Patent Publication Number: US-8127306-B2

Title: Integrating eventing in a web service application of a multi-functional peripheral

Description:
FIELD OF THE INVENTION 
     The present invention relates to multi-functional peripherals (MFPs), and more particularly to integrating an eventing specification in a Web service application. 
     RELATED CASES 
     This application is related to U.S. patent application Ser. No. 11/641,510, entitled IMPLEMENTING A WEB SERVICE APPLICATION ON A MULTI-FUNCTIONAL PERIPHERAL WITH MULTIPLE THREADS, filed on Dec. 18, 2006, the contents of which are herein incorporated by reference in their entirety for all purposes. 
     This application is related to U.S. patent application Ser. No. 11/641,355, entitled PROCESSING FAST AND SLOW SOAP REQUESTS DIFFERENTLY IN A WEB SERVICE APPLICATION OF A MULTI-FUNCTIONAL PERIPHERAL, filed on Dec. 18, 2006, the contents of which are herein incorporated by reference in their entirety for all purposes. 
     BACKGROUND 
     The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section. 
     The term “Web services” describes a standardized way of integrating Web-based applications using the XML, SOAP, and WSDL standards over a networking protocol. XML is used to tag the data, SOAP specifies how to encode a Web service request and response into an XML message, and WSDL is used for describing the services available. Web services are used for programmatic and networked entities to communicate with each other, regardless of the platform for their implementation. Because many such entities are business-related, Web services allow businesses to communicate data without intimate knowledge of each other&#39;s IT systems behind a firewall. 
     Web services share business logic, data, and processes through a programmatic interface across a network. Web services allow different applications from different sources to communicate with each other without time-consuming custom coding. And, because all communication is in XML, Web services are not tied to any one operating system or programming language. For example, Java can talk with Python and Windows applications can talk with UNIX applications. 
     Web Services specifications compose together to provide interoperable protocols for security, reliable messaging, and transactions in loosely coupled systems. Web Services specifications include both approved standards (e.g. by the World Wide Web Consortium (W3C) and the Organization for the Advancement of Structured Information Standards (OASIS)) and proposed documents and drafts that may become standards. 
     One approach for providing multiple services that take advantage of Web Services specifications is through a multi-functional peripheral (MFP). Such services may include printing, copying, faxing, scanning, and archiving. Each service may be provided by a Web service application (WSA) that executes on the MFP. Each WSA may receive hundreds of SOAP requests for services in a short amount of time. These SOAP requests vary in type, size, time to process, and require various components of an MFP. Thus, it would be helpful to provide an MFP with one or more WSAs that each process multiple client requests as efficiently as possible. 
     SUMMARY 
     Techniques for efficiently processing SOAP requests at an MFP that provides multiple services are provided. 
     According to an embodiment of the invention, a WSA comprises an event manager that implements WS-Eventing—an important Web Services specification that defines how clients may subscribe to, and be notified of, events generated by an MFP. Instead of all WSAs of an MFP sharing a single implementation of WS-Eventing, one or more WSAs have their own event manager. For example, a WSA receives an event subscription request from a client application and forwards the request to the WSA&#39;s event manager. The event manager may maintain a subscription identifier, a subscription expiration time, and information about the event sink that receives the notification. The event manager receives notification data from a platform of the MFP that indicates that the event occurred. The event manager then compares the notification data with the identification information and composes a notification message for the client application. The event manager then sends the notification message to the event sink. Some WSAs require special processing of data in an event subscription request before an event subscription response is sent to the client application. By implementing WS-Eventing within those WSAs, such special processing may be performed with much less complexity than would otherwise be required if an implementation of WS-Eventing was shared among all WSAs of an MFP. 
    
    
     
       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 that illustrates an example architecture  100  for processing SOAP requests, according to an embodiment of the invention; 
         FIG. 2  is a sequence diagram illustrating various threads that may execute within a Web service application of an MFP, according to an embodiment of the invention; 
         FIG. 3  illustrates a flow diagram that illustrates how multiple threads, within a web service application, may handle one or more requests, according to an embodiment of the invention; 
         FIGS. 4A-B  are flow diagrams that illustrate how fast and slow requests are processed differently, according to an embodiment of the invention; 
         FIG. 5  is a block diagram that illustrates the relationships of eventing components of an MFP, according to an embodiment of the invention; 
         FIG. 6  is a sequence diagram that illustrates how an event subscription request is processed by a WSA and its associated event manager, according to an embodiment of the invention; 
         FIG. 7  is a flow diagram that illustrates a multiple threaded Web service application for processing event subscription requests, according to an embodiment of the invention; and 
         FIG. 8  is a block diagram that illustrates a computer system upon which an embodiment of the invention may be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     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, 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. 
     The description herein is provided in sections organized as follows: 
     1.0 Architectural Overview
         1.1 Client   1.2 Network   1.3 Device Facility Manager   1.4 WSD Manager
           1.4.1 General API   1.4.2 General API Implementation   
           1.5 Web Service Application
           1.5.1 Abstract API   1.5.2 Abstract API Implementation   
               

     2.0 Multithreaded Implementation of a Web Service Application
         2.1 External Request Processing Thread   2.2 Internal Communications Thread   2.3 Request Processing Thread   2.4 Flow Diagram of Multiple Threads Processing a SOAP Request   2.5 Example of Multiple Threads Processing a SOAP Request   2.6 Benefits of a Multithreaded Implementation of a Web Service Application   2.7 Number of Threads       

     3.0 Processing Fast and Slow Requests Differently
         3.1 Flow Diagrams of Processing Fast and Slow Requests
           3.1.1 Fast Request Processing Thread   3.1.2 Slow Request Processing Thread   
           3.2 Benefits of Processing Fast and Slow Requests Differently       

     4.0 Integrating WS-Eventing Into a Web Service Application
         4.1 Eventing Sequence Diagram
           4.1.1 Pre-Processing   4.1.2 Post-Processing   
           4.2 Multithreaded Eventing Flow Diagram
           4.2.1 Fast Request Processing Thread   4.2.2 Event Processing Thread   4.3 Benefits of Integrating WS-Eventing Into a Web Service Application   
               

     5.0 Implementation Mechanisms 
     1.0 Architectural Overview 
       FIG. 1  is a block diagram that illustrates an example architecture  100  for processing SOAP requests, according to an embodiment of the invention. Architecture  100  includes a client  102 , an administrator  104 , an MPF  101 , a Device Facility Manager (DFM)  106 , and a plurality of Web service applications (WSAs)  108  executing on the MFP  101 . 
     The MFP  101 , as indicated by  FIG. 1 , may comprise multiple platforms (e.g. a legacy platform  130 , a Linux-based platform  140 , and a VxWorks-based platform  150 ), upon each of which one or more of the WSAs  108  may execute. The platforms depicted in  FIG. 1  are merely provided as examples, as the approach is applicable to any type of platform. 
     DFM  106  represents the MFP by responding to discovery requests, metadata requests from client  102 , and configuration and other MFP administration requests from an administrator  104 . DFM  106  may act as a repository of implementations of multiple Web Service specifications, such as WS-Discovery  112  and WS-MeX (i.e. WS-MetadataExchange)  116 . 
     Each WSA  108  executing on the MFP provides a service to client  102  that requests the service, e.g. using the SOAP protocol. Each WSA  108  may employ a service specific abstract API, such as abstract API  124 , independent from the target platform. Each WSA  108  may also employ WS-Eventing  122 . 
     Client  102  may discover that an MFP exists via a discovery request or a discovery HELLO message (i.e., a broadcast or multicast message announcing the MFP to devices on the same network). Once client  102  is aware of the existence of an MFP, client  102  sends a device metadata exchange request, e.g. via WS-MetadataExchange, to discover all the services that the MFP provides. DFM  106 , acting for the entire device, receives the request and returns metadata describing the various services provided by the MFP. Client  102  may request service metadata from a particular service application executing on the MFP, such as Web service application (WSA)  108 . WSA  108  may request the service metadata from a Web Service Device (WSD) Manager  110 , which returns the service metadata to WSA  108 . WSA  108  forwards the service metadata to client  102 . 
     Alternatively, the device metadata of the MFP and the service metadata of one or more WSAs may be sent to client  102  in the same response. 
     Based on the service metadata, client  102  generates and transmits a SOAP request corresponding to a service provided by WSA  108  which WSA  108  receives and processes. Based on a service request, WSA  108  may use an abstract API  124  to make a platform-specific call to an implementation of abstract API  124 , such as an abstract API implementation  144 . In this way, a developer of a Web service application (e.g. WSA  108 ) may focus on the development of the Web service itself and without having to know the complexities of the underlying platform upon which the Web service executes. Therefore, someone other than the Web service application developer with knowledge of the target platform may define the implementation of the corresponding abstract API. 
     1.1 Client 
     Client  102  is an application that is associated with a process that requests one or more services provided by an MFP. Client  102  is typically an application associated with the operating system that supports the initial requesting process. A purpose of client  102  is to convert a platform-specific procedure call, from a requesting process, to a SOAP request that can be processed by an application that “understands” SOAP. 
     For example, the requesting process may be associated with a Microsoft Word application and WSA  108  may provide a print service. Client  102  may then be an application associated with the operating system that supports the initial requesting process. Client  102  receives a platform-specific “print data” request sent from the requesting process. Client  102  encodes the print data request in a SOAP message that can be processed by WSA  108  that “understands” SOAP messages. 
     1.2 Network 
     SOAP communication between client  102  and an MFP may be made over a network (not shown). The network may be implemented by any medium or mechanism that provides for the exchange of data between various nodes in the network. Examples of such a network include, without limitation, a network such as a Local Area Network (LAN), Wide Area Network (WAN), Ethernet or the Internet, or one or more terrestrial, satellite, or wireless links. The network may include a combination of networks such as those described. The network may transmit data according to Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and/or Internet Protocol (IP). 
     1.3 Device Facility Manager 
     DFM  106  represents an MFP by accepting discovery requests, requests for logging information, and configuration instructions. According to an embodiment, DFM  106  also acts as a repository of implementations of multiple Web Service specifications. Thus, DFM  106  includes a shared library of routines that each implement one or more functions defined by one or more Web Services specifications (e.g. WS-Security, WS-MetadataExchange). In this way, multiple Web Service specifications may be implemented once and then shared with each of the multiple Web service applications (i.e. WSA  108 ) executing on the MFP. As a result, developers of Web service applications are not required to know many details about any of the specifications implemented on DFM  106  but can use and rely on the specifications. Some Web Service specifications implemented on DFM  106  may include, but are not limited to, WS-Discovery  112 , WS-Transfer  114 , WS-MeX (i.e. WS-MetadataExchange)  116 , and WS-Security  118 . 
     In one embodiment, DFM  106  includes library routines corresponding to the SOAP protocol. Each SOAP library routine implements one or more functions defined by one or more SOAP specifications. The SOAP library routines are used to analyze SOAP requests and package SOAP messages. Therefore, each WSA  108  may invoke one or more SOAP library routines from DFM  106  so that the SOAP library routines may be defined once and shared among all WSAs  108  executing on the MFP. Multiple versions of the SOAP protocol standard may be supported. Updates to a newer version of a SOAP protocol standard may be done with little or no modification to WSA  108 . 
     In one embodiment, a client application (e.g. client  102 ) may register to receive information pertaining to updates to one or more WSAs on the MFP. If DFM  106  receives update information pertaining to an update of a particular application and a client application is registered to receive a message indicating such update, then DFM  106  sends to the client application a message that indicates the update information. In a related embodiment, client  102  is not required to register (e.g., subscribe) to receive such update notifications. Rather, DFM  106  may be configured to automatically broadcast a message indicating an update to one or more of the WSAs. 
     In one embodiment, DFM  106  may receive update information pertaining to a WSA. For example, WSA  108  may provide a fax service and the MFP detects that the fax line is disconnected. DFM  106  should not respond to future metadata requests with device metadata that indicates that the MFP provides a fax service when the fax service is unavailable. Therefore, DFM  106 , in response to receiving update information from a WSA, updates the device and/or service metadata associated with the WSA. 
     In one embodiment, DFM  106  may receive configuration requests from an administrator application (e.g. administrator  104 ). A configuration request indicates one or more WSAs that are to be configured and/or updated. The DFM  106  handles configuration requests and performs, or causes to be performed, the configuration or update instruction on the appropriate WSA. Alternatively, as described in more detail below, DFM  106  may instruct WSD Manager  110  to handle such configuration requests. 
     In one embodiment, DFM  106  may receive and respond to log requests from an administrator application (e.g. administrator  104 ). DFM  106  retrieves logging information pertaining to the one or more WSAs executing on the MFP and sends the logging information to administrator  104 . As described in more detail below, WSD Manager  110  may retrieve and provide to DFM  106  the logging information. 
     1.4 WSD Manager 
     According to an embodiment, DFM  106  also comprises WSD Manager  110 . WSD Manager  110  provides a central point for logging information, status inquiry, and external management of the MFP, such as from an administrator  104 . Administrator  104  is an application that is configured to retrieve information pertaining to the MFP through WSD Manager  110 . For example, WSD Manager  110  may centralize all logging information coming internally from all WSAs  108  and from the various platforms upon which WSAs  108  are executing. An administrator may also configure, update, or disable a WSA  108  using WSD Manager  110 . 
     In one embodiment, WSD Manager  10  maintains overall status information, such as where the MFP is located, what WSAs are installed on the MFP, and whether the WSAs are running properly. 
     In one embodiment, WSD Manager  110  maintains the metadata for the MFP and service metadata pertaining to each service application running on the MFP. 
     1.4.1 General API 
     According to an embodiment, WSD Manager  110  retrieves general information pertaining to the MFP, such as the IP address and the model number of the MFP, through general API  120 . General API  120  defines an interface by which DFM  106  receives information specific to each platform of the MFP. In this way, a DFM developer is not required to know the details of a specific platform, only the details of the DFM that the developer is building for an MFP. (The dotted lines in  FIG. 1  are API calls from a particular API to the appropriate API implementation.) 
     1.4.2 General API Implementation 
     If general API  120  has been defined for DFM  106 , then an implementation of general API  120  for a specific platform must be defined. For example, a general API implementation  132  is defined for general API  120  on a legacy platform  130 . Similarly, a general API implementation  142  is defined for general API  120  on a Linux-based platform  140 . A corresponding general API implementation defines the functions specified in a device-specific request and implemented on the MFP. Either the developer of DFM  106  may define the implementation or someone else who has knowledge of the target platform may define the implementation. 
     1.5 Web Service Application 
     Web services application (WSA)  108  is a module that provides one or more Web services and relies on Web Services protocols and technologies, such as those protocols provided by DFM  106 . WSA  108  may also rely on a separate SOAP module (not shown) to analyze SOAP requests if WSA  108  does not include logic for analyzing SOAP requests. As indicated above, the separate SOAP module may be provided by DFM  106  and shared among all WSAs  108 . 
     WSA  108  may also comprise a WS-Eventing module  122  for responding to event requests from client  102 . Client  102  may subscribe to an event associated with the service provided by WSA  108 . For example, WSA  108  may be a printing application and an event that client  102  subscribes to is when the MFP associated with WSA  108  completes a print job. Thus, upon completion of the event, WSA  108  will send an event message to client  102  indicating that the print job is completed. Additional details about integrating WS-Eventing into a WSA are provided in a section below. 
     1.5.1 Abstract API 
     WSA  108  may also comprise an abstract API (e.g. abstract API  124 ) through which platform-specific calls may be generated. The abstract API defines an interface by which the associated WSA  108  invokes one or more functions on the MFP. Therefore, the developer of a Web service application is not required to know the underlying complexities of the target platform, but only of the new service that the developer aims to provide. 
     1.5.2 Abstract API Implementation 
     If an abstract API has been defined by a Web service application developer, then an implementation of the abstract API for a specific platform must be defined. For example, an abstract API implementation  154  is defined for abstract API  124  on VxWorks platform  150 . A corresponding abstract API implementation defines the functions specified in a platform-specific request and implemented on the MFP. Either the developer of the Web service application may define the implementation or someone else who has knowledge of the target platform may define the implementation. 
     2.0 Multithreaded Implementation of a Web Service Application 
     One goal for implementing a WSA on a MFP is to process multiple requests from multiple clients virtually simultaneously. One way in which this is made possible is by spawning and executing multiple threads, each dedicated to performing at least one particular function. 
       FIG. 2  is a sequence diagram illustrating multiple threads that may execute within a Web service application of an MFP, according to an embodiment of the invention. One thread, referred to as main thread  202 , is created when the WSA to which main thread  202  belongs (referred to hereafter as the corresponding WSA) first executes. For example, main thread  202  may be created when the MFP turns on after a shut down, or when the corresponding WSA is added to the MFP and begins to execute. Main thread  202  may periodically check the status of other threads that are executing on the corresponding WSA in case a thread is causing a memory leak or has failed in any other way. One of the purposes of main thread  202  is to create other threads for processing SOAP requests from multiple client applications. Some of those threads are now described in more detail. 
     2.1 External Request Processing Thread 
     One such thread is external request processing (ERP) thread  204 , which is responsible for receiving SOAP requests coming from a network, such as a LAN, WAN, or the Internet. ERP thread  204  examines the SOAP request and determines what other thread should further process the SOAP request. Based on the examination, ERP thread dispatches the SOAP request to one or more other threads that are simultaneously executing in the corresponding WSA. 
     2.2 Internal Communications Thread 
     Another thread is internal communications (IC) thread  206 , which is responsible for communicating with other components of the MFP that are distinct from the corresponding WSA. Such components may include the DFM and a particular platform. In one embodiment, a separate thread is created for each component of the MFP that is distinct from the corresponding WSA and which the corresponding WSA may communicate with during the processing of a SOAP request. In other words, there may be multiple IC threads. But for the purposes of illustration, only one IC thread  206  is shown. 
     2.3 Request Processing Thread 
     Another thread indicated in  FIG. 2  is request processing (RP) thread  208 , which is responsible for processing SOAP requests according to the business logic of the corresponding WSA. For example, the corresponding WSA may provide a printer service. Upon receiving a request to execute a print job, RP thread  208  may determine the size of the print job, the availability of the printer, and the priority status of the print job compared to other pending print jobs. RP thread  208  may also instruct the printer on the specifics of the print job. Another example of the business logic of a WSA is converting SOAP requests into internal requests that can be forwarded to and understood by the lower level (i.e., target platform). RP thread  208  may also perform security functions, such as verifying whether the client (which sent the SOAP request) has access to the particular resource that the client needs to execute the SOAP request. 
     Embodiments of the invention are not required to execute a main thread, such as main thread  202 . Rather, one of the other threads above may act as main thread  202  in addition to performing its other functions. For example, ERP thread  204  may be created when the corresponding WSA first executes and may cause the creation of IC thread  206  and RP thread  208 . Thereafter, ERP thread  204  waits for, receives, processes, and dispatches SOAP requests sent from client applications. 
     2.4 Flow Diagram of Multiple Threads Processing a Soap Request 
       FIG. 3  illustrates a flow diagram that illustrates how multiple threads, within a WSA, may process multiple requests, according to an embodiment of the invention. At step  302 , at least three threads are created—an ERP thread, an RP thread, and an IC thread, such as those described above. 
     At step  304 , the ERP thread opens an external network connection. At step  306 , the ERP thread listens for incoming messages and/or connections. At steps  308  and  310 , the ERP receives a SOAP request and determines which thread (e.g., an IC thread or RP thread) should subsequently process the SOAP request by checking the SOAP request&#39;s type. Such a determination may be made, for example, by comparing the SOAP request&#39;s type against a list or table of types that are each associated with a particular thread, such as an IC thread or a RP thread. 
     At step  312 , based on the determination at step  310 , the ERP thread dispatches, or transfers, the SOAP request to the appropriate thread for further processing. In one embodiment, the ERP thread enqueues the SOAP request into a queue dedicated for the appropriate thread. For example, the ERP thread enqueues the SOAP request into an RP queue for the RP thread. 
     An RP thread waits for a SOAP request dispatch (step  314 ). When the RP thread receives the SOAP request, the RP thread parses the SOAP request to determine its type (step  316 ). Alternatively, the ERP thread may send the request type along with the SOAP request so that the RP thread is not required to parse the SOAP request. 
     At step  318 , the RP thread processes the SOAP request according to the business logic of the corresponding WSA. 
     At steps  320  and  322 , the RP thread may generate a response and send the response to the client application that sent the SOAP request. Alternatively, an error may have occurred in processing the SOAP request. For example, the SOAP request may not have been formatted correctly. As another example, if a paper jam occurred in a printer, then the RP thread may send a fault to the client application instead of keeping the connection open until the printer is fixed so that the client application may send the print job to another MFP. As another example, the RP thread may determine that the number of destinations specified in an event subscription request is greater than a permissible limit. In such cases, the RP thread may generate and send a fault message to the client application that sent the SOAP request. 
     At step  324 , an IC thread may initialize internal communication with, for example, the DFM of the MFP, a WS-Eventing module, or a target platform of the MFP. At step  326 , the IC thread waits for internal communication requests from the ERP thread. At step  328 , the IC thread sends the SOAP request to the appropriate module of the MFP that is distinct from the corresponding WSA. At step  330 , the IC thread receives a response or fault message from the module. At step  332 , the IC thread sends the response or fault message to the client application that sent the SOAP request. 
     2.5 Example of Multiple Threads Processing a Soap Request 
     A client application sends a SOAP request to a socket on a MFP where an ERP thread of a WSA is listening for connections. The WSA provides a print service and the SOAP request indicates that a print job will be sent as long as the WSA is able to immediately handle the request. The SOAP request may include or follow an event subscription request that indicates that the client application wants to be notified when the print job is completed. The ERP receives the request and determines that an RP thread should process the request. The RP thread receives the SOAP request and determines whether the WSA can immediately process the request by checking the print job queue and the status of the corresponding printer. If the RP thread determines that the WSA is able to immediately process the print job, then the RP thread sends a response to the client application indicating such. 
     The client application sends the data that will be printed by the printer of the MFP along with an event subscription request. The ERP thread receives and forwards the print job data to an IC thread which communicates the data to the target platform to be printed. If the event subscription request is separate from the initial SOAP request, then the ERP thread also forwards the event subscription request to the RP thread for parsing and further processing. The RP thread may forward the event subscription request to an event manager (e.g., comprising a module or thread) that processes event subscription requests and registers those events. Upon completion, the target platform sends a notification to the IC thread that indicates that the print job is finished. The IC thread sends the notification to the event manager. The event manager generates and sends an event message to the client that notifies the client of the completion of the print job. 
     2.6 Benefits of a Multithreaded Implementation of a Web Service Application 
     Multiple benefits may be realized from embodiments of the invention. For example, because a particular thread is responsible for only a few functions, the design of the WSA is more modular and, thus, the WSA is easier to program. For example, a developer may focus on an RP thread without having to know anything about communicating with other components of the MFP. 
     Another benefit of using multiple threads is that external requests coming from the network may be queued and processed on a first come first server basis without impacting the rest of the system. Another benefit is that the target platform may generate and send events to the WSA to be queued for later handling without putting any restriction on the work that the WSA may be currently doing. 
     2.7 Number of Threads 
     The number of threads that are actually used to implement a WSA may be greater than the threads discussed above. Even with more threads, one of the goals of implementing multiple threads remains the same—improve efficiency and allow asynchronous work with other components of the MFP. Other threads that may be used to implement a WSA are described below in the following sections. However, the principal of diminishing returns applies if too many threads are created, especially when multiple threads can access the same resource, such as memory. Too many threads may result in deadlock, increased complexity of logic, increased likelihood of bugs, and resource-expensive context switches that may be required when switching between threads. One principle that may be applied when determining how many threads to execute on a MFP is to have at most one thread dedicated to handling access to each exclusive resource. 
     3.0 Processing Fast and Slow Requests Differently 
     Unfavorable conditions may arise when a WSA receives multiple SOAP requests in a relatively short period of time. Suppose a WSA receives three large print job requests following by two event subscription requests that are significantly smaller in size and will not require much time to process. If SOAP requests are handled serially, then the two subscription requests will have to wait until the three large print jobs are finished printing. If the first print job takes a significant amount of time to execute, then the four remaining requests may time out (without the WSA ever fully processing them). Therefore, according to an embodiment of the invention, different types of client requests are processed differently. 
     Requests may be categorized into at least two types. For purposes of illustration only, two types of requests are described below, although many types may be defined. One type of request will be referred to hereinafter as a “fast” request and the other type of request will be referred to hereafter as a “slow” request. 
     A fast request may be a request where the time to process the request is not long enough to cause the client application to timeout. A slow request may be the opposite—where the time to process the request is long enough to cause the client application to timeout. Some factors that may be considered in determining whether a request is “fast” or “slow” include, but are not limited to, the size of the request and any associated data, how long a client may be blocked, and whether the request depends on the availability of a particular resource, such as a printer or scanner. For example, if a SOAP request and its associated data are small enough that it can be entirely buffered (such as an event subscription request), then the SOAP request may be considered “fast” and processed as a fast request. As another example, if the request is a print job and thus requires the exclusive use of a printer, then the request may be considered “slow” and processed as a slow request. 
     Examples of fast requests may include requests to subscribe to an event, retrieve the status of a job, cancel a job, and create a job. An example of a slow request may be a send document request (i.e., part of a print job). 
     3.1 Flow Diagrams of Processing Fast and Slow Requests 
       FIGS. 4A-B  are flow diagrams that illustrate how fast and slow requests may be processed differently, according to an embodiment of the invention. At step  402 , at least three threads are created—an external request processing (ERP) thread, a fast request processing (fast RP) thread, and a slow request processing (slow RP) thread. At step  404 , the network is initialized by, for example, opening a “master” listening socket through which the corresponding WSA may receive SOAP requests. A second socket may be created that inherits the properties of the master socket. The second socket is used to accept all the data associated with a request once the request is accepted. 
     At step  406 , the ERP thread narrows or closes a TCP window associated with a connection (if needed for slow requests). This is to avoid receiving the whole request until it is known whether it is appropriate to do so (i.e., when it is a fast request). Narrowing the TCP window before an entire request is received ensures that the WSA can receive data of a new request incrementally until the ERP thread determines whether the request is fast or slow. If the TCP window is narrowed after the entire request is received, then the client considers the request “fully sent”, after which the client starts its timeout timer. If the request is a slow request, then the client will likely timeout, which is an error. A purpose of manipulating the TCP window is to prevent clients from fully sending requests until the WSA is ready to process the request (e.g., exclusive resource access has been acquired). In an alternative embodiment, however, the ERP thread does not modify the TCP window for any requests. 
     The window size of TCP connection is the amount of data (in bytes) that can be buffered during a connection. The client can send only that amount of data before it must wait for an acknowledgment and window update from the WSA. Thus, to narrow the window size of a TCP connection, the ERP thread may send an acknowledgment with a 2, for example, indicating to the client application that the WSA can only accept 2 bytes at a time, at least until further notice (e.g., when a slow RP thread is ready to process the request). A 0 byte window effectively closes the TCP window without dropping the connection with the client application. 
     At step  408 , the ERP thread listens for incoming connections. At step  410 , the ERP receives at least some data associated with a SOAP request. At step  414  of  FIG. 4B , the ERP thread parses the SOAP request and determines whether the SOAP request is “fast” or “slow”. 
     In one embodiment, the determination of what requests are fast and slow is made when at least part of the request is received at the WSA. The ERP thread may examine an “Action” field of the request (as defined by the WS-Addressing specification) to determine the action (e.g., create job) and then consult a predetermined list or table of request actions that are each associated with either a fast and slow type. 
     If the request is “fast”, then at step  416 , the entire SOAP request and any associated data may be buffered. At step  418 , the ERP thread (or another thread) determines whether a queue for fast requests is full. If the queue is full, then the request is blocked until a slot in the query is free. “Blocking” means that the ERP thread waits until the fast queue becomes available before the ERP thread does anything else. The fast request queue may be of any size. Because fast requests tend to be much smaller than slow requests, the fast request queue may maintain more SOAP requests than the slow request queue. 
     At step  420 , the ERP thread widens the TCP window if it was previously narrowed to handle a slow request. At step  422 , the ERP thread enqueues the SOAP request to the fast request queue. 
     If the request is “slow”, then at step  424 , all the data associated with the SOAP request is prevented from being fully buffered. At step  426 , the ERP thread determines whether a queue for slow requests is full. If the queue is full, then the request is rejected and the ERP thread may send a fault to the client notifying the client that too many (e.g., large) requests are pending. 
     At step  428 , the ERP thread narrows or closes the TCP window (if needed for slow requests). In one embodiment, if the ERP thread narrowed the TCP window in step  406 , then the ERP thread closes the TCP window in step  428 . 
     At step  430 , the ERP thread enqueues the SOAP request to the slow request queue. 
     3.1.1 Fast Request Processing Thread 
     At step  440 , the fast RP thread dequeues a buffered SOAP request from the fast request queue. There may be multiple ways in which the fast RP thread may choose a next particular request in the queue. For example, the next fast request to process may be determined based on FIFO, LIFO, or some other priority system unrelated to the order in which the requests were received at the WSA. 
     At step  442 , the fast RP thread processes the SOAP request based on the specific type of request, such as cancel job, event subscription request, etc. Processing of the SOAP request may involve calling a lower-level API (e.g., to the target platform via an abstract API) and forwarding the SOAP request to another module or thread, such as an event manager described in more detail below. 
     At steps  444  and  446 , the fast RP thread generates and returns a response to the client that sent the fast request. 
     3.1.2 Slow Request Processing Thread 
     At step  450 , the slow RP thread checks the availability of a resource that is required by the first request in the slow request queue. For example, suppose the next slow request to be processed in the queue is a print job and the printer is currently printing documents corresponding to a different print job. Thus, the printer is unavailable and the first slow request must wait until the print job is finished. If the resource is unavailable, then the slow RP thread may periodically check the resource&#39;s availability. The slow RP thread may also begin to process a different request in the queue if the next request is waiting for an unavailable resource. For example, if all the slow requests in the queue require exclusive usage of the same resource, then the slow requests are processed sequentially (e.g., in the order in which they were received). However, if the slow requests require difference resources, such as resource A and resource B, then, when resource A is unavailable while resource B is available, the slow requests that require exclusive usage of resource B can be processed first even though such slow requests may have been queued later. 
     Once the resource becomes available, the process proceeds to step  452 , where the slow RP thread dequeues the next request from the slow request queue. At step  454 , the TCP window associated with the connection is widened (if previously narrowed or closed). 
     At step  456 , the slow RP thread receives the remaining data associated with the dequeued SOAP request. For example, if the slow request is a send document request, then the rest of the document (if some was sent already) is sent to the WSA to be printed. 
     At step  458 , the slow RP thread processes the SOAP request based on the specific type of request. As described above with respect to fast requests, processing of the slow request may involve calling a lower-level API (e.g., to the target platform via an abstract API) and forwarding the request to another module, such as the event manager. 
     At steps  460  and  462 , the slow RP thread may generate and return a response to the client that sent the slow request. Some requests, however, may not require a response. For example, a send document request may not require the WSA to send a response to the client. 
     In some cases, there are multiple types of external resources, on which different types of slow requests are dependent. Therefore, in one embodiment, a separate thread and queue is dedicated to each type. For example, a slow request may be a request to send a fax, which requires the use of a phone line on which only one fax can be sent at a time. Thus, a separate thread dedicated to handling and queuing fax requests may be generated. As another example, a WSA that offers a scanning service may have two types of slow requests: one for scan job creation (which requires exclusive use of a scan engine), and another for retrieving a large amount of scanned data over a network (which may take a while). Processing these two types of requests separately from fast requests helps ensure that all the fast requests are processed in time and that the system operates efficiently. For instance, if a client sends a job canceling request after requesting the transfer of a large amount of scanned data, then the WSA will immediately stop sending the scanned data once it receive the job canceling request. 
     3.2 Benefits of Processing Fast and Slow Requests Differently 
     Multiple benefits may be realized from handling fast and slow requests differently. One benefit is that the WSA is enabled to serialize the processing of requests that require access to one or more dependent resources. Another benefit is that a WSA may process slow requests without sacrificing prompt handling of fast requests. Another benefit is that client connections with a WSA do not time out while a client waits for a request to be processed. In summary, processing different types of requests differently maximizes efficiency and functional throughput of a WSA executing on a MFP. 
     4.0 Integrating WS-Eventing into a Web Service Application 
     One approach for providing multiple Web Services specifications to the WSAs of an MFP is to implement the Web Services specifications in a single location of the MFP, such as in the DFM of the MFP. In that way, the Web Services are implemented once and shared among all WSAs of the MFP. However, implementing WS-Eventing in the DFM is problematic for various reasons. (The following references to WS-Eventing also apply to other eventing specifications, such as WS-Notification. Thus, an MFP may implement WS-Notification instead of WS-Eventing.) 
     WS-Eventing was initially designed to be a simple event subscription and notification service. However, some WSAs of an MFP require more than such a simple event service. Therefore, WS-Eventing must be extended to allow those WSAs (such as those that provide a scanning service) to perform additional processing of an event subscription request before an event subscription response message is sent to the requesting client. Such additional processing is required because some event subscription requests contain information that is specific to the corresponding WSA. If WS-Eventing is implemented in a DFM of an MFP, then additional communications have to be made between the WSA and DFM because the DFM is unaware of the specifics of the WSA. Therefore, the communication complexity between the WSA and DFM increases, which tends to result in errors and at least a degradation in processing time. If a client receives no subscription response message or receives an error message in response to an event subscription request, then the perceived utility of the MFP is significantly reduced. 
     Therefore, according to an embodiment of the invention, WS-Eventing is integrated into one or more WSAs of an MFP. Not only does this built-in functionality (referred to hereinafter as the “event manager”) provide a general eventing implementation (such as subscription creation, management and event delivery), the event manager may also work closely with other components of the corresponding WSA, which enables efficient bi-directional communication between those components and the event manager. 
       FIG. 5  is a block diagram that illustrates the relationships of eventing components of an MFP, according to an embodiment of the invention. Client  502  sends an event subscription request to WSA  504 . WSA  504  communicates with an event manager (EM)  506 , which implements WS-Eventing in order to pre-process the subscription request before EM  506  sends an event subscription response to client  502 . EM  506  may be implemented as a thread or module within WSA  504 . At a later point in time, device  508  sends a notification of an event to abstract layer  510  of WSA  504  which forwards the notification to WSA  504 . Abstract layer  510  may be implemented as a separate module, for example, or a thread dedicated to communicating with the various platforms of the MFP. Alternatively, device  508  sends the notification directly to WSA  504 . Device  508  may be, for example, the target platform from which at least some events are generated. 
     WSA  504  then forwards the notification to EM  506 , which is a relatively simple procedure because EM  506  may be implemented as a module or thread within WSA  504 . EM  506  processes the notification, composes an event notification message, and sends the message to client  502 . 
     4.1 Eventing Sequence Diagram 
       FIG. 6  is a sequence diagram that illustrates in further detail how an event subscription request may be processed by a WSA and its associated event manager, according to an embodiment of the invention. At step  1 , client  502  generates an event subscription request and sends the request to WSA  504 . 
     At step  2 , WSA  504  forwards the subscription request to event manager (EM)  506 . Because WSA  504  does not know the WS-Eventing specification, WSA  504  is unable to retrieve any necessary WSA-specific data contained within the subscription request. If WSA  504  provides a scanning service, then such WSA-specific data may be the destinations of a scan job. Therefore, EM  506  retrieves the WSA-specific data from the subscription request and sends it (step  3 ) to WSA  504  for processing. 
     Before step  3 , EM  506  may determine whether the event subscription request is formatted properly (i.e., according to the WS-Eventing specification). If not, then WSA  504  or EM  506  may send a fault message directly to client  502 . 
     4.1.1 Pre-Processing 
     At step  4 , WSA  504  pre-processes the WSA-specific data before EM  506  composes an event subscription response. For example, if WSA  504  provides a scanning service, then the event subscription request from client  502  identifies destinations in which to send scanned document(s). Thus, WSA  504  may include a destination manager that manages destinations specified in the WSA-specific data. If WSA  504  determines (e.g., using its destination manager) that the destinations are invalid, then WSA  504  may send a fault message (at step  5 ) directly to client  502  to notify client  502  of the error, after which processing of the event subscription request halts. Thus, no extra communication between WSA  504  and EM  506  is required, which is one of the benefits of implementing WS-Eventing in a WSA. An example of destinations being invalid is that a threshold number of destinations was exceeded in the event subscription request. 
     Alternatively, at step  4 , WSA  504  may not recognize the WSA-specific data. As a result, WSA  504  may send a fault message (at step  5 ) directly to client  502  to notify client  502  of the error. 
     Another example of pre-processing at step  4  is also in the scanning context. WSA  504  may map the destinations specified in the WSA-specific data to alternate destination addresses for security purposes. The alternate destination addresses are sent to EM  506 , which adds the alternate destination addresses to an event subscription response that is sent to client  502 . 
     Data that is passed from WSA  504  to EM  506  (e.g., alternate destination addresses) are passed as a “black box”—EM  506  has no knowledge of the content of the data and does not perform any processing on the data. EM  506  simply inserts the data as-is into a fixed location inside the event subscription response. In other words, WSA  504  and EM  506  really do not have knowledge of each other&#39;s data, which is important in integrating an event manager in a WSA such that they remain modular and independent of each other. 
     Similarly, when EM  506  extracts WSA-specific data out of a request, EM  506  simply retrieves the data at a fixed pointing in the request, without knowing the content of the data. 
     At step  5 , if no fault messages are sent, WSA  504  sends WSA-specific data to EM  506  to include in an event subscription response message (“subscribe response”). 
     At step  6 , EM  506  sends the subscribe response to client  502  that notifies client  502  that the event subscription request was successful. Because WSA  504  does not “know” the WS-Eventing specification, EM  506  is responsible for generating the subscribe response. 
     4.1.2 Post-Processing 
     At step  7 , EM  506  sends data to WSA for post-processing, after which WSA  504  may post-process the data (step  8 ). For example, in the scanning example, WSA  504  may add and save the destinations specified in the subscription request to a destination list (e.g., using a destination manager) in local memory for future use. 
     One reason for the post-processing of step  8  is that WSA  504  may need to make use of the data created by EM  506  when EM  506  composes the subscribe response. For example, WSA  504  may need to keep track of a subscription identifier that EM  506  generates to uniquely identify the accepted subscription. Therefore, post-processing should not be combined with pre-processing. However, this is an atypical case. WSA  504  generally does not know anything about eventing. 
     Another other reason for the post-processing of step  8  is to avoid an unnecessary processing step. During pre-processing (step  4 ), EM  506  needs to extract WSA-specific data, if applicable. It is optional for EM  506  to validate the event subscription request based on the WS-Eventing specification. If EM  506  determines that the event subscription request is invalid before the composition of a subscription response message, then EM  506  notifies WSA  504  about the error and WSA  504  does not need to execute the post-processing step. 
     The separation of pre- and post-processing from the event manager subscription processing is to maximize flexibility and extensibility while maintaining the modularity of the event manger and the WSA. In this way, neither the event manager nor the WSA are required to understand each other&#39;s data schema or business logic. 
     4.1 Eventing Sequence Diagram (Cont.) 
     At step  9 , device  508  generates an event and sends an indication of the event to abstract layer  510 . At step  10 , abstract layer  510  forwards the event indication to WSA  504 , which in turn generates a WSA-specific notification body and passes it to EM  506  (step  11 ). In one embodiment, WSA  504  includes an event processing thread (described in more detail below with respect to  FIG. 7 ) that receives such event indications from abstract layer  510  and delivers event notifications to the event sink. In this way, the task of processing event subscription requests may be separated from the tasks of processing generated events and delivering event notifications. Thus, the following steps may alternatively be performed by one or more event processing threads. 
     At step  12 , EM  506  composes a proper event notification message based on the notification body and sends the message to the event sink, which may include client  502  and any other clients for which client  502  may have subscribed an event. At step  13 , EM  506  updates the delivery status of that particular event (e.g., that the appropriate client was notified of the event). 
     At step  14 , EM  506  notifies WSA  504  if the event subscription expires or is unsubscribed. Some WSAs need to take certain actions when certain event subscriptions are removed due to expiration or being unsubscribed. For example, if a client registers scan destinations via an event subscription request and that subscription is deleted, then the scanner WSA needs to remove the registered scan destinations. 
     4.2 Multithreaded Eventing Flow Diagram 
     As indicated above, multiple threads may be used to process SOAP requests and, in particular, event subscription requests.  FIG. 7  is a flow diagram that illustrates a multithreaded WSA for processing event subscription requests, according to an embodiment of the invention. This flow diagram references threads that have been described above with respect to efficiently processing SOAP requests. Such threads include the external request processing (ERP) thread, the slow request processing (RP) thread, and the fast RP thread. A new thread, an event processing thread, is included which is responsible for event generation (from device to abstract layer to WSA) and event delivery (from WSA to event manager to client). 
     At step  702 , multiple threads are created. In one embodiment, the ERP thread receives SOAP requests, determines whether they can be processed as fast requests or slow requests, and enqueues the requests in the appropriate queue (not shown). 
     4.2.1 Fast Request Processing Thread 
     At step  704 , the fast RP thread dequeues a SOAP request from the fast request queue and determines the request&#39;s type at step  706 . If the SOAP request is not an event subscription request, then, at step  708  the fast RP thread appropriately processes the request by, for example, calling the abstract API of the corresponding WSA, forwarding the request to another module of the WSA or of the MFP (e.g., DFM), etc. As a result of the processing in step  708 , the fast RP thread (if required by the request) composes a response (step  710 ) and sends the response to the appropriate client (step  712 ). 
     If the fast RP thread determines (at step  706 ) that the request is an event subscription request, then the process proceeds to step  714 , where the fast RP thread pre-processes the subscription request after the event manager retrieves the WSA-specific data. For some WSAs, this step may be unnecessary. Regardless of whether the fast RP thread performs steps  714 , the event manager determines the validity of the event subscription request because (typically) only the event manager “knows” how an event subscription request is formatted. 
     At step  716 , the event manager processes the subscription request according to the WS-Eventing specification. At step  718 , the event manager generates an event subscribe response and sends the response to the appropriate client. Again, depending on the specific WSA, the fast RP thread may or may not post-process WSA-specific data from the subscribe response at step  720 . 
     After step  720 , the fast RP thread, at step  704 , dequeues another request from the fast request queue and the process continues as described above. 
     4.2.2 Event Processing Thread 
     At step  722 , the event processing thread registers an event action list from the WSA. This step allows the WSA to notify the event manager of all possible events the WSA can provide. The event manager uses this list for validating subscription request. If a particular subscription request is for an event that the WSA does not offer, then the event manager can reject the event subscription request. Thus, although the event manager is designed to keep its modularity, the event manager should “know” what events the corresponding WSA supports. 
     At step  724 , the event processing thread registers events with the abstract layer of the corresponding WSA. Although steps  722  and  724  typically occur only once, it is possible that additional events are later registered with the abstract layer. 
     At step  726 , the event processing thread waits for events from the abstract layer. At step  728 , the event processing thread receives an indication of an event from the abstract layer. 
     At step  730 , based on the received indication of the event from the abstract layer, the WSA generates a notification body and passes it to the event manager. The event manager adds the notification header and sends the resulting even notification message to the event sink. The event sink is not necessarily the same as the client application that sent the event subscription request because a client application may subscribe for events on behalf of other client applications. 
     At step  732 , the event processing thread updates the notification status of the event. Some WSAs are required to know the notification status of one or more subscribed events. For example, a printer WSA may not need to know the delivery status of an event because a printer event does not have any consequence on a subsequent client request. On the other hand, in one scenario of a scanner WSA, a client will send a subsequent request only when the client receives a certain event notification. Thus, the delivery status of such an event is important so that the scanner WSA can take further action if necessary. 
     If the event notification was not delivered, then the event processing thread may take further action, such as resending the notification or canceling the subscription. Additionally or alternatively, the event processing thread may resend an event notification multiple times in case the corresponding client is offline or not operating. 
     The event processing thread may periodically check subscription expiration times and delete any expired subscriptions. Other related subcomponents within a WSA might opt to be notified when certain types of subscriptions are removed due to expiration, a client&#39;s unsubscribing request, or other reason. 
     In a successful scenario where the event was generated and a notification response is sent to the appropriate client application(s), the updated notification status may indicate ‘COMPLETE’. For clients that are still waiting for notifications of their respective events, the corresponding statuses may indicate ‘EVENT PENDING’ with an associated expiration time. 
     4.3 Benefits of Integrating WS-Eventing into a Web Service Application 
     Integrating WS-Eventing into a WSA may provide many advantages over other possible approaches, such as implementing WS-Eventing in a DFM of an MFP. One benefit is that large amounts of data do not have to be passed across an application boundary. Another benefit is that the possibilities of communication errors between applications are eliminated, at least in the WS-Eventing context. Also, with WS-Eventing integrated in a WSA, it is relatively straightforward for the WSA to extract WSA-specific data from an event subscription request and response and to pass WSA-specific data to an event manager. Furthermore, the WSA has control over other features of the event manager. For example, the WSA can call the event manager&#39;s function to clean up expired subscriptions when WSA is idle, instead of having the event manager create a designated thread for subscription cleanup. 
     Integrating WS-Eventing in a WSA also maintains the modularity of the WSA and its associated event manager. Such modularity facilitates the straightforward design and future updating of the WSA and the event manager. Lastly, a WSA is enabled to receive timely updates from the event manager about the delivery status of event notifications. 
     5.0 Implementation Mechanisms 
     The approaches described herein may be implemented on any type of computing platform or architecture.  FIG. 8  is a block diagram that illustrates a computer system  800  upon which an embodiment of the invention may be implemented. Computer system  800  includes a bus  802  or other communication mechanism for communicating information, and a processor  804  coupled with bus  802  for processing information. Computer system  800  also includes a main memory  806 , such as a random access memory (RAM) or other dynamic storage device, coupled to bus  802  for storing information and instructions to be executed by processor  804 . Main memory  806  also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor  804 . Computer system  800  further includes a read only memory (ROM)  808  or other static storage device coupled to bus  802  for storing static information and instructions for processor  804 . A storage device  810 , such as a magnetic disk or optical disk, is provided and coupled to bus  802  for storing information and instructions. 
     Computer system  800  may be coupled via bus  802  to a display  812 , such as a cathode ray tube (CRT), for displaying information to a computer user. An input device  814 , including alphanumeric and other keys, is coupled to bus  802  for communicating information and command selections to processor  804 . Another type of user input device is cursor control  816 , such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor  804  and for controlling cursor movement on display  812 . 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  800  for implementing the techniques described herein. According to one embodiment of the invention, those techniques are performed by computer system  800  in response to processor  804  executing one or more sequences of one or more instructions contained in main memory  806 . Such instructions may be read into main memory  806  from another machine-readable medium, such as storage device  810 . Execution of the sequences of instructions contained in main memory  806  causes processor  804  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 “machine-readable medium” as used herein refers to any medium that participates in providing data that causes a machine to operation in a specific fashion. In an embodiment implemented using computer system  800 , various machine-readable media are involved, for example, in providing instructions to processor  804  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  810 . Volatile media includes dynamic memory, such as main memory  806 . Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus  802 . 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 machine-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 machine-readable media may be involved in carrying one or more sequences of one or more instructions to processor  804  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  800  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 can receive the data carried in the infra-red signal and appropriate circuitry can place the data on bus  802 . Bus  802  carries the data to main memory  806 , from which processor  804  retrieves and executes the instructions. The instructions received by main memory  806  may optionally be stored on storage device  810  either before or after execution by processor  804 . 
     Computer system  800  also includes a communication interface  818  coupled to bus  802 . Communication interface  818  provides a two-way data communication coupling to a network link  820  that is connected to a local network  822 . For example, communication interface  818  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  818  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  818  sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. 
     Network link  820  typically provides data communication through one or more networks to other data devices. For example, network link  820  may provide a connection through local network  822  to a host computer  824  or to data equipment operated by an Internet Service Provider (ISP)  826 . ISP  826  in turn provides data communication services through the world wide packet data communication network now commonly referred to as the “Internet”  828 . Local network  822  and Internet  828  both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link  820  and through communication interface  818 , which carry the digital data to and from computer system  800 , are exemplary forms of carrier waves transporting the information. 
     Computer system  800  can send messages and receive data, including program code, through the network(s), network link  820  and communication interface  818 . In the Internet example, a server  830  might transmit a requested code for an application program through Internet  828 , ISP  826 , local network  822  and communication interface  818 . 
     The received code may be executed by processor  804  as it is received, and/or stored in storage device  810 , or other non-volatile storage for later execution. In this manner, computer system  800  may obtain application code in the form of a carrier wave. 
     In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. Thus, the sole and exclusive indicator of what is the invention, and is intended by the applicants to be the invention, is the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction. Any definitions expressly set forth herein for terms contained in such claims shall govern the meaning of such terms as used in the claims. Hence, no limitation, element, property, feature, advantage or attribute that is not expressly recited in a claim should limit the scope of such claim in any way. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.