Abstract:
A method, system, and computer program product stores transaction state information in application server process cluster information, eliminating transaction state logging and reducing the processing and communications expense that is necessary for transaction processing. A method of performing transaction execution comprises the steps of beginning execution of a transaction using a first process, storing a state of execution of the transaction in user state information included in cluster information accessible to a plurality of processes including the first process, switching a context from the first process to a second process that is one of the plurality of processes having access to the user state information included in the cluster information, and continuing execution of the transaction using the second process and the user state information included in the cluster information. The step of switching the context from the first process to the second process may be performed if the first process fails.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to storing transaction state information in application server process cluster information.  
         BACKGROUND OF THE INVENTION  
         [0002]    Distributed server systems have become a standard technique for the implementation of databases, Web servers, application servers, etc. Accesses that are made to the data stored in such systems are known as transactions. Transactions that modify data stored in server systems present certain challenges in the operations of a distributed server system. In particular, modifications to data must be propagated to all necessary locations in the distributed server system in order to ensure that all portions of the system contain consistent data.  
           [0003]    Distributed or Global Transactions use a completion protocol to guarantee data consistency among multiple systems. The “standard” protocol used to achieve consistency is the two phase commit protocol. The two phase commit protocol requires that transaction state be recovered when a coordinator fails and then restarts. This is conventionally accomplished by maintaining a transaction log either in a file system, a database, or another persistent store. Disk or network writes are expensive and various optimization techniques are used to minimize the number of log entries used for transaction processing. However, a need arises for a technique by which transaction state logging can be reduced, so as to reduce the processing and communications expense that is necessary for transaction processing.  
         SUMMARY OF THE INVENTION  
         [0004]    The present invention stores transaction state information in application server process cluster information, eliminating transaction state logging and reducing the processing and communications expense that is necessary for transaction processing. The present invention takes advantage of the fact that application servers have merged with transaction monitors and distributed object technologies. The application server typically clusters state across multiple or at least one redundant process that can be used in the event of a failure in the primary process. This is a standard technique used across the industry. The present invention takes advantage of this fact and in lieu of logging transaction state to a file system or database, includes the current state of the user&#39;s transaction within the user state managed by the cluster. In this way, no transaction logging occurs and coordinator failures are handled by switching context through the existing cluster mechanism to an instance that can complete the transaction.  
           [0005]    In one embodiment of the present invention, a method of performing transaction execution comprises the steps of beginning execution of a transaction using a first process, storing a state of execution of the transaction in user state information included in cluster information accessible to a plurality of processes including the first process, switching a context from the first process to a second process that is one of the plurality of processes having access to the user state information included in the cluster information, and continuing execution of the transaction using the second process and the user state information included in the cluster information. The step of switching the context from the first process to the second process may be performed if the first process fails.  
           [0006]    The first process may be a server process, the second process may be a server process, and the plurality of processes may be server processes. The first process may be an application server process, the second process may be an application server process, and the plurality of processes may be application server processes.  
           [0007]    The transaction may be executed using a two phase transaction protocol. The first process may be a coordinator of the two phase transaction protocol, the second process may be a coordinator of the two phase transaction protocol, and the plurality of processes may be coordinators of the two phase transaction protocol. The first process may be a participant in the two phase transaction protocol, the second process may be a participant in the two phase transaction protocol, and the plurality of processes may be participants in the two phase transaction protocol. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The details of the present invention, both as to its structure and operation, can best be understood by referring to the accompanying drawings, in which like reference numbers and designations refer to like elements.  
         [0009]    [0009]FIG. 1 is an exemplary block diagram of a system architecture in which the present invention may be implemented.  
         [0010]    [0010]FIG. 2 is an exemplary block diagram of clustering in application servers, in which the present invention may be implemented.  
         [0011]    [0011]FIG. 3 a  is an exemplary data flow diagram of a two phase commit protocol.  
         [0012]    [0012]FIG. 3 b  is an exemplary data flow diagram of a two phase commit protocol.  
         [0013]    [0013]FIG. 4 is an exemplary data flow diagram of log records written in a conventional system performing the two phase transaction protocol.  
         [0014]    [0014]FIG. 5 is an exemplary flow diagram of a process of operation of the present invention.  
         [0015]    [0015]FIG. 6 is an exemplary data flow diagram of operation of the present invention.  
         [0016]    [0016]FIG. 7 is an exemplary block diagram of an application server system, in which one or more application servers may be implemented. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    The present invention provides a technique by which transaction logging can be reduced, so as to reduce the processing and communications expense that is necessary for transaction processing.  
         [0018]    The present invention takes advantage of the fact that application servers have merged with transaction monitors and distributed object technologies. The application server typically clusters session state across multiple or at least one redundant process that can be used in the event of a failure in the primary process. This is a standard technique used across the industry. The present invention takes advantage of this fact and in lieu of logging transaction state to a file system or database, includes the current state of the user&#39;s transaction within the user state managed by the cluster. In this way, no transaction logging occurs and coordinator failures are handled by switching context through the existing cluster mechanism to an instance that can complete the transaction.  
         [0019]    An exemplary system architecture  100 , in which the present invention may be implemented, is shown in FIG. 1. System  100  includes user applications  102 , application servers  104 , and backend servers  106 . User applications  102  include a plurality of applications that are operated by or interface with a user, such as web browser  108 , custom application  110 , and mail application  112 . Application servers  104  include a plurality of servers that handle application operations between user applications  102  and backend servers  106 , such as web server  114 , mail server  116 , and data mining server  118 . Application servers are typically used for complex transaction-based applications. To support high-end needs, an application server should have built-in redundancy, monitors for high-availability, high-performance distributed application services and support for complex database access.  
         [0020]    Examples of types of servers that may be included in application servers  104  include:  
         [0021]    Audio/Video Servers: Audio/Video servers bring multimedia capabilities to Web sites by enabling them to broadcast streaming multimedia content.  
         [0022]    Chat Servers: Chat servers enable a large number of users to exchange information in an environment similar to Internet newsgroups that offer real-time discussion capabilities.  
         [0023]    Fax Servers: A fax server is an ideal solution for organizations looking to reduce incoming and outgoing telephone resources but that need to fax actual documents.  
         [0024]    FTP Servers: One of the oldest of the Internet services, File Transfer Protocol makes it possible to move one or more files securely between computers while providing file security and organization as well as transfer control.  
         [0025]    Groupware Servers: A groupware server is software designed to enable users to collaborate, regardless of location, via the Internet or a corporate intranet and to work together in a virtual atmosphere.  
         [0026]    IRC Servers: An option for those seeking real-time discussion capabilities, Internet Relay Chat consists of various separate networks (or “nets”) of servers that allow users to connect to each other via an IRC network.  
         [0027]    List Servers: List servers offer a way to better manage mailing lists, whether they be interactive discussions open to the public or one-way lists that deliver announcements, newsletters, or advertising.  
         [0028]    Mail Servers: Almost as ubiquitous and crucial as Web servers, mail servers move and store mail over corporate networks (via LANs and WANs) and across the Internet.  
         [0029]    News Servers: News servers act as a distribution and delivery source for the thousands of public news groups currently accessible over the USENET news network.  
         [0030]    Proxy Servers: Proxy servers sit between a client program (typically a Web browser) and an external server (typically another server on the Web) to filter requests, improve performance, and share connections.  
         [0031]    Telnet Servers: A Telnet server enables users to log on to a host computer and perform tasks as if they&#39;re working on the remote computer itself.  
         [0032]    Web Servers: At its core, a Web server serves static content to a Web browser by loading a file from a disk and serving it across the network to a user&#39;s Web browser. This entire exchange is mediated by the browser and server talking to each other using HTTP.  
         [0033]    Backend servers  106  include a plurality of servers, such as backend business application  120  and database management systems  122  and  124 . Database management systems (DBMSs) are software that enables storing, modifying, and extracting information from a database. There are many different types of DBMSs, ranging from small systems that run on personal computers to huge systems that run on mainframes. Examples of database applications include:  
         [0034]    computerized library systems  
         [0035]    automated teller machines  
         [0036]    flight reservation systems  
         [0037]    computerized parts inventory systems  
         [0038]    From a technical standpoint, DBMSs can differ widely. The terms relational, network, flat, and hierarchical all refer to the way a DBMS organizes information internally. The internal organization can affect how quickly and flexibly you can extract information.  
         [0039]    The precise architecture of any particular application server depends upon a variety of factors, such as its overall function, expected load, required availability, and other usage, business, financial, and engineering factors. Application servers typically have web servers for static or dynamic content; distributed object request brokers (ORBs), transaction managers, data caches and resource adapters that allow the application server to communicate with transactional resource managers (databases, message queues, etc). While these components may be integrated into an application server, they are generally not related to transaction processing. For example, the web server may in fact be used for Simple Object Access Protocol (SOAP) processing and act as a transport for distributed transaction context propagation; the ORB infrastructure may do the same.  
         [0040]    Distributed or Global Transactions use a completion protocol to guarantee data consistency among multiple systems. The “standard” protocol used to achieve consistency is the two phase commit protocol. The two phase commit protocol requires that transaction state be recovered when a coordinator fails and then restarts. This is conventionally accomplished by maintaining a transaction log either in a file system, a database, or another persistent store. Disk or network writes are expensive and various optimization techniques are used to minimize the number of log entries used for transaction processing.  
         [0041]    An exemplary block diagram of clustering in application servers is shown in FIG. 2. A particular user application  102  interacts with application servers  104 . Application servers include a plurality of server processes  202 A-C. Modern application servers provide the capability to cluster server processes. Clustering involves the sharing of information among server processes so that more than one server process can respond to a particular request.  
         [0042]    In some implementations, user application  102  and process  202  interact using a stateless protocol. Every request from user application  102  includes all the information needed for application servers  104  to perform a certain task. In such an implementation, clustering is easily provided, requiring only that a request from a user application can be handled by more than one server process that are in the same cluster.  
         [0043]    However, many implementations require user applications to interact with application servers using client sessions that include data stored on the server about each specific client. In order to cluster such session-based application server processes, it is no longer enough that the document hierarchy is shared among server processes. Instead, storing the state in a server process will mean that requests sent to different processes will produce in different results. Clustering in this environment requires replicating all state information in a server process to at least one other server process, or preferably, maintaining user state information in a way that it can be managed by all server processes in a given cluster. Typically, the user application may transparently interact with any server process in the cluster, which have access to the appropriate state information. For example, user application  102  interacts with server process  202 A, which is clustered with server process  202 C. Both server process  202 A and  202 C access and manage clustered information  204 , which includes state information, such as state information  206 . Interaction between user application  102  and application servers  104  is may be directed to any clustered server process that has access to clustered information  304 , such as server processes  202 A and  202 C. At any given moment, the user application interacts with a particular server process. If the original server process fails, the user process must be transparently be redirected to another server process in the cluster that has access to the user state information. For example, user application  102  interacts with process  202 A, which has access to state information, such as state information  206 . If server process  202 A fails, then interaction with user application  102  is redirected to server process  202 C, which also has access to state information  206 .  
         [0044]    In transaction-based interactions between user application  102  and application servers  104 , information relating to the state of the transaction must be maintained in order for the transaction to properly complete, either by committing or aborting. A well-known protocol for performing transactions is the two phase transaction protocol, which is described below. Conventionally, a number of log records are stored during performance of each transaction. These log records indicate the state of the transaction and allow recovery from failures that occur at points in the transaction. However, the present invention replaces logging of transaction state to a file system or database with including the transaction state within the user state managed by the cluster. In this way, no transaction logging occurs and coordinator failures are handled by switching context through the existing cluster mechanism to an instance that can complete the transaction.  
         [0045]    An exemplary data flow diagram of a two phase commit protocol  300 , is shown in FIGS. 3 a  and  3   b . FIG. 3 a  illustrates operation of the protocol in the situation in which the transaction commits. FIG. 3 b  illustrates operation of the protocol in the situation in which the transaction aborts. FIGS. 3 a  and  3   b  are best viewed in conjunction. In protocol  300 , messages are communicated between a coordinator  302  and one or more participants  304 . Coordinator  302  is the component that runs the two-phase commit protocol on behalf of one transaction, that is, the coordinator is the component that receives the commit or abort request and drives the execution of the protocol. Participants  304  are resource managers that do work on behalf of the transaction, for example, by reading and updating resources. The goal is to ensure that the coordinator and all participants commit the transaction, as shown in FIG. 3, or the coordinator and all participants abort the transaction, as shown in FIG. 4.  
         [0046]    A resource manager is a term used to describe the role of system components that manage the operation of resources, such as DBMSs. A resource is a term used to describe an item that is managed by a resource manager, such as a database managed by a DBMS. The terms “resource manage” and “resource,” are used to broaden the description of the system components that are used in the two-phase commit protocol because, when a transaction commits, all of the shared resources it accesses need to get involved in the commitment activity, not just databases. Nondatabase resources include recoverable scratch pad areas, queues, and other communications systems.  
         [0047]    The two-phase commit protocol makes the following assumptions about each transaction T:  
         [0048]    1. Transaction T accesses resources from time to time. If it experiences a serious error at any time, such as a deadlock or illegal operation, it issues an abort operation. If it terminates normally without any errors, it issues a commit. In response to the commit, the system runs the two-phase commit protocol.  
         [0049]    2. Each resource manager can commit or abort its part of T, that is, permanently install or undo T&#39;s operations that involve this resource manager. Thus, each resource manager typically has a transactional recovery system.  
         [0050]    3. One and only one program issues the commit operation on T. That is, one program decides when to start committing T by running the two-phase commit protocol, and no other program will later start running the protocol on T independently. In some cases, a second attempt to run two-phase commit while the first attempt is still running will cause the protocol to break, that is, will cause it to commit at one resource manager and abort at another. The protocol can be programmed to cope with concurrent attempts to run two-phase commit, but we assume it does not happen.  
         [0051]    4. Transaction T has terminated executing at all resource managers before issuing the commit operation. In general, this can be hard to arrange. If the transaction does all of its communications using RPC, then it can ensure T has finished processing at all resource managers by waiting for all of those calls to return, provided that each resource manager finishes all of the work it was asked to do before returning from the call. If T uses other communications paradigms, such as peer-to-peer, then it has to ensure by some other means that T terminated. For example, the well-known LU6.2 protocol, carefully dovetails two-phase commit with the transaction termination protocol. This assumption allows us to avoid dealing with the complexity of transaction termination here.  
         [0052]    5. Every system and resource manager fails by stopping. That is, the protocol does not make mistakes when its system or a resource manager malfunctions. It either does exactly what the protocol says it should do, or it stops running. It is possible for a failure to cause the protocol to do something that is inconsistent with the specification, such as sending bogus messages.  
         [0053]    Being Prepared  
         [0054]    A participant P is said to be prepared if all of transaction T&#39;s after-images at P are in stable storage. It is essential that T does not commit at any participant until all participants are prepared. The reason is the force-at-commit rule, which says not to commit a transaction until the after-images of all of its updates are in stable storage. To see what goes wrong if you break the rule, suppose one participant, P1, commits T before another participant, P2, is pre-pared. If P2 subsequently fails, before it is prepared and after P1 commits, then T will not be atomic. T has already committed at P1, and it cannot commit at P2 because P2, may have lost some of T&#39;s updates when it failed. On the other hand, if P2 is prepared before P, commits, then it is still possible for T to bc atomic after P2 fails. When P2 recovers, it still has T&#39;s updates in stable storage (because it was prepared before it failed). After it recovers and finds out that T committed, it too can finish committing T.  
         [0055]    Ensuring that all participants are prepared before any of them commits is the essence of two-phase commit. Phase 1 is when all participants become prepared. Phase 3 is when they commit. No participant enters phase 3 until all participants have completed phase 1, that is, until all participants are prepared.  
         [0056]    The Protocol  
         [0057]    The protocol proceeds as follows  
         [0058]    1. Begin phase 1: To commit the transaction, the coordinator  302  starts by sending a REQUEST-TO-PREPARE message  306  to each participant  304 .  
         [0059]    2. The coordinator  302  waits for all participants  304  to “vote” on the request.  
         [0060]    3. In response to receiving a REQUEST-TO-PREPARE message  306 , each participant  304  votes by sending a message back to the coordinator, as follows:  
         [0061]    a. It votes PREPARED by sending a PREPARED message  308  if it is prepared to commit.  
         [0062]    b. It may vote NO by sending a NO message  314  for any reason, usually because it cannot prepare the transaction due to a local failure.  
         [0063]    c. It may delay voting indefinitely, usually because its system is overburdened with other work.  
         [0064]    4. Begin phase 3: If the coordinator  302  receives PREPARED messages  308  from all participants  304 , it decides to commit. The transaction is now officially committed. Otherwise, it either received at least one NO message  314  or gave up waiting for some participant, so it decides to abort.  
         [0065]    5. The coordinator  302  sends its decision to all participants  304  using COMMIT messages  310  or ABORT messages  316 .  
         [0066]    6. Participants  304  acknowledge receipt of the COMMIT messages  310  or ABORT messages  316  by replying with DONE messages  312 .  
         [0067]    7. After receiving DONE messages  312  from all participants  304 , the coordinator  302  can forget the transaction, mean that it can deallocate any memory it was using to keep track of information about the transaction.  
         [0068]    In conventional systems performing the two phase transaction protocol, coordinator  302  and participant(s)  304  need to write a number of log records, as shown in FIG. 4. Before coordinator  302  sends a REQUEST-TO-PREPARE message  306 , coordinator  302  logs a start-two-phase-commit record  402 , which includes a list of the participants  304 . This writing is eager in that coordinator  302  must wait until this record is in the stable log before sending a REQUEST-TO-PREPARE message  306  to any participant  304 . Otherwise, if coordinator  302  failed after sending the REQUEST-TO-PREPARE message  306  and before the log record  402  was stable, coordinator  302  would not know which participants to notify about the decision.  
         [0069]    Before sending a commit decision, coordinator  302  logs a commit record  404 . Indeed, writing the commit record  404  to the log is what actually commits the transaction. This too is eager. Otherwise, if the coordinator failed after sending the COMMIT message  310  and before flushing the commit record  404  to the log, and coordinator  302  subsequently recovered, coordinator  302  would abort the transaction during its recovery procedure, producing an inconsistent outcome (since the participant that received the COMMIT message committed). After coordinator  302  receives the DONE messages, it writes a log done record  406 , which records the fact that the transaction is finished. This is lazy, in that coordinator  302  need not wait until log done record  406  is stable before proceeding to further processing.  
         [0070]    When a participant  304  receives a REQUEST-TO-PREPARE message  306  from the coordinator  302 , participant  304  writes a prepared record  408  to the log. This is eager, that is, participant  304  waits until the prepared record  408  is in the stable log before sending PREPARED message  308  to the coordinator  302 . Otherwise, if participant  304  failed after sending PREPARED message  308  and before flushing the prepared record  408  to the log, and participant  304  subsequently recovered, participant  304  would abort the transaction during its recovery procedure (since there is no prepared or commit record in the log). But since participant  304  sent PREPARED message  308 , it gave permission to the coordinator  302  to commit the transaction, which would produce an inconsistent outcome. Participant  304  writes a log record, a commit or abort record  410 , after participant  304  receives the decision message from the coordinator  302 . The log committed or aborted record  410  is eager, since once participant  304  sends DONE, participant  304  gives permission to the coordinator  302  to forget the transaction. If participant  304  fails after sending DONE message  312  and before the decision message is stable, then at recovery time participant  304  might not be able to find out what the decision was. Moreover participant  304  holds locks for the transaction until after it commits or aborts, so the sooner it logs the decision, the sooner participant  304  can release locks.  
         [0071]    In the present invention, the log records shown in FIG. 4 are not needed. Instead, the state of the transaction is stored in the user state included in the cluster information. An example of a process  500  of operation of the present invention is shown in FIG. 5. It is best viewed in conjunction with FIG. 6, which is a data flow diagram of operation of the present invention. Process  500  begins with step  502 , in which a transaction, preferably using the two phase transaction protocol, begins execution by a server process. In step  504 , the execution state of the transaction is stored in the user state included in the cluster information accessible by the server process. For example, for a server process acting as a coordinator  302 , transaction state information is stored in user state information  602  included in the cluster information  604  accessible by the coordinator  302 . The transaction state information, for example, where the two phase transaction protocol is used, includes start state  606 , commit or abort state  608 , and done state  610 . For a server process acting as a participant  304 , transaction state information is stored in user state information  612  included in the cluster information  614  accessible by the participant  304 . The transaction state information, for example, where the two phase transaction protocol is used, includes prepared state  616  and committed or aborted state  618 . Depending on the recovery protocol, some additional information about the branches may also need to be maintained in user state information  612 .  
         [0072]    In step  506 , the coordinator or participant fails during a transaction. In step  508 , the context is switched to another process that has access to the cluster information of the failed coordinator or participant. For example, if coordinator  302  fails, the context is switched to another process that has access to cluster information  604  and which can act as the coordinator. Likewise if participant  304  fails, the context is switched to another process that has access to cluster information  614  and which can act as the participant. In step  510 , the transaction continues with the process to which the context has been switched acting as the coordinator or participant. Since the process to which the context has been switched has access to the cluster information, the transaction can continue from the last state stored in the user state information before the coordinator or participant failed.  
         [0073]    An exemplary block diagram of an application server system  700 , in which one or more application servers may be implemented, is shown in FIG. 7. System  700  is typically a programmed general-purpose computer system, such as a personal computer, workstation, server system, and minicomputer or mainframe computer. System  700  includes one or more processors (CPUs)  702 A- 702 N, input/output circuitry  704 , network adapter  706 , and memory  708 . CPUs  702 A- 702 N execute program instructions in order to carry out the functions of the present invention. Typically, CPUs  702 A- 702 N are one or more microprocessors, such as an INTEL PENTIUM® processor. FIG. 7 illustrates an embodiment in which System  700  is implemented as a single multi-processor computer system, in which multiple processors  702 A- 702 N share system resources, such as memory  708 , input/output circuitry  704 , and network adapter  706 . However, the present invention also contemplates embodiments in which System  700  is implemented as a plurality of networked computer systems, which may be single-processor computer systems, multi-processor computer systems, or a mix thereof.  
         [0074]    Input/output circuitry  704  provides the capability to input data to, or output data from, database/System  700 . For example, input/output circuitry may include input devices, such as keyboards, mice, touchpads, trackballs, scanners, etc., output devices, such as video adapters, monitors, printers, etc., and input/output devices, such as, modems, etc. Network adapter  706  interfaces database/System  700  with Internet/intranet  710 . Internet/intranet  710  may include one or more standard local area network (LAN) or wide area network (WAN), such as Ethernet, Token Ring, the Internet, or a private or proprietary LAN/WAN.  
         [0075]    Memory  708  stores program instructions that are executed by, and data that are used and processed by, CPU  702  to perform the functions of system  700 . Memory  708  may include electronic memory devices, such as random-access memory (RAM), read-only memory (ROM), programmable read-only memory (PROM), electrically erasable programmable read-only memory (EEPROM), flash memory, etc., and electro-mechanical memory, such as magnetic disk drives, tape drives, optical disk drives, etc., which may use an integrated drive electronics (IDE) interface, or a variation or enhancement thereof, such as enhanced IDE (EIDE) or ultra direct memory access (UDMA), or a small computer system interface (SCSI) based interface, or a variation or enhancement thereof, such as fast-SCSI, wide-SCSI, fast and wide-SCSI, etc, or a fiber channel-arbitrated loop (FC-AL) interface.  
         [0076]    In the example shown in FIG. 7, memory  708  includes a plurality of application servers  712 A-N and operating system  714 . Application servers  712 A-N are software that handle application operations between user applications and backend servers. Application servers are typically used for complex transaction-based applications. Operating system  714  provides overall system functionality.  
         [0077]    Each application server, such as application server  712 A, includes a plurality of server processes  716 A-N and cluster information  718 A-N. Modern application servers provide the capability to cluster server processes. Clustering involves the sharing of information, such as cluster information  718 A- 718 N, among server processes, such as server processes  716 A-N, so that more than one server process can respond to a particular request.  
         [0078]    As shown in FIG. 7, the present invention contemplates implementation on a system or systems that provide multi-processor, multi-tasking, multi-process, and/or multi-thread computing, as well as implementation on systems that provide only single processor, single thread computing. Multi-processor computing involves performing computing using more than one processor. Multi-tasking computing involves performing computing using more than one operating system task. A task is an operating system concept that refers to the combination of a program being executed and bookkeeping information used by the operating system. Whenever a program is executed, the operating system creates a new task for it. The task is like an envelope for the program in that it identifies the program with a task number and attaches other bookkeeping information to it. Many operating systems, including UNIX®, OS/2®, and WINDOWS®, are capable of running many tasks at the same time and are called multitasking operating systems. Multi-tasking is the ability of an operating system to execute more than one executable at the same time. Each executable is running in its own address space, meaning that the executables have no way to share any of their memory. This has advantages, because it is impossible for any program to damage the execution of any of the other programs running on the system. However, the programs have no way to exchange any information except through the operating system (or by reading files stored on the file system). Multi-process computing is similar to multi-tasking computing, as the terms task and process are often used interchangeably, although some operating systems make a distinction between the two.  
         [0079]    It is important to note that while the present invention has been described in the context of a fully functioning data processing system, those of ordinary skill in the art will appreciate that the processes of the present invention are capable of being distributed in the form of a computer readable medium of instructions and a variety of forms and that the present invention applies equally regardless of the particular type of signal bearing media actually used to carry out the distribution. Examples of computer readable media include recordable-type media such as floppy disc, a hard disk drive, RAM, and CD-ROM&#39;s, as well as transmission-type media, such as digital and analog communications links.  
         [0080]    Although specific embodiments of the present invention have been described, it will be understood by those of skill in the art that there are other embodiments that are equivalent to the described embodiments. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiments, but only by the scope of the appended claims.