Patent Publication Number: US-8533742-B2

Title: Distributed messaging system supporting stateful subscriptions

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is related to U.S. patent application Ser. No. 10/841,297, entitled “CONTINUOUS FEEDBACK-CONTROLLED DEPLOYMENT OF MESSAGE TRANSFORMS IN A DISTRIBUTED MESSAGING SYSTEM,” filed on an even date herewith, status abandoned; assigned to the same assignee, and incorporated herein by reference. 
     This application is a continuation of application Ser. No. 10/841,916, filed May 7, 2004, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to data processing systems and, in particular, to messaging systems in a distributed processing environment. Still more particularly, the present invention provides a distributed messaging system supporting stateful subscriptions. 
     2. Description of Related Art 
     A publish-subscribe messaging middleware is a system in which there are two types of clients. Publishers generate messages, also referred to as events, containing a topic and some data content. Subscribers request a criterion, also called a subscription, specifying what kind of information, based on published messages, the system is to deliver in the future. Publishers and subscribers are anonymous, meaning that publishers do not necessarily know how many subscribers there are or where they are and, similarly, subscribers do not necessarily know where publishers are. 
     A topic-based, or content-based, publish-subscribe system is one in which the delivered messages are a possibly filtered subset of the published messages and the subscription criterion is a property that can be tested on each message independent of any other message. For example, a filter may determine whether “topic=stock-ticker” or “volume&gt;1000.” Content-based or topic-based publish-subscribe systems are referred to herein as “stateless.” 
     There are pre-existing and emerging alternative technologies to solve the deficiencies of content-based publish-subscribe systems. Message mediators may be introduced into the flow of traditional messaging middleware. This is a useful concept; however, in their current manifestations, mediators are complex to program, require external database services in order to store and access state, and groups of mediators cannot be automatically combined. 
     Traditional database systems may also be used. Each published message can give rise to a cascade of transactions updating the message history. Subscriptions can be expressed as views of these histories. Technologies are being developed to allow views to be updated incrementally. Such an approach is easier to program; however, it can be costly and slow if each new message results in a transaction involving a large number of subscribers. 
     An emerging technology still being researched is continuous queries on data streams. These approaches preserve the simpler programming model of the database system approach above and attempt to reduce the cost of traditional databases by a combination of approaches, including batching message updates and restricting the available operations to ones allowing the use of bounded-sized, in-memory sliding windows. However, this approach is restricting and limited. 
     SUMMARY OF THE INVENTION 
     The present invention solves the disadvantages of the prior art and provides a distributed messaging system supporting stateful subscriptions. A stateful publish-subscribe system extends the functionality of the content-based approach to include more general state-valued expressions. Stateful subscriptions may refer to one or more message histories and may include more complex expressions. Therefore, subscribers may receive different information than that provided in the published messages. A plurality of broker machines is provided to deliver messages sent by publishing clients toward subscribing clients based upon the contents of the messages and stateful transformations requested by the subscribing clients. These broker machines form an overlay network. Subscription specifications are analyzed by a compiler and converted into a collection of transform objects and view objects. The messaging system builds a structure containing all transform objects and view objects needed for all intermediate and subscribed views of all subscriptions. This messaging system uses this structure to allocate transform objects and view objects to broker machines in the overlay network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  depicts a pictorial representation of a network of data processing systems in which the present invention may be implemented; 
         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; 
         FIG. 3  is a block diagram of a data processing that may serve as a client of a service in accordance with a preferred embodiment of the present invention; 
         FIG. 4  illustrates a broker network for a publish-subscribe system in accordance with a preferred embodiment of the present invention; 
         FIG. 5  illustrates how a stateful publish-subscribe service of the present invention appears to clients; 
         FIG. 6  illustrates an example of a operator that transforms input view objects to an output view object in accordance with a preferred embodiment of the present invention; 
         FIG. 7  illustrates an example dataflow hypergraph distributed over multiple brokers in accordance with a preferred embodiment of the present invention; 
         FIG. 8  depicts a process for deploying transform objects and view objects when a dataflow specification is a declarative specification in accordance with a preferred embodiment of the present invention; 
         FIG. 9  is a flowchart illustrating the configuration and deployment of a stateful publish-subscribe system in accordance with a preferred embodiment of the present invention; and 
         FIG. 10  is a flowchart illustrating the operation of a stateful publish-subscribe system at runtime in accordance with a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides a method, apparatus and computer program product for supporting stateful subscriptions in a distributed messaging system. The data processing device may be a stand-alone computing device or may be a distributed data processing system in which multiple computing devices are utilized to perform various aspects of the present invention. Therefore, the following  FIGS. 1-3  are provided as exemplary diagrams of data processing environments in which the present invention may be implemented. It should be appreciated that  FIGS. 1-3  are only exemplary and are not intended to assert or imply any limitation with regard to the environments in which the present invention may be implemented. Many modifications to the depicted environments may be made without departing from the spirit and scope of the present invention. 
     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. 
     In the depicted example, servers  112 ,  114 ,  116  are connected to network  102  along with storage unit  106 . In addition, clients  122 ,  124 , and  126  are connected to network  102 . These clients  122 ,  124 , and  126  may be, for example, personal computers or network computers. In the depicted example, servers  112 ,  114 ,  116  provide data, such as boot files, operating system images, and applications to clients  122 ,  124 ,  126 . Clients  122 ,  124 , and  126  are clients to servers  112 ,  114 ,  116 . Network data processing system  100  may include additional servers, clients, and other devices not shown. 
     In accordance with a preferred embodiment of the present invention, network data processing system  100  provides a distributed messaging system that supports stateful subscriptions. A subset of clients  122 ,  124 ,  126  may be publishing clients, while others of clients  122 ,  124 ,  126  may be subscribing clients, for example. Published events may also be generated by one or more of servers  112 ,  114 ,  116 . 
     A stateful publish-subscribe system is a distributed messaging system in which at least one subscription is stateful. Other subscriptions may be content-based or, in other words, stateless. In other words, a stateful publish-subscribe system must compute information that requires multiple messages of one or more streams. For example, a stateful subscription may request, “Give me the highest quote within each one-minute period.” A stateful subscription may entail delivering information other than simply a copy of the published messages. For example, a stateful subscription may request, “Tell me how many stocks fell during each one-minute period.” 
     The stateful publish-subscribe system is implemented within an overlay network, which is a collection of service machines, referred to as brokers, that accept messages from publisher clients, deliver subscribed information to subscriber clients, and route information between publishers and subscribers. One or more of servers  112 ,  114 ,  116 , for example, may be broker machines. 
     Both content-based and stateful publish-subscribe systems support a message delivery model based on two roles: (1) publishers produce information in the form of streams of structured messages; and, (2) subscribers specify in advance what kinds of information in which they are interested. As messages are later published, relevant information is delivered in a timely fashion to subscribers. 
     Content-based subscriptions are restricted to Boolean filter predicates that can only refer to fields in individual messages. For example, a content-based subscription may request, “Deliver message if traded volume&gt;1000 shares.” On the other hand, stateful subscriptions are more general state-valued expressions and may refer to one or more messages, either by referring to multiple messages of a single message stream or by referring to multiple message streams or both. In a content-based publish-subscribe system, because subscriptions can only specify filtering, all published messages are either passed through to subscribers or filtered out. Therefore, messages received by subscribers are identically structured copies of messages published by publishers. In contrast, in a stateful publish-subscribe system, subscriptions may include more complex expressions and, therefore, subscribers may receive information that is not identical to the published messages with different formatting. For example, a published message may have only integer prices, while subscriptions to average prices may have non-integer averages. 
     Published message streams are associated with topics. Each topic is associated with a base relation. A base relation is a table of tuples, each tuple corresponding to an event in the particular message stream. Subscriptions are expressed as view expressions in a relational algebraic language, although other representations may be used, such as eXtensible Markup Language (XML), for example. The language defines a cascade of views of base relations and derived views computed from either base relations or other views. At compile-time, the set of subscriptions is compiled into a collection of objects that are deployed and integrated into messaging brokers. At run-time, publishers and subscribers connect to these brokers. Published events are delivered to objects associated with base relations. The events are then pushed downstream to other objects that compute how each derived view changes based on the change to the base relation. Those derived views associated with subscriptions then deliver events to the subscriber informing the subscriber of each change in state. 
     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 Transmission Control Protocol/Internet Protocol (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. 
     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. 
     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 clients  108 - 112  in  FIG. 1  may be provided through modem  218  and network adapter  220  connected to PCI local bus  216  through add-in connectors. 
     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 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. 
     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. The data processing system depicted in  FIG. 2  may be, for example, an IBM eServer™ pSeries® system, a product of International Business Machines Corporation in Armonk, N.Y., running the Advanced Interactive Executive (AIX) operating system or LINUX operating system. An object oriented programming system such as Java may run in conjunction with the operating system and provides calls to the operating system from Java programs or applications executing on data processing system  300 . “JAVA” is a trademark of Sun Microsystems, Inc. 
     With reference now to  FIG. 3 , a block diagram of a data processing that may serve as a client of a service in accordance with a preferred embodiment of the present invention. Data processing system  300  is an example of a computer, such as client  108  in  FIG. 1 , in which code or instructions implementing the processes of the present invention may be located. In the depicted example, data processing system  300  employs a hub architecture including a north bridge and memory controller hub (MCH)  308  and a south bridge and input/output (I/O) controller hub (ICH)  310 . Processor  302 , main memory  304 , and graphics processor  318  are connected to MCH  308 . Graphics processor  318  may be connected to the MCH through an accelerated graphics port (AGP), for example. 
     In the depicted example, local area network (LAN) adapter  312 , audio adapter  316 , keyboard and mouse adapter  320 , modem  322 , read only memory (ROM)  324 , hard disk drive (HDD)  326 , CD-ROM driver  330 , universal serial bus (USB) ports and other communications ports  332 , and PCI/PCIe devices  334  may be connected to ICH  310 . PCI/PCIe devices may include, for example, Ethernet adapters, add-in cards, PC cards for notebook computers, etc. PCI uses a cardbus controller, while PCIe does not. ROM  324  may be, for example, a flash binary input/output system (BIOS). Hard disk drive  326  and CD-ROM drive  330  may use, for example, an integrated drive electronics (IDE) or serial advanced technology attachment (SATA) interface. A super I/O (SIO) device  336  may be connected to ICH  310 . 
     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 XP™, which is available from Microsoft Corporation. An object oriented programming system such as Java may run in conjunction with the operating system and provides 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 programming 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 . The processes of the present invention are performed by processor  302  using computer implemented instructions, which may be located in a memory such as, for example, main memory  304 , memory  324 , or in one or more peripheral devices  326  and  330 . 
     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 memory, equivalent non-volatile 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. 
     For example, data processing system  300  may be a personal digital assistant (PDA), which is configured with flash memory to provide non-volatile memory for storing operating system files and/or user-generated data. 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 tablet computer, laptop computer, or telephone device in addition to taking the form of a PDA. 
     In accordance with a preferred embodiment of the present invention, a plurality of broker machines are responsible for delivery of message sent by publishing clients towards subscribing clients based upon the content of the messages and the stateful transformations requested by the subscribing clients. These broker machines form an overlay network. Some broker machines may be specialized for hosting publishing clients, referred to as publisher hosting brokers (PHB), and others for hosting subscribing clients, referred to as subscriber hosting brokers (SHB). Between the PHBs and the SHBs, there may be any number of intermediate nodes that include routing and filtering. The brokers at the intermediate nodes are referred to as intermediate brokers or IBs. For expository purposes, this separation is assumed; however, in actual deployment, some or all of the broker machines may combine the functions of PHB, SHB, and/or IB. 
       FIG. 4  illustrates a broker network for a publish-subscribe system in accordance with a preferred embodiment of the present invention. A publishing client, such as one of publishers  402   a - 402   d , establishes a connection to a PHB, such as PHB  404   a  or PHB  404   b , over a corresponding one of client connections  406   a - 406   d . The client connection may be, for example, any reliable first-in/first-out (FIFO) connection, such as a Transmission Control Protocol/Internet Protocol (TCP/IP) socket connection. Independently, a subscribing client, such as one of subscribers  412   a - 412   d , establishes a connection to a SHB, such as SHB  410   a  or SHB  410   b , over a corresponding one of client connections  414   a - 414   d , which may be similar to client connections  406   a - 406   d . The PHBs and SHBs are connected, via intermediate brokers  408   a - 408   b , through broker-to-broker links. 
     The publish-subscribe system of the present invention may include a fault-tolerant protocol that tolerates link failures and message re-orderings, in which case it is not necessary for the broker-to-broker connections to use reliable FIFO protocols, such as TCP/IP, but may advantageously use faster, less reliable protocols. Each broker machine may be a stand-alone computer, a process within a computer, or, to minimize delay due to failures, a cluster of redundant processes within multiple computers. Similarly, the links may be simple socket connections, or connection bundles that use multiple alternative paths for high availability and load balancing. 
     In the example depicted in  FIG. 4 , one or more execution engine may run on the brokers in the broker network and be interconnected. The one or more execution engines may be interconnected to form a distributed execution engine. The execution engines running on the plurality of broker machines receive input messages, process the input messages using the transform objects, and route output messages toward subscribers. 
       FIG. 5  illustrates how a stateful publish-subscribe service of the present invention appears to clients. Clients are unaware of the physical broker network or its topology. A client application may connect to any broker in the role of publisher and/or subscriber. Publishing clients are aware only of particular named published message streams, such as message streams  502 ,  504 . Multiple clients may publish to the same message stream. 
     Administrators and clients may defined derived views based on functions of either published message streams or of other derived views. In the depicted example, message streams may be represented as relations. Derived views are represented as relations derived from published message streams or from other derived views by means of relational algebraic expressions in a language, such as Date and Darwen&#39;s Tutorial-D, Structured Query Language (SQL), or XQUERY. For example, derived view  510  is defined as a function of stream relations  502  and  504  by means of a JOIN expression with relations  502  and  504  as inputs and relation  510  as an output. Similarly, relation  512 , indicated as a subscriber view, is derived from relation  510  by client-specified relational expressions. For example, subscriber view  512  may be a request to group the stock trades of relation  510  by issue and hour and compute the running total volume and max and min price for each issue-hour pair. 
     Each subscribing client subscribes to a particular derived view. As published events enter the system from publishing clients, they are saved in their respective streams. The system is then responsible for updating each derived view according to the previously specified relational expressions and then delivering client messages to each subscriber representing the changes to the state of the respective subscribed view. 
     In a preferred embodiment of the present invention, subscription specifications are analyzed by a compiler and converted into a collection of transform objects and view objects. Each operator that derives a view from one or more inputs corresponds to a transform object. Each view corresponds to a view object. View objects hold the state of a view. Transform objects express the logic for incrementally updating an output view constituting the result of an operator in response to individual changes to input views constituting the arguments to that operator. 
       FIG. 6  illustrates an example of a operator that transforms input view objects to an output view object in accordance with a preferred embodiment of the present invention. In the depicted example, views  610  and  620  are view objects that are inputs to some operator, such as, for example, a JOIN operator. Transform  650  is a transform object for that operator, which produces a derived view shown as view object  670 . When one of the input objects changes, either because it itself is a published input stream or because it is a derived view that has changed as a result of changes to its inputs, messages reflecting the changes are sent to transform object  650 . Transform  650  receives the messages representing changes to its inputs  610 ,  620 , computes how the result of the operator changes given the announced changes it has received, and then delivers the computed results to its output view object  670  in the form of change messages. Output view object  670  then propagates in its turn such change messages, either to further transforms, if view object  670  is an intermediate view, or to subscribers, if view object  670  is a subscriber view. 
       FIG. 6  illustrates the objects and message pathways for a single transform implementing a single computational operation. When subscriptions are entered, the mechanism of the present invention builds a structure containing all of the transform objects and view objects needed for all intermediate and subscribed views of all subscriptions. This structure is called a dataflow hypergraph. The dataflow hypergraph has nodes corresponding to each view object and hyperedges, which may possibly have more than one input feeding an output, representing each transform object associated with an operation in the subscription specification. 
     The view objects and transform objects are then allocated to actual brokers in the overlay network, either manually by an administrator or automatically via a service, such as the one described in U.S. patent application Ser. No. 10/841,297, entitled “CONTINUOUS FEEDBACK-CONTROLLED DEPLOYMENT OF MESSAGE TRANSFORMS IN A DISTRIBUTED MESSAGING SYSTEM,” and filed on an even date herewith, status abandoned. The published streams and the subscribed views may be constrained to be located on brokers where the publishers and subscribers actually connect. The placement of the intermediate transform objects and view objects is not constrained. That is intermediate transform objects and view objects may be placed wherever suitable, taking into consideration the capacities of the broker machines and the links, as well as the desired performance. After such allocation of objects to brokers, the result is a distributed transform graph. 
       FIG. 7  illustrates an example dataflow hypergraph distributed over multiple brokers in accordance with a preferred embodiment of the present invention. In the depicted example, the physical network consists of brokers  710 ,  720 ,  730 , and  740 . There are three publishing clients  702 ,  704 , and  706 , and one subscribing client  750 . The publishing clients are publishing to three separate published message streams: “buys”  722  on broker  720 , “sells”  734  on broker  730 , and “matches”  712  on broker  710 . The subscribing client  750  subscribes to derived view  748  on broker  740 . 
     Broker  710  also includes transforms  714  and  716 , which feed change messages to brokers  720  and  730 , respectively. Broker  720  includes view objects  724 ,  726  and transform objects  725 ,  727 . As an example, view object  726  represents an intermediate derived view or relation, which is based on transform  725 , published stream  722 , and view  724 . Broker  730  includes views  732  and  736 , in addition to published stream  734 , and also includes transforms  735 ,  737 . Broker  740  includes views  742 ,  744 ,  748 , and transform  746 . View  748  is a subscriber view for subscriber  750 . As stated above, multiple publisher clients may provide messages for a single message stream, and multiple subscriber clients may subscribe and receive updates from the same view. 
     As shown in  FIG. 7 , the transform graph consists of multiple transform and view objects distributed over all brokers. The paths between objects will sometimes lie within a broker, as is the case between transform object  725  and intermediate view object  726 . In other cases, such as the path between transform  727  and intermediate view object  742  (shown with a dotted line), the path must cross over an inter-broker link. It is clear to those versed in the art that the within-broker communications between objects may use cheaper communications means, such as parameter passing between objects, whereas inter-broker communications requires generating physical messages or packets that will cross the link. In a preferred embodiment, the protocols of all view objects and transform objects will be able to recover from lost, out-of-order, or duplicate messages, and, therefore, will work correctly regardless of which paths between objects cross broker boundaries and which do not. 
     In order to support stateful subscriptions, a history of states is stored in a data storage device. For example, messages from the “matches” published stream  712  are stored in storage  782 , messages from the “buys” published stream  722  are stored in storage  784 , and messages from the “sells” published stream  734  are stored in storage  786 . Storage  782 ,  784 ,  786  may be a portion of system memory or may be a persistent storage, such as a hard drive, or a combination thereof. In a system guaranteeing reliable service, published messages will be logged to persistent storage before being propagated. Other states, such as views  742 ,  744 , are preferentially stored in main memory and are not required to be stored persistently. 
       FIG. 8  depicts a process for deploying transform objects and view objects when a dataflow specification is a declarative specification in accordance with a preferred embodiment of the present invention. A declarative specification is a program, such as middleware program source  802 . Program source  802  may be written in a language similar to the SQL syntax, for example. Program source  802  may be compiled using compiler  804  into a dataflow hypergraph. Compiler  804  includes particular algorithms for compiling relational algebraic operators into tailored object code that implements transform objects specialized to incrementally evaluate operators, such as join, select, aggregate, etc., for relations of known data types and key signatures. The object code may be, for example, Java™ programming language code. Compiler  804  represents values of derived states as monotonic knowledge, that is, values in a monotonic domain. A monotonic domain means a set of values that can change value only in one direction. The compiler uses each algebraic expression and the domain of its inputs to determine how to represent the monotonic domain of its result. Because the different values of any field belong to a monotonic domain, the system can always detect which of two values of a single data field is older. As another example, the system distinguishes internally between a “missing value,” which is missing because its value is not yet known (and may later change), and one that is missing because it is known to be absent (and will not change). As yet another example, the system distinguishes between a field having a value that is currently ten, but which may get larger later, from a field whose value is currently ten, but is final and will never change. 
     Compiler  804  generates an object for each relation and an object for each transform. Each algebraic operation with n operands is associated with one transform object with n input interfaces and one output interface to a view object. For example, the join of SELLS and a second relation are compiled into the transform object  735  and the view object  736 . The compiler may then generates a script that is executed at deployment time to instantiate all transform and view objects, connecting an instance of the output interface from each relation to an instance of the input to each transform that uses the relation as an input. Base relations are fed from an “input transform” object, which delivers published messages to the relation. The objects form the knowledge flow graph, or hypergraph, in which published messages enter at base relations and each view object passes the changes to its state to the transform objects that use that relation as an operand. These transform objects then compute the incremental change to the relations to which they are associated and pass that information down to other view objects. Information flows down the graph until it reaches relations associated with subscriptions and then the messages are delivered to “output transforms,” which convert the state change information into messages to subscribing clients. 
     Each relational operator, such as join, project, group-by, select, etc., is associated with a template used by the compiler to produce an incremental transform. As discussed above, a transform has one or more relations feeding inputs to it and a single relation receiving its output. The incremental transform is an object that, given a message saying that one component of its input has changed, computes the appropriate changes to its output and invokes a method in the output relation indicating what has changed. 
     The hypergraph may be placed manually by an administrator, or, alternatively, the hypergraph may be optimized using transform placement service  806  and automatically deployed using deployment service  808 . The hypergraph is optimized and deployed to broker network  824  by passing broker instructions  812  to broker network  824  and receiving performance information  814  from broker network  824 . An optimization step consolidates multiple subscriptions to exploit common computations and performs other simplifications to reduce the total computational load on the system. A placement step allocates the transform objects of the knowledge flow graph to particular brokers for the purpose of load balancing, reduction of bandwidth, and overall reduction of delays between publishers and subscribers. 
     Broker network  824  receives the knowledge flow graph at deployment time. At execution time, publishers  822  publish message streams to broker network  824 . The brokers receive events and propagate messages representing view changes towards the subscribers. Subscribers  828  may then receive messages from subscriber views in broker network  824 . 
       FIG. 9  is a flowchart illustrating the configuration and deployment of a stateful publish-subscribe system in accordance with a preferred embodiment of the present invention. The process begins and receives middleware programming source (block  902 ). The process compiles the middleware programming source into a knowledge flow graph (block  904 ). Then, the process receives performance information from the broker network (block  906 ) and optimizes the knowledge flow graph based upon the broker network performance information (block  908 ). Thereafter, the process deploys the transform objects and the view objects in the knowledge flow graph to brokers (block  910 ) and the process ends. 
       FIG. 10  is a flowchart illustrating the operation of a stateful publish-subscribe system at runtime in accordance with a preferred embodiment of the present invention. The process begins and publishing clients publish messages to publisher hosting brokers (block  1002 ). Transform objects calculate changes in views based on changes in base relations and/or changes in intermediate views (block  1004 ). The transform objects generate view change messages that propagate toward the subscriber clients. Then, subscriber clients receive view change messages from subscriber hosting brokers representing the subscribed views (block  1006 ). The process shown in  FIG. 10  repeats as long as messages continue to be published and the publish-subscribe system is operational. 
     The stateful publish-subscribe system of the present invention is able to be deployed on a wide-area, distributed overlay network of broker machines, communicating by message passing, and being subjected to the possibility of duplicate and out-of-order messages between links. The distributed messaging system of the present invention allows subscriptions expressed as relational algebraic expressions on published message histories. Relational algebra may include, in particular, operators such as select, project, join, extend, group-by, sum, count, and average, for example. The relational algebraic expressions may be mapped to form various query languages, such as SQL and XQUERY. Furthermore, the messaging system of the present invention may allow service specifications that are deterministic and “eventually consistent,” meaning that multiple identical subscriptions eventually receive the same result, but weaker, and hence cheaper to implement, than fully consistent database systems. 
     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 as a floppy disk, a hard disk drive, a RAM, CD-ROMs, DVD-ROMs, and transmission-type media, such as digital and analog communications links, wired or wireless communications links using transmission forms, such as, for example, radio frequency and light wave transmissions. The computer readable media may take the form of coded formats that are decoded for actual use in a particular data processing system. 
     The description of the present invention has been presented for purposes of illustration and description, and 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.