of computers in which the present invention may be implemented. Network data processing system  100  contains a network  102 , which is the medium used to provide communications links between various devices and computers connected together within network data processing system  100 . Network  102  may include connections, such as wire, wireless communication links, or fiber optic cables.  
         [0021]    In the depicted example, server  104  is connected to network  102  along with storage unit  106 . In addition, clients  108 ,  110 , and  112  are connected to network  102 . These clients  108 ,  110 , and  112  may be, for example, personal computers or network computers. In the depicted example, server  104  provides data to clients  108 - 112 . Clients  108 ,  110 , and  112  are clients to server  104 . Network data processing system  100  may include additional servers, clients, and other devices not shown.  
         [0022]    In particular, network data processing system  100  may include server cluster  120 . A cluster of computer systems provides fault tolerance and/or load balancing. If one system fails, one or more additional systems are still available. Load balancing distributes the workload over multiple servers. Server  104  may be part of server cluster  120  or may process requests for the server cluster. As such, the server cluster may appear to clients  108 ,  100 ,  112  as a single machine, i.e. having a single Internet Protocol (IP) address.  
         [0023]    In the depicted example, network data processing system  100  is the Internet with network  102  representing a worldwide collection of networks and gateways that use the TCP/IP suite of protocols to communicate with one another. At the heart of the Internet is a backbone of high-speed data communication lines between major nodes or host computers, consisting of thousands of commercial, government, educational and other computer systems that route data and messages. Of course, network data processing system  100  also may be implemented as a number of different types of networks, such as for example, an intranet, a local area network (LAN), or a wide area network (WAN). FIG. 1 is intended as an example, and not as an architectural limitation for the present invention.  
         [0024]    Referring to FIG. 2, a block diagram of a data processing system that may be implemented as a server, such as server  104  in FIG. 1, is depicted in accordance with a preferred embodiment of the present invention. Data processing system  200  may be a symmetric multiprocessor (SMP) system including a plurality of processors  202  and  204  connected to system bus  206 . Alternatively, a single processor system may be employed. Also connected to system bus  206  is memory controller/cache  208 , which provides an interface to local memory  209 . I/O bus bridge  210  is connected to system bus  206  and provides an interface to I/O bus  212 . Memory controller/cache  208  and I/O bus bridge  210  may be integrated as depicted.  
         [0025]    Peripheral component interconnect (PCI) bus bridge  214  connected to I/O bus  212  provides an interface to PCI local bus  216 . A number of modems may be connected to PCI local bus  216 . Typical PCI bus implementations will support four PCI expansion slots or add-in connectors. Communications links to network computers  108 - 112  in FIG. 1 may be provided through modem  218  and network adapter  220  connected to PCI local bus  216  through add-in boards.  
         [0026]    Additional PCI bus bridges  222  and  224  provide interfaces for additional PCI local buses  226  and  228 , from which additional modems or network adapters may be supported. In this manner, data processing system  200  allows connections  15  to multiple network computers. A memory-mapped graphics adapter  230  and hard disk  232  may also be connected to I/O bus  212  as depicted, either directly or indirectly.  
         [0027]    Those of ordinary skill in the art will appreciate that the hardware depicted in FIG. 2 may vary. For example, other peripheral devices, such as optical disk drives and the like, also may be used in addition to or in place of the hardware depicted. The depicted example is not meant to imply architectural limitations with respect to the present invention.  
         [0028]    The data processing system depicted in FIG. 2 may be, for example, an IBM e-Server pseries system, a product of International Business Machines Corporation in Armonk, New York, running the Advanced Interactive Executive (AIX) operating system or LINUX operating system.  
         [0029]    With reference now to FIG. 3, a block diagram illustrating a data processing system is depicted in which the present invention may be implemented. Data processing system  300  is an example of a client computer. Data processing system  300  employs a peripheral component interconnect (PCI) local bus architecture. Although the depicted example employs a PCI bus, other bus architectures such as Accelerated Graphics Port (AGP) and Industry Standard Architecture (ISA) may be used. Processor  302  and main memory  304  are connected to PCI local bus  306  through PCI bridge  308 . PCI bridge  308  also may include an integrated memory controller and cache memory for processor  302 . Additional connections to PCI local bus  306  may be made through direct component interconnection or through add-in boards. In the depicted example, local area network (LAN) adapter  310 , SCSI host bus adapter  312 , and expansion bus interface  314  are connected to PCI local bus  306  by direct component connection. In contrast, audio adapter  316 , graphics adapter  318 , and audio/video adapter  319  are connected to PCI local bus  306  by add-in boards inserted into expansion slots. Expansion bus interface  314  provides a connection for a keyboard and mouse adapter  320 , modem  322 , and additional memory  324 . Small computer system interface (SCSI) host bus adapter  312  provides a connection for hard disk drive  326 , tape drive  328 , and CD-ROM drive  330 .  
         [0030]    Typical PCI local bus implementations will support three or four PCI expansion slots or add-in connectors.  
         [0031]    An operating system runs on processor  302  and is used to coordinate and provide control of various components within data processing system  300  in FIG. 3. The operating system may be a commercially available operating system, such as Windows 2000, which is available from Microsoft Corporation. An object oriented programming system such as Java may run in conjunction with the operating system and provide calls to the operating system from Java programs or applications executing on data processing system  300 . “Java” is a trademark of Sun Microsystems, Inc. Instructions for the operating system, the object-oriented operating system, and applications or programs are located on storage devices, such as hard disk drive  326 , and may be loaded into main memory  304  for execution by processor  302 .  
         [0032]    Those of ordinary skill in the art will appreciate that the hardware in FIG. 3 may vary depending on the implementation. Other internal hardware or peripheral devices, such as flash ROM (or equivalent nonvolatile memory) or optical disk drives and the like, may be used in addition to or in place of the hardware depicted in FIG. 3. Also, the processes of the present invention may be applied to a multiprocessor data processing system.  
         [0033]    As another example, data processing system  300  may be a stand-alone system configured to be bootable without relying on some type of network communication interface, whether or not data processing system  300  comprises some type of network communication interface. As a further example, data processing system  300  may be a Personal Digital Assistant (PDA) device, which is configured with ROM and/or flash ROM in order to provide nonvolatile memory for storing operating system files and/or user-generated data.  
         [0034]    The depicted example in FIG. 3 and above-described examples are not meant to imply architectural limitations. For example, data processing system  300  also may be a notebook computer or hand held computer in addition to taking the form of a PDA. Data processing system  300  also may be a kiosk or a Web appliance.  
         [0035]    Returning to FIG. 1, server  104  may support a messaging system, such as Java Messaging Service. A messaging system is software that provides a message delivery system. The messaging system stores messages in a message store, such as database  106 , until they are consumed by subscribers. Messages are sorted and stored by topic and each of the servers in server cluster  120  subscribes to each of the topics. Each of the servers in the server cluster is a node, referred to as a Cnode. A cluster contains many server machines, and these machines operating in tandem give the view of a single server. This permits servicing of many more clients. The present invention addresses the problem of trying to keep the data identical on all the separate server machines in the cluster—even when the data is changing—so that identical requests from clients to any machine will be identical.  
         [0036]    The present invention also allows changes to be made from a single session, using locks, across several requests and allows the requesting computer to see the changing, but still uncommitted, data while all others still see the committed data. In accordance with a preferred embodiment of the present invention, data is represented in memory as a document object model (DOM). A DOM is represented as a tree structure. Nodes in a DOM tree are referred to herein as Dnodes.  
         [0037]    With reference now to FIGS.  4 A- 4 D, block diagrams depict data flow between a cluster node and a messaging service in accordance with a preferred embodiment of the present invention. Cnode  402  includes edit thread  404 , transaction thread  406 , user thread  408 , result thread  410 , and consumption thread  420 . Messaging service  412  includes edit topic  414 , result topic  416 , and transaction topic  418 . A session is created when a client makes a request to a server in the cluster. A session is identified by a session ID. When a request is received, the Cnode creates a message, or “work item,” that is published to a topic. A work item message may contain a filter value, a brand, the task to be performed, and a session identification (ID) identifying the session making the request. The publisher of the work item message is referred to as the originating Cnode.  
         [0038]    Cnode  402  includes consumption thread  420 , which consumes work item messages from topics and passes them to the appropriate thread. All Cnodes are publishers to, and consumers of, work item messages from the edit topic  414  and the transaction topic  418 . The edit topic requires no filtering. Work item messages in the edit topic are consumed by all Cnodes. The transaction topic employs filters. Work item messages in the transaction topic are consumed only by the Cnode with an ID matching the filter value.  
         [0039]    Events that modify or lock an unlocked Dnode are published to edit topic  414 . Work item messages published to the edit topic are “branded” with the ID of the publishing (originating) Cnode. Events that modify or unlock a locked Dnode are published to transaction topic  418 . Work item messages published to the transaction topic have a filter value of the ID of the locking Cnode, the Cnode that originally received the request to lock a given Dnode. Thus, only the Cnode that locked a Dnode may modify or unlock that Dnode.  
         [0040]    All Cnodes are publishers to result topic  416 . The result topic employs filters, but the filter may be the ID of the locking Cnode or it may be ‘all’, indicating that all Cnodes will consume the result item. In other words, result items may be consumed by all Cnodes or only the locking Cnode. All Cnodes are consumers of the Result topic. Result items are published to the result topic. Result items contain either a response from a task, the resulting modification from the task, or both. In accordance with a preferred embodiment of the present invention, the data is represented by a DOM and the resulting modification comprises a data stream representing a Dnode or subtree. A result item also contains a reference to the user thread from the originating Cnode. A result item is labeled with one of five “task types”: Edit, Lock, Unlock, Transaction, and Response. A result item also has a “Success” flag that indicates whether the task was successful or not. Unsuccessful result items do not contain a result.  
         [0041]    Cnodes maintain edit queue  405  of work item messages received from edit topic  414 . Cnodes also maintain transaction buffer  407  of modified Dnodes associated with a locked subtree. Only the locking Cnode, the Cnode that originally received the lock request, maintains the modified Dnodes in transaction buffer  407 . When a DOM subtree is locked, the locking Cnode makes a copy of the locked subtree; it is this copy that is maintained and modified in the transaction buffer. The nodes in a locked subtree are herein referred to collectively as a lock domain.  
         [0042]    Modifications resulting from work item messages published in the edit topic are propagated upon completion. Modifications resulting from work item messages published in the transaction topic exist only in the modified Dnodes in the transaction buffer of the locking Cnode. All modifications for a locked subtree are propagated as a unit once the transaction has been committed, such as by unlocking the Dnode. Modifications are propagated by publishing the modifications to result topic  416 .  
         [0043]    Particularly, with reference to FIG. 4A, a data flow diagram is shown illustrating the processing of an unlocked Dnode modification request. Cnode  402  processes a request to modify an unlocked Dnode as described by the following steps:  
         [0044]    1. The user thread receives a request to modify an unlocked Dnode.  
         [0045]    2. The user thread publishes a work item message to the edit topic.  
         [0046]    3. The topic consumption thread consumes the work item message from edit topic  414 .  
         [0047]    4. The consumption thread dispatches the work item message to the edit thread for processing. The edit thread receives the work item message and places the work item message in the edit queue.  
         [0048]    5. When the work item message becomes the top item in the edit queue, the Cnode whose brand is affixed to that work item message processes the work item message in the edit queue and publishes an edit result item in the result topic.  
         [0049]    6. The topic consumer thread consumes the edit result item.  
         [0050]    7. The edit result item is dispatched to the result thread.  
         [0051]    8. If the brand affixed to the result item is that of the current Cnode, the user thread referenced in the result item is interrupted with a response, and the changes are committed to secondary storage. The result threads of all Cnodes remove the corresponding work item message from their edit queue and commit the modification to their DOM in memory.  
         [0052]    Events on unlocked Dnodes from multiple sources must be executed in the order received. Publishing them to the edit topic accomplishes this ordering and all Cnodes execute items with their own brand in their edit queue as those rise to the top of the edit queue. When the result thread receives an edit result item for an unlocked Dnode with the brand of another Cnode, then the result thread commits the modification and removes the work item message from the edit queue.  
         [0053]    Turning now to FIG. 4B, a data flow diagram is shown illustrating the processing of a Dnode lock request. Cnode  402  processes a request to lock a Dnode as described by the following steps:  
         [0054]    1. The user thread receives a request to lock an unlocked Dnode.  
         [0055]    2. The user thread publishes a work item message to the edit topic.  
         [0056]    3. The topic consumption thread consumes the work item message.  
         [0057]    4. The consumption thread hands the work item message to the edit thread which places the work item message in the edit queue.  
         [0058]    5. The edit thread processes the work item message in the edit queue and publishes a lock result item in the result topic. If the node is already locked the request is rejected and the success flag in the result item indicates an unsuccessful result.  
         [0059]    6. The topic consumer thread consumes the lock result item.  
         [0060]    7. The consumption thread hands the lock result item to the result thread. If the brand affixed to the result item is that of the current Cnode, the user thread referenced in the result item is interrupted with a response. There is no change in any secondary storage item during a lock.  
         [0061]    8. The result thread commits the lock of the subtree in the DOM. The locking Cnodes makes a copy of the newly locked Dnode and assigns it to its transaction buffer. All Cnodes record the lock. Along with a reference to the locked data (the root node of the locked subtree, in the preferred embodiment), the locking facility employed by the Cnodes must record the Locking Cnode&#39;s ID and the Session ID of the locking session.  
         [0062]    Since a lock operation is executed against an unlocked Dnode, the execution of the work item message must be synchronized to avoid later edit requests from modifying the Dnode before it is locked. The execution of lock requests and requests that modify unlocked Dnodes are processed on a first-in-first-out (FIFO) basis and synchronized by the edit queue.  
         [0063]    With reference now to FIG. 4C, a data flow diagram is shown illustrating the processing of a locked Dnode modification request. Cnode  402  processes a request to modify a locked Dnode as described by the following steps:  
         [0064]    1. The user thread receives a request to modify a locked Dnode.  
         [0065]    2. The user thread publishes a work item message to the transaction topic and assigns the locking Cnode&#39;s ID as a filter value.  
         [0066]    3. The consumption thread of the locking Cnode consumes the work item message from the transaction topic.  
         [0067]    4. The consumption thread dispatches the work item message to the transaction thread.  
         [0068]    5. The transaction thread processes the work item message against the cloned Dnode in the transaction buffer. Next, the transaction thread publishes a transaction result item to the result topic, with a task type of “Response”.  
         [0069]    6. The consumption thread consumes the transaction result item.  
         [0070]    7. The consumption thread dispatches the transaction result item to the result thread.  
         [0071]    8. The Result thread interrupts the user thread with a response.  
         [0072]    Modify or unlock requests on locked Dnodes are restricted to the locking session. This single source of modifications accomplishes the ordered execution achieved for unlocked Dnodes via the edit queue. By treating events for locked and unlocked Dnodes differently, edit topic events are synchronized and transaction topic events can execute without regard to time or timing. This improves throughput, because transaction topic events do not have to wait for any other event to execute.  
         [0073]    With reference to FIG. 4D, a data flow diagram is shown illustrating the processing of a Dnode unlock request. Cnode  402  processes a request to unlock a locked Dnode as described by the following steps:  
         [0074]    1. The user thread receives a request to unlock a locked Dnode.  
         [0075]    2. If the request comes from the locking session, the user thread publishes a work item message to the transaction topic and assigns the locking Cnode&#39;s ID as a filter value. If the request is not from the locking session, then the request is denied and an error is generated.  
         [0076]    3. The consumption thread of the locking Cnode consumes the work item message in the transaction topic.  
         [0077]    4. The consumption thread hands the work item message to the transaction thread.  
         [0078]    5. The transaction thread publishes to the result topic a result item for the unlock request, with a filter set to ‘all’, that includes the Dnode (DOM subtree) from its transaction buffer that is associated with the unlock request.  
         [0079]    6. The consumption thread consumes the transaction result item containing the modified DOM subtree.  
         [0080]    7. The consumption thread hands the transaction result item to the result thread.  
         [0081]    8. If the brand is that of the current Cnode, the result thread commits all the modifications to secondary storage. All result threads perform the modification update to their DOM in memory. If the brand is that of the current Cnode, the result thread interrupts the associated user thread with a response. The result threads of all Cnodes unlock the associated Dnode.  
         [0082]    In an alternative embodiment, the DOM subtree may be streamed and a copy added to the transmitting message. The transaction thread then publishes the unlock result to the result topic, with the filter set to ‘all’, as a separate message. The consumption thread then consumes the result item for the unlock request as a separate message. If the brand is that of the current Cnode, the result thread interrupts the associated user thread with a response. The result threads of all Cnodes unlock the associated Dnode.  
         [0083]    With reference now to FIG. 5, a flowchart illustrating the operation of a user thread is shown in accordance with a preferred embodiment of the present invention. The process begins when a user request to read, lock, unlock, or modify a Dnode is received. A user thread is created for each request. A determination is made as to whether the Dnode is locked (step  502 ). If the Dnode is not locked, a determination is made as to whether the request is a modify or lock request (step  504 ). If the request is not a modify or lock request, the process executes the request (step  506 ), and ends.  
         [0084]    If the request is a modify or lock request in step  504 , the process creates a work item message (step  508 ), sets the brand to be equal to the Cnode ID (step  510 ), and sets the request ID (step  512 ). The request ID allows items in the edit queue to be matched with result items. When a result item is consumed—and the task is therefore complete—the result thread looks up the work item message in the edit queue and removes it. Next, the process sets the reference thread (step  514 ) and publishes the work item message the edit topic with the filter set to all Cnodes (step  516 ). The reference thread is the ID for the user thread that initiated a request. This allows the result thread for the originating Cnode to find the correct user thread to ‘wake up’ and respond to. Thereafter, the user thread process sleeps until interrupted by the result thread (step  518 ).  
         [0085]    If the Dnode is locked in step  502 , a determination is made as to whether the request is a modify or unlock request (step  520 ). If the request is not a modify or unlock request, the process executes the request (step  506 ) and ends. If the request is a modify or unlock of a locked Dnode request, a determination is made as to whether the requester&#39;s HTTP session is the locking session for the Dnode (step  522 ). If the session associated with the request is not the locking session, the process rejects the request (step  524 ) and ends.  
         [0086]    If the session is the locking session in step  522 , the process creates a work item message (step  526 ), sets the request ID (step  528 ), and sets the reference thread (step  530 ). Then, the process publishes the work item message to the transaction topic with the filter set to the locking Cnode ID (step  532 ) and proceeds to step  518  to sleep until interrupted by the result thread.  
         [0087]    Turning now to FIG. 6, a flowchart is shown depicting the operation of a topic consumer thread in accordance with a preferred embodiment of the present invention. The process begins and consumes a work item message from a topic (step  602 ). A determination is made as to whether the topic is the edit topic (step  604 ). If the topic is the edit topic, the process hands the work item message to the edit thread (step  606 ) and ends.  
         [0088]    If the topic is not the edit topic in step  602 , a determination is made as to whether the topic is the transaction topic (step  608 ). If the topic is the transaction topic, the process hands the work item message the transaction thread (step  610 ) and ends. If the topic is not the transaction topic in step  608 , then the topic is the result topic and the process hands the work item message to the result thread (step  612 ) and ends.  
         [0089]    With reference to FIG. 7, a flowchart is shown illustrating the operation of an edit thread in accordance with a preferred embodiment of the present invention. The process begins and consumes a work item message from the edit topic (step  702 ). All work item messages in the edit topic have the filter set to “All.” Next, the process places the work item message on the edit queue (step  704 ) and a determination is made as to whether the top work item message in the edit queue has a brand equal to the ID of the Cnode (step  706 ).  
         [0090]    If the brand is equal to the ID of the Cnode, the process performs the requested task (step  708 ). The requested task may be performed directly against the data on that Cnode, or against a clone of the data. If the first approach is taken, the result thread does not update the data on the Cnode whose brand is associated with that work item message. If the second approach is taken, then the result thread performs the update in the same manner as all other Cnodes in the cluster.  
         [0091]    If the task is successful, the process streams the modified node and publishes the result in a response to the result topic with the filter set to all Cnodes and the success flag set (step  710 ). Thereafter the process ends. If the brand is not equal to the ID of the Cnode in step  706 , the process waits until the item is removed from the queue (step  712 ) and ends. The item is removed from the queue by the result thread in response to an edit result from the appropriate Cnode.  
         [0092]    Whenever an item is removed from the edit queue, the Edit Thread is asked to check the edit queue (step  706 ) to determine if the new work item message at the top of the queue has a brand equal to the ID of the current Cnode. The behavior of the Edit Thread then proceeds as described in the remaining steps of FIG. 7.  
         [0093]    Turning now to FIG. 8, a flowchart depicting the operation of a transaction thread is illustrated in accordance with a preferred embodiment of the present invention. The process begins and consumes a work item message from the transaction topic (step  802 ). Work item messages in the transaction topic are branded with the ID of the locking Cnode; therefore, the transaction thread consumes work item messages branded with its Cnode ID only. A determination is made as to whether the request is a valid unlock request (step  804 ).  
         [0094]    If the request is a valid unlock request, the process streams the modified nodes of the lock domain, present in the transaction buffer of the current Cnode, and publishes the result to the result topic with the filter set to “All” (step  806 ). Then, the process publishes an unlock result and response to the result topic (step  808 ), removes the lock domain from the transaction buffer (step  810 ), and ends. If the request is not a valid unlock request in step  804 , the process performs the requested task on a clone of the Dnode in the transaction buffer (step  812 ), publishes a response to the result topic with the filter equal to the originating Cnode ID and the success flag set (step  814 ), and ends.  
         [0095]    With reference now to FIG. 9, a flowchart is shown illustrating the operation of a result thread in accordance with a preferred embodiment of the present invention. The process begins and consumes a result from the result topic (step  902 ). A determination is made as to whether the result is a response result (step  904 ). If the result is a response result, the process interrupts the user thread with the response (step  906 ) and ends.  
         [0096]    If the result is not a response result in step  904 , a determination is made as to whether the brand of the result is equal to the ID of the Cnode (step  908 ). If the brand is equal to the ID of the Cnode, a determination is made as to whether the result is a transaction result (step  910 ). If the result is not a transaction result, then it is an Edit Result, and the process removes the request from the edit queue (step  912 ). For either type of result, a determination is then made as to whether the result is marked as a success (step  914 ).  
         [0097]    If the result is not marked as a success, the process interrupts the user thread with the response (step  906 ) and ends. If the result is marked as a success in step  914 , the process interrupts the user thread with the response, persists the result to secondary storage (step  916 ), refreshes the Dnode with the result (step  918 ), and ends. Thus, the permanent copy of the DOM is only updated by the Cnode making the modification.  
         [0098]    If the brand is not equal to the ID of the Cnode in step  908 , a determination is made as to whether the result is a transaction result (step  920 ). If the result is a transaction result, the process proceeds to step  924  to determine if the result is a success. If the result is a success, the process refreshes the Dnode (step  918 ) with the result, and ends. Thus, every Cnode updates its copy of the DOM in memory. If the result is not a success, the process ends.  
         [0099]    If the result is not a transaction result in step  920 , then it is an Edit Result, and the process removes the request from the edit queue (step  922 ) and a determination is made as to whether the result is marked as a success (step  924 ). If the result is marked as a success, the process refreshes the Dnode with the result (step  918 ), and ends. If the result is not marked a success, the process ends.  
         [0100]    Thus, the present invention solves the disadvantages of the prior art by providing a message based data sharing model. By separating requests for locked and unlocked Dnodes, the requests may be treated differently. All non-transaction requests are filtered through the edit topic and thereby ordered. Transaction requests need not wait for queued requests to be executed, thereby speeding up execution times. Transaction requests are consumed only by the originating Cnode, thereby reducing message traffic. The present invention readily supports locking, transactions, and rollback.  
         [0101]    Rollbacks are possible because transactions only modify cloned data in the transaction buffer. If any part of a transaction is not successful, the modified data in the transaction buffer can be purged from memory, leaving the original data untouched. All modifications in a transaction are persisted as a unit; thus, the number of persistence writes to alternate storage is reduced. By first executing the event, the validity of the event can be ascertained and the results can be provided to the remaining nodes in the cluster without each node having to perform the same work. The performance savings for authorization may be particularly important.  
         [0102]    The data sharing model of the present invention also does not use a master/slave relationship within the cluster, thus eliminating errors resulting from such an arrangement. The cluster operates asynchronously where possible but maintains the synchronous order of request execution where needed. Secondary storage and all cluster nodes are updated simultaneously, reducing the opportunity for data loss to an insignificant level. No committed data can be lost.  
         [0103]    Java Messaging Service provides scalability. An application built around a messaging architecture scales well as both clients and servers are added to the system. An application will also scale well as the number of messages increases. Cluster size is limited only by limits to the number of topic consumers. Although unlikely, if the number of allowed topic consumers is insufficient, messages may be published to backup sets of edit and transaction topics and the consumers may be partitioned between the two topic sets. Java Messaging Service also masks both heterogeneity and change. The common element in a messaging application is the message. As long as components can read and understand the messages, the platform on which they reside and the languages in which they are written are unimportant.  
         [0104]    It is important to note that while the present invention has been described in the context of a fully functioning data processing system, those of ordinary skill in the art will appreciate that the processes of the present invention are capable of being distributed in the form of a computer readable medium of instructions and a variety of forms and that the present invention applies equally regardless of the particular type of signal bearing media actually used to carry out the distribution. Examples of computer readable media include recordable-type media such a floppy disc, a hard disk drive, a RAM, and CD-ROMs and transmission-type media such as digital and analog communications links.  
         [0105]    The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.