Patent Publication Number: US-6710786-B1

Title: Method and apparatus for incorporating state information into a URL

Description:
This application is a continuation of U.S. patent application Ser. No. 08/962,028, entitled METHOD AND APPARATUS FOR INCORPORATING STATE INFORMATION INTO A URL, which was filed on Oct. 31, 1997, and to U.S. patent application Ser. No. 08/794,269, entitled WEB REQUEST BROKER CONTROLLING MULTIPLE PROCESSES, which was filed on Feb. 3, 1997, the contents of which are hereby incorporated in their entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to processing transactions in networked computer systems, and more specifically to processing multiple-request transactions in a stateless web environment. 
     BACKGROUND OF THE INVENTION 
     The World Wide Web includes a network of servers on the Internet, each of which is associated with one or more HTML (Hypertext Markup Language) pages. The HTML pages associated with a server provide information and hypertext links to other documents on that and (usually) other servers. Servers communicate with clients by using the Hypertext Transfer Protocol (HTTP). The servers listen for requests from clients for their HTML pages, and are therefore often referred to as “listeners”. 
     Users of the World Wide Web use a client program, referred to as a browser, to request, decode and display information from listeners. When the user of a browser selects a link on an HTML page, the browser that is displaying the page sends a request over the Internet to the listener associated with the Universal Resource Locator (URL) specified in the link. In response to the request, the listener transmits the requested information to the browser that issued the request. The browser receives the information, presents the received information to the user, and awaits the next user request. 
     Traditionally, the information stored on listeners is in the form of static HTML pages. Static HTML pages are created and stored at the listener prior to a request from a web browser. In response to a request, a static HTML page is merely read from storage and transmitted to the requesting browser. Currently, there is a trend to develop listeners that respond to browser requests by performing dynamic operations. For example, a listener may respond to a request by issuing a query to a database, dynamically constructing a web page containing the results of the query, and transmitting the dynamically constructed HTML page to the requesting browser. To perform dynamic operations, the functionality of the listener must be enhanced or augmented. Various approaches have been developed for extending listeners to support dynamic operations. 
     One of the major characteristics of the web is that it provides a stateless environment. That is, HTTP communicates information on a message-by-message basis without any mechanism for designating relationships between messages. This means that a process servicing a current request cannot determine whether the current request came from the same client as a previous request. In addition, the servicing process cannot determine how or if the current request relates to a previous request. 
     A disadvantage with using a stateless environment is that it is difficult to process multiple-request transactions. A multiple-request transaction is a set of operations that (1) are specified in more than one request, and (2) must be performed as an atomic unit of work. For example, a multiple-request transaction could consist of three separate operations, such as buying stock item A, selling stock item B and updating the inventory to reflect the number of stock items on hand. Each of these three operations may be specified in a separate request, but each operation should only be performed if all three operations can be performed. In order to properly determine that buying stock item A, selling stock item B and updating the inventory are from the same single transaction requires that transaction state information be retained by the servicing process that receives the three requests. 
     One possible solution to the stateless problem is to spawn a servicing process for each request-issuing source (each “client”). Each time a request from a client is received, the same servicing process is called upon to process the request. Because the same process is invoked for a given client, the transaction state information for a particular transaction can be maintained by the associated servicing process, thus allowing for the processing of multiple-request transactions. 
     This solution has significant drawbacks, however. First, maintaining a separate servicing process for each client is wasteful since most clients do not continually make requests to the servicing process. Between client requests, the servicing process simply waits, consuming system resources, without performing any work. A second drawback with this solution is that it is non-scalable. If a servicing process is spawned and maintained for each client, system resources would quickly be consumed, even for a relatively small number of clients. Therefore, spawning a servicing process for each client is not a viable solution for large scale systems. 
     A second possible solution is to require each servicing process to maintain the current state of the transactions that it is currently processing. By maintaining transaction state information, each servicing process can ensure that multiple-request transactions are processed correctly. However, a drawback associated with requiring each servicing process to maintain transaction state information is that puts a burden on the developer of each servicing process to write extra code in order to maintain the required transaction state information. 
     Based on the foregoing, it is desirable to provide a mechanism for processing multiple-request transactions in a stateless environment that does not require a servicing process to maintain transaction state information. 
     SUMMARY OF THE INVENTION 
     The present invention provides an effective and highly scalable mechanism for supporting multiple-request operations (including but not limited to transactions) in a stateless environment, such as the web. According to the invention, a server is preferably used to coordinate the overall processing of client requests. Preferably, the server performs this coordination function in such a way that: (1) state information associated with multiple-request operations is maintained by the clients making the requests; (2) the clients are unaware that they are maintaining operation-specific state information; and (3) the server itself is not required to persistently maintain the state information, thereby remaining stateless. 
     In a preferred embodiment, processing of a client request is performed as follows. The server receives a request from a client, and if the request is for a multiple-request operation, the server initiates an operation. Once the operation is initiated, the server may either forward the request to another entity (such as an application) for processing, or the server may process the request itself. After the request is processed, the server assembles a set of state information associated with the operation. This state information may include the identity of the client, the ID and status of the operation, what has already transpired in the operation, and any other context information associated with the operation. Once assembled, the state information is incorporated into a URL. This URL, along with the response to the client request, is sent back to the client to be maintained by the client. This state information is preferably not persistently maintained by the server. 
     When the client submits a second request relating to the same operation, the client sends the URL that was previously provided by the server which contains the state information. Upon receiving the second request, the server extracts the state information from the URL, and uses it to resume the previously initiated operation. With the benefit of this state information, the server can resume the operation at the exact point at which the previous request stopped. Once the operation is resumed, the server either processes the request, or forwards it to another entity for processing. After the second request is processed, the server updates the state information associated with the operation, and incorporates the updated state information into another URL. This URL, along with the response to the second request, is sent back to the client to be maintained by the client. The client will send this URL in a future request to resume the operation. This process repeats until the operation is either completed or canceled. 
    
    
     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 upon which an embodiment of the invention may be implemented; 
     FIG. 2 is a block diagram of a distributed application server according to an embodiment of the invention; 
     FIG. 3A is a portion of a flow chart illustrating steps for handling a browser request according to an embodiment of the invention; 
     FIG. 3B is another portion of the flow chart illustrating steps for handling a browser request according to an embodiment of the invention; 
     FIG. 4 is a block diagram of a table containing information maintained by a dispatcher according to an embodiment of the invention; 
     FIG. 5 is a block diagram of a table containing information maintained by a resource manager according to an embodiment of the invention. 
     FIG. 6 is a block diagram of a distributed application server for processing transactions according to an embodiment of the invention; 
     FIG. 7A is a portion of a flow diagram illustrating steps for processing multiple-request transactions in a stateless environment according to an embodiment of the invention; 
     FIG. 7B is another portion of the flow diagram illustrating steps for processing multiple-request transactions in a stateless environment according to an embodiment of the invention; 
     FIG. 7C is another portion of the flow diagram illustrating steps for processing multiple-request transactions in a stateless environment according to an embodiment of the invention; 
     FIG. 7D is another portion of the flow diagram illustrating steps for processing multiple-request transactions in a stateless environment according to an embodiment of the invention; 
     FIG. 7E is another portion of the flow diagram illustrating steps for processing multiple-request transactions in a stateless environment according to an embodiment of the invention; 
     FIG. 7F is another portion of the flow diagram illustrating steps for processing multiple-request transactions in a stateless environment according to an embodiment of the invention; 
     FIG. 7G is another portion of the flow diagram illustrating steps for processing multiple-request transactions in a stateless environment according to an embodiment of the invention; 
     FIG. 7H is another portion of the flow diagram illustrating steps for processing multiple-request transactions in a stateless environment according to an embodiment of the invention; and 
     FIG. 7I is another portion of the flow diagram illustrating steps for processing multiple-request transactions in a stateless environment according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A method and apparatus for processing multiple-request transactions over a network 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  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  to perform specific operations in response to messages from browsers. According to one embodiment of the invention, the operations are performed 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 . 
     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. 
     Functional Overview of Application Server 
     FIG. 2 is a block diagram of a system  200  designed according to an embodiment of the invention. The system  200  includes a plurality of browsers  202 ,  204  and  206  that communicate with a plurality of listeners  210 ,  216  and  222  over the Internet  208  according to the HTTP protocol. In response to requests from the browsers, the listeners cause a web application server  280  to invoke software modules, referred to herein as cartridges. In the illustrated embodiment, web application server  280  has initiated the execution of three cartridges  230 ,  234  and  238 . 
     The web application server  280  is composed of numerous components, including transport adapters  212 ,  218  and  224 , dispatchers  214 ,  220  and  226 , an authentication server  252 , a virtual path manager  250 , a resource manager  254 , a configuration provider  256  and a plurality of cartridge execution engines  228 ,  232  and  236 . The various components of the web application server  280  shall be described hereafter in greater detail. 
     Significantly, the numerous components of web application server  280  communicate through an inter-machine communication mechanism, such as an Object Request Broker  282 . Using an inter-machine communication mechanism, cartridge instances that perform the operations specified in browser requests may execute on different machines than the listeners that receive the requests and the browsers that issue the requests. Because the cartridge instances are on different machines than the listeners, the listeners are better insulated against faulty cartridge instances, thus enhancing the reliability and security of the system. In addition, the scalability of the system is greatly increased by spreading the processing burden of executing the cartridge instances among many machines, rather than the same machine that is executing the listener. The ability to distribute cartridge instance execution across multiple machines allows numerous types of load balancing techniques to be used in deciding when and where to spawn new cartridge instances. 
     A typical operation within system  200  generally includes the following stages: 
     A browser transmits a request over the Internet  208 . 
     A listener receives the request and passes it through a transport adapter to a dispatcher. 
     The dispatcher communicates with virtual path manager  250  to identify a cartridge selected by the browser request and to determine whether the cartridge requires authentication. 
     If the cartridge requires authentication, the dispatcher communicates with the authentication server  252  to determine whether the browser is authorized to access the selected cartridge. 
     If the authentication server  252  determines that the browser is not authorized to access the selected cartridge, the browser is notified that access has been denied. 
     However, if access is authorized or the virtual path manager  250  determines that authentication is not required, the dispatcher does one of two things. If the dispatcher knows about an unused instance for that cartridge, the dispatcher sends the request to that instance. If there are no unused cartridge instances for that cartridge, the dispatcher asks the resource manager  254  to create a new cartridge instance. After the instance starts up successfully, the cartridge notifies the resource manager of its existence. The resource manager  254  then notifies the dispatcher of the new instance. The dispatcher creates a revised request based on the browser request and sends the revised request to the new instance. 
     The cartridge instance handles the revised request and sends a response to the dispatcher. 
     The dispatcher passes the response back through the listener to the client. 
     These stages shall be described in greater detail hereafter. 
     Cartridges 
     Cartridges are modules of code for performing specific application or system functions. A cartridge forms the basic unit of distribution in the system  200 . According to one embodiment of the invention, cartridges are named using Universal Resource Locators (URLs). Thus, a cartridge name (i.e. URL) has two parts: the IP address of the server on which the cartridge resides, and the virtual path in the server directory structure of the compiled cartridge code. Because cartridges are named using URLs, the cartridge name space is global and cartridges may be accessed using the same messaging techniques as are used to access other web resources, such as documents. 
     According to one embodiment of the invention, each cartridge has a standard interface which provides a common overall structure for all cartridges. The standard interface defines the interface of routines that are invoked by the web application server  280  under particular conditions. According to one embodiment of the invention, the abstract cartridge interface is as follows: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                  interface Cartridge 
               
               
                   
                  { 
               
               
                   
                  boolean init( ); 
               
               
                   
                  boolean authenticate(in Principal user_passwd); 
               
               
                   
                  boolean exec(in Request req_obj, out Response resp_obj); 
               
               
                   
                  boolean shutdown( ); 
               
               
                   
                 } 
               
               
                   
                   
               
            
           
         
       
     
     The init( ) routine is responsible for intializing the cartridge instance. This may include invoking the constructors of several subobjects, preforking threads and acquiring all other required shared resources. 
     The shutdown( ) routine is responsible for cleaning up all of the resources and shutting down the cartridge instance. Once the shutdown( ) routine is invoked on a cartridge instance, it immediately becomes unavailable for servicing subsequent requests. 
     The authenticate( ) routine validates whether the client requesting the services of the cartridge is authorized to use those services. 
     The exec( ) routine is the generic way to dispatch all service requests to the cartridge. 
     Exemplary Cartridges 
     Each cartridge is either configured as a cartridge that performs a well-defined function, or as a programmable cartridge that acts as an interpreter or a routine environment for an application. An example of a programmable cartridge is a PL/SQL runtime, configured to process database queries according to the Oracle-based Programming Language using Structured Query Language (PL/SQL). The PL/SQL runtime executes a browser request having a database query. The PL/SQL runtime processes the request, for example, by accessing a database server in communication with the cartridge instance via a data link. 
     Another example of a programmable cartridge is a JAVA runtime interpreter. The JAVA runtime interpreter cartridge enables web application developers to write server-side JAVA applications to process browser requests. Similarly, a custom server may be configured as a cartridge in order to provide dynamic operations such as, for example, accessing processes executed by a third party server. 
     Dispatchers 
     Dispatchers are software modules configured to route the requests received by listeners to the appropriate cartridges. According to one embodiment of the invention, dispatchers are implemented as server-side program extensions (i.e. “plug-ins”). As such, the dispatchers are loaded into and execute within the same address space as the listeners to which they belong. The dispatchers may be linked with the listener code at compile time or dynamically loaded at runtime. 
     In the illustrated embodiment, dispatchers  214 ,  220  and  226  are associated with listeners  210 ,  216  and  222 , respectively. Dispatchers  214 ,  220  and  226  selectively route browser requests received by listeners  210 ,  216  and  222  to cartridges. 
     For example, assume that listener  210  receives a browser request over the Internet  208  delivered in the form of a Uniform Resource Locator (URL). The browser request serves as an identifier for a web object, for example an HTML page or an operation to be performed. The listener  210  hands off the browser request to dispatcher  214  without any attempt at interpreting the browser request. Upon receiving the browser request, the dispatcher  214 : 
     (1) communicates with virtual path manager  250  to identify a cartridge selected by the browser request and to determine whether the cartridge requires authentication, 
     (2) if the cartridge requires authentication, communicates with the authentication server  252  to determine whether the browser is allowed to access the selected cartridge, 
     (3) if access is authorized, communicates with the resource manager to determine the specific instance of the selected cartridge to which the browser request should be sent, and 
     (4) creates and dispatches a revised browser request for execution by the specified instance of the cartridge. 
     The revised browser request repackages information received in the original browser request. The revised browser request may include, for example, a context object that contains data required for the proper operation of the cartridge. The data required for proper operation of a cartridge may include, for example, a transaction ID that identifies a transaction with which the browser request is associated. 
     If the cartridge replies to the request, the cartridge sends the reply to the dispatcher and the dispatcher passes the reply up to the listener for transmission to the browser that initiated the request. 
     Configuration Provider 
     According to one embodiment of the invention, cartridges that are to be used with web application server  280  are first registered with web application server  280 . During the registration process, information about the cartridges is supplied to the configuration provider  256 . Configuration provider  256  stores the information as metadata  258  for later access by the components of the web application server  280 . 
     The metadata  258  may include, for example, 
     (1) the cartridge name; 
     (2) the minimum number of required instances; 
     (3) the maximum number of instances; 
     (4) the location of the code that implements the cartridge; 
     (5) the program-dependent function names used by the cartridge execution engine to execute the callback functions (initialization, request handler, shutdown); 
     (6) a list of machines for running the cartridge; 
     (7) the idle time for the cartridge (the amount of time instances of the cartridge are allowed to remain idle before they are shut down); 
     (8) an object identifier; and 
     (9) data indicating the type of authentication service, if any, to be used with the cartridge. 
     The object identifier specifies the data that must be supplied by a browser request for requesting performance of an operation by the corresponding cartridge. The object type may be a specific word, a URL, or may include a virtual path such as “/java”. 
     Once the configuration provider  256  has stored the configuration information for a particular cartridge in the metadata  258 , that cartridge is automatically registered when web application server  280  is started. 
     After a cartridge is registered with the web application server  280 , the resource manager  254  initiates the minimum instances for the cartridge. Once the minimum number of instances has been initiated, the web application server  280  is prepared to process browser requests. 
     The Virtual Path Manager 
     As mentioned above, dispatchers communicate with the virtual path manager  250  to determine where to route each revised browser request. Specifically, each browser request typically includes a URL. Upon receiving a browser request, the dispatcher sends the URL in the request to the virtual path manager  250 . The virtual path manager  250  responds by sending the dispatcher data that identifies the cartridge, if any, associated with the URL. 
     In order to supply the required information to dispatchers, virtual path manager  250  consults the metadata  258  that maps URLs to cartridges. In response to receiving a browser request, the virtual path manager  250  uses the mapping data to determine the cartridge, if any, to which the URL contained in the browser requests corresponds. 
     For example, if the browser request is a URL request beginning with the virtual path “/java”, the mapping may indicate that the JAVA interpreter cartridge is configured to handle requests having the virtual path “/java”. 
     According to one embodiment of the invention, the virtual path manager  250  also determines whether the cartridge associated with the URL requires authentication. If the cartridge requires authentication, the virtual path manager  250  indicates in the response that the virtual path manager  250  sends to the dispatcher that authentication is required. If authentication is not required, the dispatcher creates and sends a revised browser request to an instance of the cartridge without invoking the authentication server  252 . If authentication is required, the dispatcher sends the revised request to an instance of the cartridge only after the authentication server indicates that the revised request may be submitted to an instance of the cartridge. 
     The Resource Manager 
     The resource manager  254  of the web application server  280  manages the execution of each of the cartridges by initiating a predetermined minimum number of instances for the cartridges, load balancing between the instances of each cartridge, and initiating new instances of cartridges as necessary up to a predetermined maximum number of instances of a given cartridge. 
     For example, assume that the metadata for a particular cartridge (C 1 ) includes the following information: 
     Name=C 1   
     Minimum Instances=10 
     Maximum Instances=50 
     Host Machines=M 1 , M 3 , M 8   
     Idle time=30 seconds 
     Based on this metadata, when cartridge C 1  is first registered, resource manager  254  will initiate ten instances of C 1 . Resource manager  254  will initiate the ten instances on the machines associated with the labels M 1 , M 3  and M 8 . 
     Upon receipt of requests from dispatchers to access C 1 , resource manager  254  determines whether any existing instance of C 1  is available for use. If no instance of C 1  is available when a request is received, resource manager  254  determines whether the maximum number of instances of C 1  are already running. If the maximum number of instances of C 1  are not already running, then resource manager  254  initiates a new instance of C 1  on one of the possible host machines and transmits a message that identifies the new instance to the dispatcher that issued the request. If the maximum number of instances of C 1  are already running, then resource manager  254  sends a message to the dispatcher that issued the request to indicate that the request cannot be handled at this time. 
     Load Balancing 
     According to one embodiment of the invention, resource manager  254  applies a set of load balancing rules to determine where to initiate instances of cartridges where there is more than one possible host machine. Thus, in the above example, M 1 , M 2  and M 3  are all capable of executing instances of cartridge C 1 . If M 1 , M 2  and M 3  have the same processing capacity, it may be desirable to distribute the instances evenly across the three machines. However, if M 1  has ten times the processing power of M 2  and M 3 , it may be desirable to initiate all instances of C 1  on M 1  up to a certain point, and then to distribute additional instances evenly among M 1 , M 2  and M 3 . 
     To assist resource manager  254  in determining how to load balance among possible machines, the metadata stored for each cartridge may include additional details. For example, the metadata may specify a separate minimum and maximum number of instances for each machine. Resource manager  254  may then distribute new instances among the machines based on which machine has the lowest ratio of actual instances to maximum instances. 
     The metadata may also specify an order for the machines that can run a cartridge. The machine at the N+1 position in the order is only used to execute instances of the cartridge when the machine at the Nth position in the order is already executing its maximum number of instances. 
     Cartridge Instance Status Tracking 
     According to one embodiment of the invention, the resource manager  254  maintains state information to keep track of cartridge instances that have been created. The state information includes data that identifies the instance, identifies the machine executing the instance, and identifies the listener to which the instance has been assigned. 
     FIG. 5 illustrates a table  500  that may be maintained by resource manager  254  to store this state information. Table  500  includes an instance column  502 , a cartridge column  504 , a listener column  506  and a machine column  508 . Each row of table  500  corresponds to a distinct cartridge instance. Within the row for a given cartridge instance, cartridge column  504  identifies the cartridge associated with the cartridge instance and instance column  502  indicates the instance number of the cartridge instance. For example, row  510  corresponds to an instance of cartridge C 1 . Therefore, cartridge column  504  of row  510  indicates cartridge C 1 . Instance column  520  of row  510  indicates that the cartridge instance associated with row  510  is instance  1  of cartridge C 1 . 
     Listener column  506  indicates the listener to which the cartridge instance associated with a row has been assigned. Machine column  508  indicates the machine on which the cartridge instance associated with a row is executing. For example, the cartridge instance associated with row  510  has been assigned to listener  210  and is executing on machine M 1 . 
     Similar to resource manager  254 , each dispatcher maintains state information for the cartridge instances that have been assigned to the listener to which the dispatcher is attached. Such state information may be maintained, for example, in a table  400  as shown in FIG.  4 . Similar to table  500 , table  400  includes an instance column  402  and a cartridge column  404  that respectively hold instance numbers and cartridge identifiers. However, while table  500  includes one entry for every cartridge instance assigned by resource manager  254 , table  400  only includes entries for cartridge instances that have been assigned to a particular listener. For example, table  400  includes entries for only those cartridge instances listed in table  500  that have been assigned to listener  210 . 
     In addition to instance column  402  and cartridge column  404 , table  400  includes a status column  406 . For each row, the status column  406  holds a value that indicates the status of the instance associated with the row. For example, the status column  406  of row  408  indicates that instance  1  of cartridge C 1  is currently busy. In the illustrated embodiment, the status column  406  holds a flag that indicates that a cartridge instance is either BUSY or FREE. The significance of the cartridge status shall now be describe with reference to the operation of resource manager  254  and dispatchers  214  and  220 . 
     Interaction Between Dispatchers and the Resource Manager 
     As explained above, dispatchers communicate with resource manager  254  when they need to send a revised browser request to a particular cartridge. According to one embodiment of the invention, dispatchers first determine whether an instance of the appropriate cartridge (1) has already been assigned to it and (2) is available to process the new revised browser request. If an appropriate cartridge instance has already been assigned to the dispatcher and is currently available to process the new revised browser request, then the dispatcher forwards the revised browser request to the cartridge instance without further communication with resource manager  254 . 
     For example, assume that listener  210  receives a browser request that, according to virtual path manager  250 , must be processed by cartridge C 1 . Assume also that table  400  reflects the current list and status of cartridge instances that have been assigned to listener  210 . Upon receiving the browser request from listener  210 , dispatcher  214  inspects table  400  to locate a FREE instance of cartridge C 1 . In the illustrated table  400 , row  410  indicates that instance  3  of cartridge C 1  is currently FREE. Consequently, dispatcher  214  forwards a revised browser request directly to instance  3  of cartridge C 1  without further communication with resource manager  254 . In response to sending the revised browser request, dispatcher  214  changes the status value in status column  406  of row  410  to BUSY. 
     If a listener has not already been assigned an appropriate cartridge instance that is currently available, then the dispatcher associated with the cartridge requests a cartridge instance from the resource manager  254 . If the resource manager  254  determines that an instance of the required cartridge is not available and the number of existing instances of the required cartridge is below the maximum, then the resource manager  254  initiates a new cartridge. Upon initiating a new cartridge, the resource manager  254  inserts an entry for the new cartridge instance in table  500 . 
     Assume, for example, that listener  210  receives a browser request that must be processed by cartridge C 3 . Assume also that instance  3  of cartridge C 3  has not yet been initiated. Under these conditions, dispatcher  214  sends to resource manager  254  a request for a handle to an instance of cartridge C 3 . In response to this request, resource manager  254  initiates instance  3  of cartridge C 3  on machine M 3 . In addition, resource manager  254  inserts into table  500  the entry found at row  512 . 
     After inserting row  512  for instance  3  of cartridge C 3  in table  500 , resource manager  254  sends back to the dispatcher  214  a handle to the newly created instance. In response to receiving this handle, dispatcher  214  inserts an entry (row  412 ) for the new instance in its status table  400 . The dispatcher  214  then transmits a revised browser request to instance  3  of cartridge C 3 . 
     Releasing Cartridge Instances 
     According to one embodiment of the invention, listeners do not automatically release ownership of cartridge instances when the cartridge instances finish responding to outstanding browser requests. For example, assume that instance  3  of cartridge C 3  receives a revised browser request, processes the revised browser request, and sends a response back to dispatcher  214 . Dispatcher  214  passes the response to listener  210  to be sent back to the browser that issued the browser request. 
     At this point, listener  210  no longer requires ownership of instance  3  of cartridge C 3 . However, rather than transferring ownership of instance  3  of cartridge C 3  back to resource manager  254 , dispatcher  214  merely changes the status column  406  of row  412  from BUSY to FREE. 
     Changing the value in status column  406  of row  412  to FREE indicates that instance  3  of cartridge C 3  is no longer working on a request, and is therefore ready to handle subsequent requests. However, because table  400 , which indicates that instance  3  of cartridge C 3  is available, is maintained locally by dispatcher  214 , instance  3  of cartridge C 3  is only available for subsequent browser requests arriving at listener  210 . Row  512  of table  500  maintained by resource manager  254  continues to indicate that instance  3  of cartridge C 3  is owned by listener  210 . 
     Because listeners do not automatically release cartridge instances every time a request is serviced, overhead associated with communication between the resource manager  254  and the various dispatchers is significantly reduced. For example, assume that a listener  210  receives ten successive requests that must be communicated to cartridge C 3 . Rather than communicating with resource manager  254  for each of the ten requests, dispatcher  214  may communicate with resource manager  254  in response to the first request. The subsequent nine requests can be handled by dispatcher  214  without communicating with resource manager  254  because the dispatcher  214  uses the same instance of C 3  that processes the first request to process the nine subsequent requests. 
     While not automatically releasing listener ownership of cartridge instances when each request is serviced can increase the efficiency of web application server  280 , listeners cannot maintain ownership of cartridge instances indefinitely. For example, instances that have not been used for long periods of time should be passed back to the resource manager  254  so they can be de-allocated to free up resources. In addition, it is not efficient for one listener to maintain ownership of the instance of a cartridge that it has not used for a relatively long time when other listeners require instances of that cartridge. 
     Consequently, resource manager  254  communicates to each listener a maximum idle time for each cartridge instance passed to the listener. The maximum idle time indicates the maximum amount of time a cartridge instance can go unused before the listener must release ownership of the cartridge instance. For example, assume that the resource manager  254  indicates to listener  210  that the maximum amount of idle time for instance  3  of cartridge C 3  is 10 minutes. Based on this information, listener  210  may continue to use instance  3  of cartridge C 3  to process browser requests for cartridge C 3  as long as instance  3  of cartridge C 3  does not remain idle or FREE for more than 10 minutes. 
     If instance  3  of cartridge C 3  is idle for more than 10 minutes, dispatcher  214  removes row  412  from table  400  and sends a message to resource manager  254  that listener  210  is releasing ownership of instance  3  of cartridge C 3 . In response to this message, resource manager  254  updates row  512  to indicate that instance  3  of cartridge C 3  is not owned by any listener and may thus be reassigned to another listener or terminated. 
     In an alternative embodiment, dispatchers do not automatically release cartridge instances when the idle time for the cartridge instance has expired. Instead, the dispatcher sends a message to resource manager  254  offering to release the expired instance. Resource manager  254  may respond to the offer by requesting that the listener release the cartridge instance, or by allowing the listener to retain ownership of the expired cartridge instance. 
     According to one embodiment of the invention, resource manager  254  maintains a queue of the requests that cannot be immediately serviced. When it becomes possible to service a queued request, the request is removed from the queue and processed. 
     For example, assume that listener  222  receives a browser request that must be processed by cartridge C 1 , and that listener  222  has not been assigned any instances of cartridge C 1 . Dispatcher  226  sends a request for an instance of C 1  to resource manager  254 . Assume further that a maximum of 50 instances of C 1  are allowed, and that 50 instances of C 1  have been assigned to listener  210 . Under these conditions, resource manager  254  cannot service the request from listener  222 . Therefore, resource manager  254  puts the request on a queue. When listener  210  releases an instance of C 1 , resource manager  254  communicates to listener  222  that an instance of C 1  is available. 
     Under certain conditions, resource manager  254  may preemptively cause a listener to release a cartridge instance. For example, resource manager  254  may detect a system overload situation and respond by terminating a set of cartridge instances, either before or after informing the listeners that currently have been assigned the cartridge instances that the cartridge instances are going to be terminated. 
     Resource manager  254  may also preemptively cause listeners to release cartridge instances to implement fairness policies between listeners. For example, resource manager  254  may cause a listener that holds the most instances of a given cartridge to release an instance of the cartridge when another listener has waited more than a predetermined threshold of amount of time for an instance of the cartridge. For example, if listener  210  has been assigned 50 instances of cartridge C 1  and C 1  has a maximum of 50 instances, then resource manager  254  may cause listener  210  to release an instance of C 1  ten seconds after receiving a request for an instance of C 1  from another listener. 
     Cartridge Execution Engines 
     According to one embodiment of the invention, each cartridge instance is composed of a cartridge execution engine and a cartridge. A cartridge execution engine is a code module that insulates cartridges from the complexities of the web application server  280  and the inter-module communication mechanism. A cartridge is made available to a cartridge execution engine by storing in a function table pointers to the cartridge functions. According to one embodiment, all cartridges provide the functions specified in the exemplary cartridge interface described above. By having all cartridges support the same interface, a single standard cartridge execution engine can be used with all cartridges. 
     According to one embodiment of the invention, cartridges are implemented as shared libraries, and cartridge execution engines are executable programs that invoke the routines in the shared libraries using the standard cartridge interface. The cartridge execution engine provides the interface between cartridges and the dispatcher, directs cartridge flow of control, and provides services for cartridges to use. 
     When the resource manager  254  requires the creation of a new cartridge instance, the resource manager  254  causes a cartridge execution engine to be instantiated. In turn, the instance of the cartridge execution engine thus created causes the appropriate cartridge to be instantiated. The resource manager  254  can cause the cartridge execution engine to be instantiated, for example, by invoking a “cartridge execution engine factory” that resides on the machine on which the cartridge is to be executed. The instance of the cartridge execution engine can cause the cartridge to be instantiated, for example, by making a call to one of the routines in the shared library that constitutes the cartridge. 
     As shown in FIG. 2, the web application server  280  includes cartridge execution engines  228 ,  232  and  236  for each of the cartridges  230 ,  234  and  238 . The cartridge execution engines control execution of the instances of the corresponding cartridges by making calls into the cartridges through the standard cartridge interface. By establishing basic callback functions between the cartridge execution engine and a cartridge, any cartridge can be integrated into the web application server  280  by configuring the cartridge to respond to the callback functions, and then registering the cartridge in the configuration provider  256 , as described below. 
     Thus, if the dispatcher  214  determines that the PL/SQL runtime cartridge is the appropriate cartridge to process a request, the dispatcher  214  dispatches the request to a cartridge instance that includes a cartridge execution engine associated with the PL/SQL runtime cartridge. If a new instance needs to be initiated, the resource manager  254  creates a new instance of the PL/SQL runtime cartridge in a separate address space and dispatches the request to the cartridge execution engine  228  of the new instance. The address space used to execute the instance of the program may be within memory of the computer system upon which one or more of the components of web application server  280  is executing, or on another computer system. 
     In response to a message from a dispatcher, the cartridge execution engine issues a request handler callback function to the cartridge, causing the cartridge to process the request. The cartridge executing the request returns the result to the cartridge execution engine, which forwards the result to the dispatcher. In the event that the web application server  280  detects a fault in the operation, the cartridge execution engine issues a shutdown function of the cartridge. 
     Hence, the cartridge execution engine provides an application programming interface to the web application server  280  that specifies predetermined operations to be performed. Use of the standard cartridge interface enables programmers of the cartridges to configure each cartridge for high-level integration into the web application server  280  independent of the protocols used by the particular web listener with which the cartridge will be used. 
     Transport Adapters 
     Listeners enable the use of server-side plug-ins by providing a programming interface and protocol for use by such plug-ins. Unfortunately, the programming interfaces and protocols provided by listeners vary from listener to listener. For example, Netscape Server Application Programming Interface (NSAPI), Internet Server Application Programming Interface (ISAPI) and Application Development Interface (ADI) are three examples of distinct programming interfaces currently provided by listeners. 
     Transport adapters insulate dispatchers from the proprietary protocols and interfaces used by web listeners. Specifically, each transport adapter is configured to recognize the protocols of different listeners, and to convert the browser requests received from the listeners into converted browser requests having a standard dispatcher protocol that is independent from the protocol of the listener. Similarly, transport adapters convert the replies from the dispatcher to the transport protocol of the listeners. 
     Hence, the transport adapter enables the web application server  280  to be used with listeners from different vendors. Moreover, transport adapters may be configured to accommodate different server architectures and operating systems. 
     Operation of the Web Application Server 
     FIGS. 3A and 3B are a flow diagram illustrating a method of responding to a browser request according to an embodiment of the present invention. The browser request is received in step  350  by a listener. For the purposes of explanation, it shall be assumed that the browser request was issued by browser  202  and received by listener  210 . 
     Upon receiving the browser request, the listener  210  forwards the request to the web application server  280  in step  352 . Specifically, listener  210  passes the request to the transport adapter  212  using the proprietary programming interface of the listener  210 . The transport adapter  212  converts the request as necessary to pass the request to dispatcher  214  using a standard dispatcher programming interface. 
     Dispatcher  214  identifies the request object type that corresponds to the browser request in step  354  based on the virtual path specified in the browser request by communicating with the virtual path manager  250 . If the request object type corresponds to a cartridge, the virtual path manager also indicates to the dispatcher  214  whether authentication is required. 
     The dispatcher  214  determines in step  356  if the request object type corresponds to an identifiable cartridge. If the request object type does not correspond to an identifiable cartridge, the request is returned to the listener  210  in step  358  (see FIG.  3 B). If in step  358  the listener  210  recognizes the request as a request for a static HTML page, the listener accesses the static HTML page, and sends the HTML page to the browser  202  in step  360 . If the browser request is not recognized by the listener  210 , the reply is sent to the browser  202  in step  360  indicating that the request was unrecognizable. 
     If in step  356  the dispatcher  214  determines that the request must be sent to a cartridge, then the dispatcher performs any necessary authentication by communicating with the authentication server  252 . The authentication process will be described in greater detail hereafter. In addition, if in step  356  it is determined that listener  210  has not been assigned any instances of that cartridge that are currently FREE, then the dispatcher  214  communicates with the resource manager  254  to be assigned an instance of the cartridge  230  to which the browser request can be sent. 
     In step  362 , shown in FIG. 3B, the resource manager  254  determines whether an instance of the identified cartridge is available (unowned) among the existing number of instances. For the purposes of explanation, it shall be assumed that the request is associated with cartridge  230 , and that cartridge  230  is a PL/SQL runtime cartridge. 
     If in step  362  the resource manager identifies an available instance, for example instance  260  of the PL/SQL runtime  230 , the resource manager  254  informs the dispatcher  214  that the request should be sent to instance  260 . The dispatcher  214  then creates and sends a revised browser request to the cartridge execution engine  228  of the instance  260  in step  368  to cause the available instance  260  to process the request, as described below. 
     However, if in step  362  no instance of the cartridge  230  is available, the resource manager  254  determines in step  364  if the existing number of instances exceeds a maximum prescribed number. If the existing number of instances exceeds the maximum prescribed number in step  364 , the resource manager  254  indicates to the dispatcher  214  that the request cannot be processed at this time. In response, the dispatcher  214  returns the request to the listener  210  in step  358 , after which the web listener  210  sends a reply to the browser  202  over the network in step  360  indicating the request was not processed. 
     Alternatively, when a cartridge instance is not currently available to handle a request, listener  210  may place the request on a waiting list for that cartridge instance. When a cartridge instance becomes available, the revised browser request is removed from the waiting list and forwarded to the cartridge instance. If the revised browser request remains on the waiting list for more than a predetermined amount of time, listener  210  may remove the request from the waiting list and send a message to the browser  202  to indicate that the request could not be processed. 
     If in step  364  the existing number of instances does not exceed the maximum prescribed number, the resource manager  254  initiates a new instance of the identified program and informs the dispatcher  214  that a revised browser request based on the browser request should be sent to the new instance. The dispatcher  214  then dispatches a revised browser request to the cartridge execution engine of the new instance. 
     For example, assume that the resource manager  254  initiated instance  260  in response to the browser request. During the initialization, the stored sequences of instructions for the PL/SQL runtime are accessed to create a new instance  260  of the cartridge  230  in an address space that is separate from the address space in which dispatcher  214  is executing. According to one embodiment, initialization is performed by loading the cartridge execution engine  228  and having the cartridge execution engine call the initialization routine in cartridge  230 . 
     Once the new instance  260  is running, the dispatcher  214  dispatches the request to the cartridge execution engine  228  associated with the new instance  260  in step  368 . The cartridge execution engine  228  sends a callback message to the new instance  260  requesting execution of the request. In the callback message, the cartridge execution engine  228  passes any parameters necessary for the instance  260  to process the request. Such parameters may include, for example, passwords, database search keys, or any other argument for a dynamic operation executed by the instance  260 . 
     The instance  260  then executes the request. During the execution of the request by the instance in step  368 , the dispatcher  214  monitors the instance to determine the occurrence of a fault in step  370 . If in step  370  the dispatcher  214  detects a fault, the dispatcher  214  calls the corresponding cartridge execution engine  228  in step  372  to abort the instance  260  having the fault. The corresponding cartridge execution engine  228  in turn issues a shut down command across the API to the faulty instance. The instance, responding to the shut down command by the cartridge execution engine  228 , will shut down without affecting any other process in any other address space. 
     If in step  370  no fault is detected, the dispatcher  214  receives a reply from the instance  260  upon completion of execution in step  374 . The dispatcher  214  in step  376  forwards the reply to the listener  210 , which responds to the browser with the reply from the executed instance  260 . After executing the instance  260 , the dispatcher  214  in step  378  maintains the instance in the memory, as shown in step  378  to enable execution of a subsequent request. 
     Distributed Architecture of Web Server 
     Significantly, the various components of the web application server  280  communicate with each other using a communication mechanism that does not require the components to be executing in the same address space or even on the same machine. In the illustrated embodiment, the components of the web application server  280  are configured to communicate through an Object Request Broker (ORB)  282 . Object Request Brokers are described in detail in “Common Object Request Broker: Architecture and Specification (CORBA)”. This and other documents relating to CORBA are well known and readily available from a variety of sources. 
     While the embodiments of the present invention shall be described with reference to communications through a CORBA-compliant ORB, other cross-platform communication mechanisms may be used. For example, the components of web application server  280  may alternatively communicate with each other using Remote Procedure Calls (RPC), a UNIX pipe, Microsoft COM. 
     Because the various components of the web application server  280  communicate with each other using a machine independent communication mechanism, there are no inherent restrictions with respect to where the components are located with respect to each other. For example, listeners  210 ,  216  and  222  may be executing on the same machine, or on three completely different machines, each with a different operating system. Similarly, the authentication server  252 , virtual path manager  250 , resource manager  254  and configuration provider  256  may be executing on the same machine or on four different machines. Further, those four different machines may not have any overlap with the three machines executing listeners  210 ,  216  and  222 . 
     Cartridge execution engines  228 ,  232  and  236  incorporate all of the necessary logic to communicate with the other components of the web application server  280  through the object request broker  282 . Consequently, the location of the cartridge instances themselves is not inherently restricted by the communication mechanism. Thus, instance  260  may be executing in a completely different machine and operating system than dispatchers from which it receives requests. Likewise, instance  260  may be on a different machine and operating system than the resource manager  254  or any of the other components of the web application server  280 , including instances of other cartridges that are being managed by the same web application server  280 . 
     Significantly, the location-independence enjoyed by cartridges used by web application server  280  is achieved through the cartridge execution engine communication logic, not through any custom programming in the cartridges themselves. Consequently, the cartridges do not need to be specially designed for execution in a distributed application server environment. Cartridge designers are thus insulated from the complexities of a distributed system, and can concentrate their efforts on the logic associated with the tasks for which the cartridges were created. 
     Processing Transactions 
     According to an embodiment of the invention, transactions are implemented in a stateless environment through the use of metadata that indicates specific information for specific types of transactions. A piece of information about a transaction that is supplied in the metadata is referred to herein as an attribute of the transaction. The use of metadata to indicate specific attributes of a transaction allows for a system in which cartridges are not required to persistently maintain state information. Transactions in such a system are declarative rather than programmatic in that the messages themselves indicate the transactions to which they belong. For example, the metadata for two particular types of transactions, TX 1  and TX 2 , could be as follows: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 [TX1] 
               
            
           
           
               
               
            
               
                   
                 [STOREFRONT] 
               
               
                   
                 name = STOREACCOUNTS 
               
               
                   
                 belong-to-list = /STOREFRONT 
               
            
           
           
               
               
            
               
                   
                 /BANKING 
               
            
           
           
               
               
            
               
                   
                 resource-list = /SEARS 
               
            
           
           
               
               
            
               
                   
                 /BANK1 
               
            
           
           
               
               
            
               
                   
                 begin = /storefront/open session 
               
               
                   
                 commit = /storefront/commit session 
               
               
                   
                 rollback = /storefront/rollback session 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 [TX2] 
               
            
           
           
               
               
            
               
                   
                 [EMPLOYEE] 
               
               
                   
                 name = EMPLOYEEACCOUNTS 
               
               
                   
                 belong-to-list = /EMPLOYEE 
               
            
           
           
               
               
            
               
                   
                 /BANKING 
               
            
           
           
               
               
            
               
                   
                 resource-list = /PERSONNEL 
               
            
           
           
               
               
            
               
                   
                 /BANK1 
               
            
           
           
               
               
            
               
                   
                 begin = /employee/open session 
               
               
                   
                 commit = /employee/commit session 
               
               
                   
                 rollback = /employee/rollback session 
               
               
                   
                   
               
            
           
         
       
     
     For each type of transaction the metadata includes various attributes. According to one embodiment, the attributes include a cartridge name, a transaction name, a belong-to-list, a resource-list, begin, commit and rollback TRANSACTION URLs. In the example given above, the cartridge name for TX 1 , is STOREFRONT, the transaction name is STOREACCOUNTS, the belong-to-list consists of /STOREFRONT and /BANKING, the resource-list consists of /SEARS and /BANK 1 , the begin transaction URL is /storefront/open session, the commit transaction URL is /storefront/commit session and the rollback transaction URL is /storefront/rollback session. 
     The cartridge name attribute identifies the particular type of cartridge that the dispatcher communicates with to perform the operations of the transaction. The transaction name attribute uniquely identifies the type of transaction relative to other transaction types. The belong-to-list of a transaction type lists the cartridges that may participate in the performance of the transaction. The resource-list is the list of resources that are affected by the performance of transactions that are of the transaction type. The begin transaction URL is the URL that signals that a transaction of this type is about to begin. The commit transaction URL is the URL that signals that a transaction of this type that is currently in progress should be committed. The rollback transaction URL is the URL that signals that a transaction of this type that has already started should be rolled back. How each of these attribute values is used during the performance of a transaction shall be described in greater detail below. 
     Transaction Overview 
     FIG. 6 is a block diagram of a system  600  that provides for the processing of multiple-request transactions in a stateless environment according to one embodiment of the invention. FIG. 6 is similar to FIG.  2  and therefore like components have been numbered alike. Within this document, the term browser request and the term transaction request are used interchangeably. The term multiple-request transaction is used to refer to a single transaction that is comprised of two or more browser requests. 
     As described earlier, cartridge execution engine  228  communicates with a plurality of dispatchers (e.g. one or more of dispatchers  214 ,  220  and  226 ) through object request broker  282  to receive browser messages. These browser messages may be sent from a plurality of browsers connected to the Internet. In addition to the plurality of dispatchers, cartridge execution engine  228  also communicates with a cartridge  230 , configuration provider  256  and transaction manager  606 . As previously described above, the cartridge  230  represents a module of code that is either configured as a cartridge that performs a well-defined function, or as a programmable cartridge that acts as an interpreter or a routine environment for an application. The combination of cartridge execution engine  228 , transaction manager  606  and cartridge  230  constitute a cartridge instance. 
     A particular cartridge may be associated with a plurality of database servers for access to a plurality of databases. In this example, cartridge  230  has the ability to process database transactions according to the Structured Query Language (SQL) by accessing database  610  and database  614  through database server  608  and database server  612  respectively. 
     Transaction manager  606  represents a coordinating module that is associated with cartridge execution engine  228  and functions to coordinate the execution of multiple-request transactions in the stateless web environment. In coordinating the execution of multiple-request transactions, transaction manager  606  retains no state information for the multiple-request transactions. The transaction manager  606  communicates with cartridge execution engine  228  to receive transaction control messages. Using the information contained in the transaction control messages, the transaction manager  606  interacts with database servers  608  and  612  to cause changes made during multiple-request transactions to respective databases  610  and  614  to be either committed or rolled back as an atomic unit of work. 
     Identifying Transactions 
     Browser requests that are associated with multiple-request transactions include a globally unique transaction ID. The globally unique transaction ID within a browser request is used to identify the multiple-request transaction to which the browser request belongs. According to one embodiment, when a browser request is received that contains a begin transaction URL, the transaction manager creates a globally unique transaction ID. This globally unique transaction ID is returned to the sending browser, and is sent by the browser in subsequent browser requests that are associated with the same multiple-request transaction. 
     In certain embodiments, when returned to the browser from which a multiple-request transaction was initiated, the globally unique transaction ID associated with the particular multiple-request transaction is stored as cookie information on the client executing the browser. When a subsequent browser request is sent by the browser, the dispatcher determines if the subsequent request contains a begin transaction URL. If the request does not contain a begin transaction URL, then the dispatcher obtains the globally unique transaction ID associated with the browser request by reading the sending browser&#39;s cookie information using the HTTP protocol standards. 
     For example, when a browser  202  sends a first browser request associated with a transaction and a begin transaction URL, the transaction manager  606  creates a unique browser identifier and sends it to the dispatcher  214 . The dispatcher  214  then causes the globally unique transaction ID to be stored as cookie information on browser  202 . When browser  202  sends a second browser request that is associated with the same transaction, the dispatcher  214  obtains the globally unique transaction ID contained in the cookie information of browser  202 . 
     Using the globally unique transaction ID, the database servers that ultimately process the browser request can determine that both the first browser request and the second browser request are associated with the same multiple-request transaction. Because a particular browser may be executing more than one transaction at a time, in certain embodiments, the cartridge name for the particular transaction is contained within each globally unique transaction ID and is used to help identify the particular transaction to which the globally unique transaction ID corresponds. 
     In certain situations cookie information may not be available on a particular browser. For example, a particular browser may not support the use of cookies or a particular user may choose to deny access to the browser cookie information. Therefore, in certain embodiments, the transaction identifiers are embedded in the messages returned to a browser, and sent out by the browser in subsequent browser requests. This can be accomplished by annotating the URLs that are associated hyperlinks of the HTML page that is returned to the browser  202 . Based upon the globally unique transaction ID that is sent out as part of the browser request URL, the database servers that ultimately perform the operations specified in the browser requests can use the globally unique transaction ID to identify the multiple-request transaction to which each particular browser request belongs. 
     Transaction Cartridge Instantiation 
     Each browser request contains URL information that is sent from the sending browser in response to a user of the browser selecting a hypertext link on an HTML page. The URL information includes a Uniform Resource Indicator (URI) portion and a header section. The URI portion includes transaction state information and a cartridge name. The transaction state information is used to identify the particular state of a multiple-request transaction. The cartridge name is used to identify the cartridge type and allows the cartridge execution engine to identify the metadata that is associated with the browser request. 
     The header section is used to store a globally unique transaction ID that is used by the database servers to identify the multiple-request transaction that is associated with a particular transaction request. 
     When a listener receives the browser request, it passes the browser request to the dispatcher. The dispatcher then communicates with the virtual path manager to determine the cartridge type that is associated with the browser request. In one embodiment, the dispatcher forwards the information contained in the URI to the virtual path manager. Using the information in the URI, the virtual path manager communicates with the configuration provider to determine the cartridge type that is associated with the browser message. 
     Once the cartridge type is identified, the virtual path manager returns data that identifies the cartridge type to the dispatcher. The dispatcher then searches a cartridge instance pointer list that includes pointers to cartridge instances that have previously been associated with the particular dispatcher. If the dispatcher locates a pointer to a cartridge instance that is of the cartridge type that is associated with the browser request, the dispatcher uses the pointer to send a revised browser message to the cartridge instance. 
     If the dispatcher does not locate a pointer to the type of cartridge instance that is associated with the browser request, the dispatcher communicates with the resource manager to obtain a cartridge instance of that type. In obtaining the cartridge instance, the dispatcher sends a message to the resource manager that includes the cartridge type that was previously identified by the virtual path manager. 
     Upon receiving the dispatcher message, the resource manager determines if a cartridge instance of the request type is available for use by searching a cartridge instance pointer table. If a cartridge instance pointer of the requested type is located in the cartridge instance pointer table, the resource manager sends a pointer to the available cartridge instance back to the dispatcher. 
     However, if a cartridge instance of the requested type is not available, the resource manager causes a cartridge instance of the request type to be instantiated. In one embodiment of the invention, the resource manager causes a cartridge instance of the requested type to be instantiated by requesting a particular cartridge factory process to create a cartridge instance of the request type. Cartridge factory processes may be located across multiple machines. When a particular cartridge factory process is requested to instantiate a cartridge instance, it instantiates the cartridge instance on the same machine that the cartridge factory is currently executing on. Therefore, the resource manager selects which cartridge factory to use based on the particular machine the resource manager chooses to instantiate the cartridge instance. 
     Upon receiving a request to instantiate a cartridge instance, the cartridge factory process instantiates an instance of a cartridge execution engine. Once the cartridge execution engine is instantiated, the cartridge execution engine obtains the transaction information, if any, that is associated with the requested cartridge type. For example, if the requested cartridge type is of type STOREFRONT as described in TX 1  above, the cartridge execution engine obtains and stores the metadata information that is associated with TX 1 . This metadata information is used by the cartridge instance to process transactions. 
     After obtaining the metadata information, the cartridge execution engine instantiates a cartridge of the requested cartridge type. The instance of the cartridge that is created is dynamically linked with the cartridge execution engine. The cartridge execution engine then instantiates a transaction manager. The transaction manager instance is dynamically linked with the cartridge and the cartridge execution engine to form a cartridge instance. 
     Once the cartridge instance is formed, the transaction manager uses the metadata information that was previously stored by the cartridge execution engine to open connections with the databases that were identified in the resource-list of the metadata. These connections are retained by the transaction manager and later used to provide database handles to the associated cartridge and to control the processing of multiple-request transactions. For example, if the requested cartridge type is of type STOREFRONT, the resource-list is associated with a SEARS and BANK 1  database. Using the resource-list information, the transaction manager opens a connection with the SEARS database and the BANK 1  database by respectively establishing connections with the database servers associated with the SEARS and BANK 1  databases. These connections are retained by the transaction manager and are used for processing transactions of type TX 1 . 
     After the transaction manager establishes its connections with the appropriate databases (i.e. through the database servers associated with the appropriate databases), the cartridge execution engine notifies the cartridge factory that a cartridge instance has been instantiated by returning a pointer to the cartridge instance back to the cartridge factory. Upon receiving the cartridge instance pointer, the cartridge factory sends the cartridge instance pointer to the resource manager. 
     The resource manager then registers the cartridge instance pointer into its cartridge instance pointer table. The resource manager then sends the cartridge instance pointer to the dispatcher. Upon receiving the cartridge instance pointer from the resource manager, the dispatcher stores the cartridge instance pointer into its associated cartridge instance pointer list. The dispatcher then uses the cartridge instance pointer to send a revised browser message to the cartridge instance. 
     Creating Revised Browser Messages 
     Upon obtaining a cartridge instance pointer, the dispatcher creates a revised browser message using the information associated with the browser request. This revised browser message includes the URI, header information, the cartridge type and a dispatcher pointer that allows messages to be returned to the dispatcher. For example, a revised message for a transaction of type TX 1  as described above, may include the following information: 
     URI=/storefront/open_session 
     header=NULL 
     cartridge name=[STOREFRONT] 
     dispatcher pointer=address XXXXX 
     In this example, the URI is a begin transaction URI (a URI that is used by the cartridge execution engine to identify the beginning of a multiple-request transaction). Because the URI a begin transaction URI, a globally unique transaction ID has not yet been associated with the multiple-request transaction. Hence, the header that would contain the transaction ID is set to NULL. For ongoing multiple-request transactions (i.e. when the browser request does not contain a URI of /storefront/open_session)and in which cookies are used to store the globally unique transaction ID, the header will contain the unique transaction ID. This unique transaction ID allows the database servers to associate a transaction request with an ongoing multiple-request transaction. 
     The cartridge name identifies the cartridge type and is used by the cartridge execution engine to identify the metadata that contains information about the transaction type associated with the particular browser request. In this example, the cartridge name of STOREFRONT identifies the metadata associated with TX 1  as being associated with the browser request. 
     After creating the revised browser message, the dispatcher uses the previously obtained cartridge instance pointer to send the revised browser message to the cartridge instance. When the cartridge instance receives the revised browser message, the cartridge instance uses the cartridge type information to identify the metadata that is associated with the browser request. After identifying the metadata, the cartridge execution engine uses the URI information to determine the state of the transaction associated with the browser request. 
     For example, it shall be assumed that the browser request included a URI of “/storefront/open_session” and a cartridge type of STOREFRONT. 
     By looking at the metadata associated with the cartridge type of STOREFRONT (i.e. the metadata described in TX 1  above), the cartridge execution engine  230  determines that the URI of/storefront/open_session corresponds to a “begin” transaction state. Using this same mechanism, the cartridge execution engine  230  can determine that a browser request containing a URI of /storefront/commit_session corresponds to a “commit” transaction state and that a browser request containing a URI of /storefront/rollback_session corresponds to a “rollback” transaction state. 
     In the case where the URI does not include a particular state (i.e. a URI consisting only of /storefront), the cartridge execution engine  230  assumes that the browser request is associated with an ongoing multiple-request transaction that is not ready to be either committed or rolled backed. 
     When the cartridge execution engine receives a revised browser message that is not associated with a “begin” transaction, the cartridge execution engine checks the header to determine if it specifies a globally unique transaction ID. If the header specifies a globally unique transaction ID, then cookie information was used to store the globally unique transaction ID. If the header does not specify a globally unique transaction ID, the cartridge execution engine then searches the URI to identify the globally unique transaction ID that is associated with the browser request. Once the cartridge execution engine locates the globally unique transaction ID, the cartridge execution engine includes the transaction ID in the transaction control messages that are sent to the transaction manager. The transaction manager then uses the globally unique transaction ID in communicating with the associated database servers to cause multiple-request transactions to be either committed or rolled back as an atomic unit of work. 
     Processing Transactions 
     FIG. 7A through 7I are a flow diagram illustrating a method for processing multiple-request transactions in a stateless environment according to an embodiment of the invention. 
     At step  702 , a revised browser message that was directed to cartridge  230  is intercepted by cartridge execution engine  228 . For the purposes of explanation, it shall be assumed that the revised browser message was sent by dispatcher  214  and that the revised browser message is associated with transaction TX 1  as described above. 
     At step  704 , cartridge execution engine  228  determines if the revised browser message is associated with a transaction. If the revised browser message is not associated with a transaction, at step  706 , cartridge execution engine  228  forwards the revised browser message to cartridge  230  for cartridge  230  to perform the requested non-transactional functions associated with the revised browser message. Once the cartridge performs the requested non-transactional functions, control returns to step  702  in order for the cartridge execution engine  228  to intercept the next revised browser message. 
     Otherwise, if the revised browser message is associated with a transaction, at step  708 , cartridge execution engine  228  determines the state of the transaction by first determining whether the revised browser message is associated with a begin transaction URI. In determining whether the revised browser message is associated with a begin transaction URI, the cartridge execution engine  228  uses the cartridge name to identify the previously stored metadata that includes the transaction attributes of the transaction type identified in the revised browser message. Using the previously stored metadata, the cartridge execution engine  228  determines if the revised browser message is associated with a begin transaction URI. 
     For example, it shall be assumed that the revised browser message contained a cartridge name of STOREFRONT and a URI of /storefront/open_session. Using the STOREFRONT cartridge name, the cartridge execution engine  228  determines that the revised browser message is associated with the metadata for transaction TX 1 . Using this metadata, the cartridge execution engine  228  determines that the URI of /storefront/open_session is associated with a begin transaction. 
     If the cartridge execution engine  228  determines that the revised browser message is not associated with a begin transaction, then control proceeds to step  744 . 
     If the cartridge execution engine  228  determines that the revised browser message is associated with a begin transaction, then at step  712 , the cartridge execution engine  228  includes a begin transaction identifier (tx_begin) in a transaction control message. The cartridge execution engine  228  then sends the transaction control message to the transaction manager  606 . 
     At step  714 , upon receiving the begin transaction identifier, the transaction manager  606  creates a globally unique transaction ID that is used to identify subsequent browser requests that are associated with this multiple request transaction. In certain embodiments of the invention, the transaction ID is formed using the browser IP address, the transaction name and a particular timestamp value. 
     At step  716 , the cartridge execution engine  228  sends an operation message to cartridge  230  that is formed from information that is contained in the revised browser message. The operation message also includes a dispatcher pointer that identifies the dispatcher that sent the revised browser request (dispatcher  214 ). This pointer allows the cartridge  230  to write information back to the dispatcher. At step  718 , upon receiving the operation message, the cartridge  230  sends a message to the transaction manager  606  requesting handles for access to the databases that are associated with the transaction. 
     At step  720 , transaction manager  606  returns handles to the appropriate database servers to allow the cartridge  230  to process the transaction request. For example, assuming database  610  is associated with the SEARS database and database  614  is associated with the BANK 1  database, transaction manager  606  will return handles to database server  608  and  612  respectively. 
     At step  722 , cartridge  230  uses the handles returned from transaction manager  606  to execute the operations identified in the operation message that was sent by the cartridge execution engine  228 . 
     At step  724 , the cartridge  230  determines whether the sending browser allows cookie information to be associated with the browser. If the browser does not allow for cookie information to be associated with the browser, at step  726 , the cartridge  230  causes the hyperlinks of the HTML page that was generated in response to executing this transaction request to be annotated to include the globally unique transaction ID. By annotating the hyperlinks of the HTML page, the URIs contained in subsequent browser request will contain the globally unique transaction ID. 
     At step  728 , the cartridge  230  uses the dispatcher pointer to return back to the dispatcher  214  the HTML page that was generated in response to executing the transaction request. The cartridge  230  then notifies cartridge execution engine  228  that execution of the transaction request is complete. 
     At step  730 , the cartridge execution engine  228  sends a message to the transaction manager  606  requesting it to suspend the transaction. At step  732 , the transaction manager  606  sends a suspend request to database servers  608  and  612  to cause them to suspend execution of the transaction. The suspend request includes the globally unique transaction ID so that the database servers  608  and  612  know which transaction to suspend. By sending a suspend request to database servers  608  and  612 , it allows other browsers to execute transactions that are associated with databases  610  and  614 . 
     At step  734 , transaction manager  606  sends the globally unique transaction ID to the cartridge execution engine  228 . At step  736 , the cartridge execution engine  228  determines whether the sending browser allows for cookie information to be associated with the browser. If the browser does not allow for cookie information to be associated with the browser, at step  738 , the dispatcher  214  is notified that the processing of the revised browser request is complete. Control then returns to step  702  to intercept another revised browser message. 
     If the browser does allow for cookie information to be associated with the browser, at step  740 , the cartridge execution engine  228  uses the globally unique transaction ID to create cookie information to be associated with the sending browser. 
     At step  742 , cartridge execution engine  228  forwards the cookie information to dispatcher  214  so that it may be transmitted to the sending browser and notifies the dispatcher  214  that the processing of the revised browser request is complete. Control then returns to step  702  to intercept another revised browser message. 
     At step  744 , the cartridge execution engine  228  determines whether the revised browser message is associated with a commit transaction URI. In determining whether the revised browser message is associated with a commit transaction URI, the cartridge execution engine  228  uses the cartridge name to identify the previously stored metadata for the type of transaction associated with the revised browser message. Using the previously stored metadata, the cartridge execution engine  228  determines if the revised browser message is associated with a commit transaction URI. 
     For example, it shall be assumed that the revised browser message contained a cartridge name of STOREFRONT and a URI of /storefront/commit_session. Using the STOREFRONT cartridge name, the cartridge execution engine  228  determines that the revised browser message is associated with the metadata for transaction TX 1 . Using this metadata, the cartridge execution engine  228  determines that the URI of /storefront/commit_session is associated with a commit transaction. 
     If the cartridge execution engine  228  determines that the revised browser message is not associated with a commit transaction, then control proceeds to step  774 . 
     If the cartridge execution engine  228  determines that the revised browser message is associated with a commit transaction, then at step  746 , the cartridge execution engine  228  determines whether the header section of the revised browser message contains cookie information. If cartridge execution engine  228  determines that the header section of the revised browser message contains cookie information, then at step  748  the cartridge execution engine  228  extracts the globally unique transaction ID from the cookie information. Control then proceeds to  752 . 
     If cartridge execution engine  228  determines that the header section of the revised browser message does not contain cookie information, then at step  750  the cartridge execution engine  228  extracts the globally unique transaction ID from the annotated URI. 
     At step  752 , the cartridge execution engine  228  packages a resume transaction identifier (tx_resume) into a transaction control message. The cartridge execution engine  228  then sends the transaction control message to the transaction manager  606 . 
     At step  754 , upon receiving the resume transaction identifier, the transaction manager  606  sends a resume request to database servers  608  and  612  to cause them to resume execution of the transaction. The resume request includes the globally unique transaction ID which allows the database servers  608  and  612  to identify the multiple-request transaction that is associated with the current transaction request. 
     At step  756 , the cartridge execution engine  228  sends an operation message to cartridge  230  that is based on the transaction information contained in the revised browser message. The operation message also contains a dispatcher pointer that identifies the dispatcher that sent the revised browser request (dispatcher  214 ) and allows the cartridge  230  to write information back to the dispatcher. At step  758 , upon receiving the operation message, the cartridge  230  sends a message to the transaction manager  606  requesting handles for access to the databases that are associated with the transaction. 
     At step  760 , transaction manager  606  returns handles to the appropriate database servers to allow the cartridge  230  to process the transaction request. For example, assuming database  610  is associated with the SEARS database and database  614  is associated with the BANK 1  database, transaction manager  606  will return handles to database server  608  and  612  respectively. 
     At step  762 , cartridge  230  uses the handles returned from transaction manager  606  to execute the operation specified by the operation message that was sent by the cartridge execution engine  228 . 
     At step  764 , the cartridge  230  determines whether the sending browser allows cookie information to be associated with the browser. If the browser does not allow for cookie information to be associated with the browser, at step  766 , the cartridge  230  causes the globally unique transaction ID to be removed from the annotated hyperlinks of any HTML page that is associated with the transaction. By removing the transaction ID annotations from the hyperlinks of the HTML page, subsequent browser requests that are issued in response to selection of a hyperlink from the HTML page will not contain the globally unique transaction ID and, therefore, will not be mistakenly associated with this multiple-request transaction. 
     At step  768 , the cartridge  230  uses the dispatcher pointer to return the HTML page generated in response to performing the operation specified in the browser request to the dispatcher  214  and notifies cartridge execution engine  228  that execution of the transaction request is complete. 
     At step  770 , the cartridge execution engine  228  sends a transaction control message to the transaction manager  606  requesting it to commit the transaction. At step  771 , the transaction manager  606  sends a commit request to database servers  608  and  612  to cause all changes made in response to the various browser requests that belonged to the multiple-request transaction to be committed as an atomic unit of work. The commit request includes the globally unique transaction ID which allows the database servers  608  and  612  to identify the associated multiple-request transaction. 
     At step  772 , the cartridge execution engine  228  notifies the dispatcher  214  that the processing of the revised browser request is complete and signals the dispatcher  214  to cause the cookie information associated with the committed multiple-request transaction to be removed from the sending browser. By removing the transaction ID from the cookie information associated with the sending browser, subsequent browser requests will not contain the globally unique transaction ID and, therefore, will not be mistakenly associated with the committed multiple-request transaction. Control then returns to step  702  to intercept another revised browser message. 
     At step  774 , the cartridge execution engine  228  determines whether the revised browser message is associated with a rollback transaction URI. In determining whether the revised browser message is associated with a rollback transaction URI, the cartridge execution engine  228  uses the cartridge name to identify the previously stored metadata that corresponds to the transaction type indicated in the revised browser message. Using the previously stored metadata, the cartridge execution engine  228  determines if the revised browser message contains a rollback transaction URI. 
     For example, it shall be assumed that the revised browser message contained a cartridge name of STOREFRONT and a URI of /storefront/ rollback_session. Using the STOREFRONT cartridge name, the cartridge execution engine  228  determines that the revised browser message is associated with the metadata for transaction TX 1 . Using this metadata, the cartridge execution engine  228  determines that the URI of /storefront/ rollback_session is associated with a rollback transaction. 
     If the cartridge execution engine  228  determines that the revised browser message is not associated with a rollback transaction, then control proceeds to step  804 . 
     If the cartridge execution engine  228  determines that the revised browser message is associated with a rollback transaction, then at step  776 , the cartridge execution engine  228  determines whether the header section of the revised browser message contains cookie information. If cartridge execution engine  228  determines that the header section of the revised browser message contains cookie information, then at step  778  the cartridge execution engine  228  extracts the globally unique transaction ID from the cookie information. Control then proceeds to  782 . 
     If cartridge execution engine  228  determines that the header section of the revised browser message does not contain cookie information, then at step  780  the cartridge execution engine  228  extracts the globally unique transaction ID from the annotated URI. 
     At step  782 , the cartridge execution engine  228  incorporates a resume transaction identifier (tx_resume) in a transaction control message. The cartridge execution engine  228  then sends the transaction control message to the transaction manager  606 . 
     At step  784 , upon receiving the resume transaction identifier, the transaction manager  606  sends a resume request to database servers  608  and  612  to cause them to resume execution of the transaction. The resume request includes the globally unique transaction ID which allows the database servers  608  and  612  to identify the multiple-request transaction that is associated with the current transaction request. 
     At step  786 , the cartridge execution engine  228  sends an operation message to cartridge  230  that is based on the transaction information contained in the revised browser message. The operation message also contains a dispatcher pointer that identifies the dispatcher that sent the revised browser request (dispatcher  214 ) and allows the cartridge  230  to write information back to the dispatcher. At step  788 , upon receiving the operation message, the cartridge  230  sends a message to the transaction manager  606  requesting handles for access to the databases that are used in the specified type of transaction. 
     At step  790 , transaction manager  606  returns handles to the appropriate database servers to allow the cartridge  230  to process the transaction request. For example, assuming database  610  is associated with the SEARS database and database  614  is associated with the BANK 1  database, transaction manager  606  will return handles to database server  608  and  612  respectively. 
     At step  792 , cartridge  230  uses the handles returned from transaction manager  606  to execute the transaction information associated with the operation message that was sent by the cartridge execution engine  228 . 
     At step  794 , the cartridge  230  determines whether the sending browser allows cookie information to be associated with the browser. If the browser does not allow for cookie information to be associated with the browser, at step  796 , the cartridge  230  causes the globally unique transaction ID to be removed from the annotated hyperlinks of any HTML page to be returned to the browser. By removing the transaction ID annotations from the hyperlinks of the HTML page, subsequent browser requests will not contain the globally unique transaction ID and, therefore, will not be mistakenly associated with this multiple-request transaction. 
     At step  798 , the cartridge  230  uses the dispatcher pointer to return the HTML page that is associated with executing the transaction back to the dispatcher  214  and notifies cartridge execution engine  228  that execution of the transaction request is complete. 
     At step  800 , the cartridge execution engine  228  sends a transaction control message to the transaction manager  606  requesting it to rollback the transaction. At step  801 , the transaction manager  606  sends a rollback request to database servers  608  and  612  to cause all changes made in response to the browser requests that belong to the multiple-request transaction to be rolled back as an atomic unit of work. The roll back request includes the globally unique transaction ID which allows the database servers  608  and  612  to identify and roll back the correct multiple-request transaction. 
     At step  802 , the cartridge execution engine  228  notifies the dispatcher  214  that the processing of the revised browser request is complete and signals the dispatcher  214  to cause the cookie information associated with the rolled back multiple-request transaction to be removed from the sending browser. By removing the transaction ID from the cookie information associated with the sending browser, subsequent browser requests will not contain the globally unique transaction ID and, therefore, will not be mistakenly associated with the rolled back multiple-request transaction. Control then returns to step  702  to intercept another revised browser message. 
     At step  804 , the cartridge execution engine  228  determines whether the header section of the revised browser message contains cookie information. If cartridge execution engine  228  determines that the header section of the revised browser message contains cookie information, then at step  806  the cartridge execution engine  228  extracts the globally unique transaction ID from the cookie information. Control then proceeds to  810 . 
     If cartridge execution engine  228  determines that the header section of the revised browser message does not contain cookie information, then at step  808  the cartridge execution engine  228  extracts the globally unique transaction ID from the annotated URI. 
     At step  810 , the cartridge execution engine  228  packages a resume transaction identifier (tx_resume) in a transaction control message. The cartridge execution engine  228  then sends the transaction control message to the transaction manager  606 . 
     At step  812 , upon receiving the resume transaction identifier, the transaction manager  606  sends a resume request to database servers  608  and  612  to cause them to resume execution of the transaction. The resume request includes the globally unique transaction ID which allows the database servers  608  and  612  to identify the multiple-request transaction that is associated with the current transaction request. 
     At step  814 , the cartridge execution engine  228  sends an operation message to cartridge  230  that is based on the transaction information contained in the revised browser message. The operation message also contains a dispatcher pointer that identifies the dispatcher that sent the revised browser request (dispatcher  214 ) and allows the cartridge  230  to write information back to the dispatcher. At step  816 , upon receiving the operation message, the cartridge  230  sends a message to the transaction manager  606  requesting handles for access to the databases that are associated with the transaction. 
     At step  818 , transaction manager  606  returns handles to the appropriate database servers to allow the cartridge  230  to process the transaction request. For example, assuming database  610  is associated with the SEARS database and database  614  is associated with the BANK 1  database, transaction manager  606  will return handles to database servers  608  and  612  respectively. 
     At step  820 , cartridge  230  uses the handles returned from transaction manager  606  to execute the operation specified in the operation message that was sent by the cartridge execution engine  228 . 
     At step  822 , the cartridge  230  determines whether the sending browser allows cookie information to be associated with the browser. If the browser does not allow for cookie information to be associated with the browser, at step  824 , the cartridge  230  causes the hyperlinks of an HTML page generated in response to performing the operation to be annotated to include the globally unique transaction ID. By annotating the hyperlinks of the HTML page, the URIs in subsequent browser requests that are issued in response to selecting the links in the HTML page will contain the globally unique transaction ID. 
     At step  826 , the cartridge  230  uses the dispatcher pointer to return the HTML page thus generated back to the dispatcher  214  and notifies cartridge execution engine  228  that execution of the transaction request is complete. 
     At step  828 , the cartridge execution engine  228  sends a message to the transaction manager  606  requesting it to suspend the transaction. At step  830 , the transaction manager  606  sends a suspend request to database servers  608  and  612  to cause them to suspend execution of the transaction. The suspend request includes the globally unique transaction ID which allows the database servers  608  and  612  to accurately identify the multiple-request transaction to be suspended. Control then returns to step  702  to intercept another revised browser message. 
     Transaction Timeouts 
     According to one embodiment of the invention, a timeout value is associated with each transaction. The timeout value is used by to identify multiple-request transactions that have not been active for a specified time period. In one embodiment, each database server maintains a timeout value for the multiple-request transactions that are being serviced by the database server. Thus, whenever a multiple-request transaction begins to execute, the associated database server initializes the timeout value for the particular transaction. Upon receiving a resume transaction request that is associated with a globally unique transaction ID, the database server resets the timeout value for the multiple-request transaction that is associated with the globally unique transaction ID. If a multiple-request transaction times out, the database server causes all changes made as part of the multiple-request transaction to be rolled back as an atomic unit of work. Once the multiple-request transaction is rolled back, a message is then sent to the associated browser to indicate the state of the transaction. 
     Conducting Transactions in a Stateless Web Environment 
     The present invention provides a practical and highly scalable mechanism for conducting multiple-request transactions in a stateless environment, such as the web. According to the invention, a transaction manager is used to coordinate the overall transaction process. Preferably, the transaction manager coordinates the process in such a way that state information is maintained for a transaction without requiring the transaction manager itself to persistently maintain the state information. 
     In a preferred embodiment, processing of a client request is performed as follows. The transaction manager receives a request from a client, and if the request is a transaction request, the manager initiates a transaction with a transaction processing mechanism, such as a database management system (DBMS). Once the transaction is initiated, the manager preferably forwards the request to another entity, such as an application, which actually processes the request. After the request is processed, control is returned to the manager, and at that point, the manager assembles a set of state information associated with the transaction. This state information may include the identity of the client, the ID and status of the transaction, and what has already transpired in the transaction. Once assembled, the state information, along with the response to the client request, is sent back to the client to be maintained by the client. The state information may be sent to the client in the form of a “cookie” or it may be incorporated into a URL that is returned to the client. While it is possible to do so, state information is preferably not persistently maintained by the manager or by the application that processed the request. 
     When the client submits a second request relating to the same transaction, the client sends along the state information previously provided by the manager. Upon receiving the second request, the manager extracts the state information from the request, and uses it to resume the previously initiated transaction with the DBMS. Once the transaction is resumed, the manager sends the second request, including the state information, to another entity (the same or a different application) for processing. After the second request is processed, the manager updates the state information associated with the transaction, and sends the updated state information, along with the response to the second request, to the client. The client will send this updated state information in a future request to resume the transaction. This process repeats until the transaction is either committed or rolled back. 
     The present invention provides several significant advantages. First, note that the transaction manager and the applications that process the requests remain stateless. That is, the transaction manager and the applications are not required to maintain any of the state information for the transaction. All of that information is maintained by the client. This means that no overhead is incurred for storing the information. More importantly, the fact that the client maintains its own state information means that any request from the client can be processed by any thread, process, or node. This significantly improves scalability because it eliminates the need to have a dedicated process or thread for each client. 
     Another point to note is that even though the client is maintaining the state information, the client is not aware that it is maintaining transaction-specific state information. As discussed above, the state information is provided to the client by the transaction manager. The client simply sends this information back to the transaction manager when it makes its next request. The client is not, nor does it need to be, aware that it is maintaining state information. This is a very advantageous aspect of the present invention because it obviates the need to put any state management logic on the client. This in turn means that no changes or additions need to be made to the client for the present invention to operate properly. 
     Hence, the present invention provides a practical, scalable, and effective mechanism for conducting transactions in a stateless environment. These and other advantages of the invention will become apparent as the invention is described in further detail. 
     Incorporation of State Information in URLs 
     The present invention provides an effective and highly scalable mechanism for supporting multiple-request operations (including but not limited to transactions) in a stateless environment, such as the web. According to the invention, a server is preferably used to coordinate the overall processing of client requests. Preferably, the server performs this coordination function in such a way that: (1) state information associated with multiple-request operations is maintained by the clients making the requests; (2) the clients are unaware that they are maintaining operation-specific state information; and (3) the server itself is not required to persistently maintain the state information, thereby remaining stateless. 
     In a preferred embodiment, processing of a client request is performed as follows. The server receives a request from a client, and if the request is for a multiple-request operation, the server initiates an operation. Once the operation is initiated, the server may either forward the request to another entity (such as an application) for processing, or the server may process the request itself. After the request is processed, the server assembles a set of state information associated with the operation. This state information may include the identity of the client, the ID and status of the operation, what has already transpired in the operation, and any other context information associated with the operation. Once assembled, the state information is incorporated into a URL. This URL, along with the response to the client request, is sent back to the client to be maintained by the client. This state information is preferably not persistently maintained by the server. 
     When the client submits a second request relating to the same operation, the client sends the URL that was previously provided by the server which contains the state information. Upon receiving the second request, the server extracts the state information from the URL, and uses it to resume the previously initiated operation. With the benefit of this state information, the server can resume the operation at the exact point at which the previous request stopped. Once the operation is resumed, the server either processes the request, or forwards it to another entity for processing. After the second request is processed, the server updates the state information associated with the operation, and incorporates the updated state information into another URL. This URL, along with the response to the second request, is sent back to the client to be maintained by the client. The client will send this URL in a future request to resume the operation. This process repeats until the operation is either completed or canceled. 
     The present invention provides several significant advantages. First, note that the server remains stateless. That is, the server is not required to maintain any of the state information for the transaction. All of that information is maintained by the client. This means that no overhead is incurred for storing the information. More importantly, the fact that the client maintains its own state information means that any request from the client can be processed by any thread, process, or node. This significantly improves scalability because it eliminates the need to have a dedicated process or thread for each client. 
     Another point to note is that even though the client is maintaining the state information, the client is not aware that it is maintaining operation-specific state information. As discussed above, the state information is provided by the server to the client in the form of a URL. The client simply sends this URL whenever it requests service from the server. The client treats this URL like any other URL. The client is not, nor does it need to be, aware that this URL contains state information. This is a very advantageous aspect of the present invention because it obviates the need to put any state management logic on the client. This in turn means that no changes or additions need to be made to the client for the present invention to operate properly. 
     Hence, the present invention provides a practical, scalable, and effective mechanism for supporting multiple-request operations in a stateless environment. These and other advantages of the invention will become apparent as the invention is described in further detail. 
     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.