Patent Publication Number: US-6910070-B1

Title: Methods and systems for asynchronous notification of database events

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to methods and systems to notify system users and/or applications of changes to the state of a computer system and/or to the data residing within the system. More particularly, the present invention relates to computer-implemented methods and systems to notify database users and/or applications of events within the database data or to the database system itself. 
     2. Description of the Related Art 
     Databases typically constitute the entire repository of information in an enterprise. As such, databases have become an integral and vital part of corporate infrastructure, storing data relating to virtually every aspect of a company&#39;s logical and physical structure, including organization, human resources, product lines, customers and financial information, for example. Such a database may be accessed by a variety of entities (including the database administrator, the company&#39;s employees, outside suppliers and/or computer programs both within and without the company), each such entity typically retrieving different data and information therefrom. Users and/or programs that must access the database often have an interest in events within the computerized database system itself (hereafter, “system events”) or in the data that is resident in the database system (hereafter, “data events”). An event, within the context of the present invention, denotes a significant change of some sort. The change may be in the system state or configuration or in any other parameter of interest. Users of systems are typically interested in knowing about and acting on any changes of significance that takes places in the system that might potentially affect them. 
     For example, users and/or programs accessing the database may want to be notified upon the occurrence of specified systems events, such as whether the database is about to shutdown or startup, is running out of disk space or rollback segments or upon the occurrence of logons and logoffs, for example. Likewise, inventory managers, for example may want to be notified upon the occurrence of specified data events, such as when the inventory for a specific part falls below a critical threshold, so that additional parts can be ordered in a timely manner. In each case, some action may be taken based upon such system- or data-related information extracted or otherwise obtained from the database. 
     The extraction of such information from a database may be carried out, for example, by carrying out the steps outlined in  FIGS. 1   a  or  1   b . As shown in  FIG. 1   a , the interested user may query the database (step S 1   a ) and may take some action based upon the database query result (step S 2   a ). For example, the database query may include a number of Structured Query Language (SQL) or Procedural Language/Structure Query Language (PL/SQL) statements that may ascertain the status of one or more tables within the database. This sequence may then be repeated, for example, until the database query reveals that the event of interest has occurred. Such a repetitive cycle, however, is believed to be inefficient, wasteful and time consuming. Another method of obtaining information relative to events of interest within a database is shown in  FIG. 1   b . To notify a user of the occurrence of an event, a trigger may be declared, as shown in step S 1   b . A trigger is a procedure that is implicitly executed when, for example, certain Data Manipulation Language (DML) statements (such as INSERT, UPDATE or DELETE, for example) are issued against an associated table in the database, as disclosed, for example, in Chapter 17 of  Oracle 8  Server Concepts , release 8.0 {circle around (C)} 1997 Oracle Corporation (or later versions thereof), which publication is incorporated herein by reference in its entirety. The trigger, as shown at step S 2   b , may include logic, such as SQL or PL/SQL statements that are executed when one of the aforementioned statements are issued against the table or tables of interest. Thereafter, another application may check a trigger flag (step S 3   b ) that is updated by the trigger declared in step S 1   b  and a still further application may take appropriate action based upon the state of the trigger flag, as shown at step S 4   b . Again, the user is not notified that the event of interest occurred until some outward manifestation of the event is apparent of until a check of the trigger flag reveals that the state thereof has changed. The user may have to repeatedly check the state of the trigger flag to ascertain whether is has changed state. This cyclic polling process is also believed to be inherently inefficient, wasteful of database resources and time consuming. Such a database request—reply synchronous process forces the user to “pull” the specific information of interest from the database by issuing a request therefor, even through the events of interest most often occur asynchronously.  FIGS. 1   a  and  1   b  assume that the user (whether system administrator, database user or application) is tightly integrated with the database. For example, the user in the examples of  FIGS. 1   a  and  1   b  may be required to log onto the database to query the database and/or to code the appropriate trigger. 
       FIG. 2  shows another paradigm in database event notification. Indeed,  FIG. 2  shows an example of retrieving information from a database wherein the database “pushes” information from the database to a number of loosely coupled users. As depicted therein, the contents of a database  210  (or a selected portion thereof) are pushed or broadcast (as symbolized by the broadcast antenna tower  220 ), to the Internet  230  and/or across another communications channel. A number of clients, such as shown at  245 ,  255  and  265  filter the pushed database contents through corresponding client-side filters  240 ,  250  and  260  coupled between the Internet  230  and the clients  245 ,  255  and  265 . Assuming, for example, that the database  210  is or includes a repository of news information, a large amount of information may be continually and indiscriminately pushed or broadcast through the Internet  230  to clients  245 ,  255  and  265  using the service. However, client  245  may only be interested in receiving a small portion of the pushed news information. For example, client  245  may only be interested in basketball information. More specifically, client  245  may one be interested in information directly relating to the Boston Celtics, for example. In that case, the client-side filter  240  may be caused to filter out any information relating to the Boston Celtics (using specified keywords, for example) from the stream of information pushed by database  210 , to deliver only Celtics news to the client  245 . Similarly, client  255  might be running an automated stock trading program, configured to purchase a specific stock (e.g. XYZ Corp.) when a predetermined combination of market conditions exist (e.g., when XYZ trades for under $20 and ABC Corp. trades for over $55). However, the system  200  of  FIG. 2  is not equipped to selectively push data relating only to the share price of XYZ and ABC corporations directly to client  265 , but only to aim a great flow of information, fire hose-like, toward the clients  245 ,  255  and  265 . In effect, the client  265  is inundated with a great deal of information of which it has no interest and may have to receive the entire stock ticker in order to extract therefrom the information of interest. Such a flood of unwanted information needlessly consumes bandwidth and may clog corporate Intranets. This problem is exacerbated by the fact that the database  210  typically pushes it information out at regular intervals, irrespective of whether the information of interest to the clients  245 ,  255  or  265  has been updated since the last push, thereby often pushing duplicative and extraneous information onto the Internet and ultimately to the clients  245 ,  255  or  265 . Because of its inherent limitations, the paradigm of  FIG. 2  has largely been abandoned, although no suitable alternative is believed to have emerged in its stead. 
     What are needed, therefore, are methods and systems to notify users of database systems of interesting changes to the database system or data that is efficient and economical in terms of bandwidth, time and effort. In particular, what are needed are methods and systems allowing the detection of system and data events, as well as the publication and delivery of notifications thereof only to those clients having expressed an interest therein. Moreover, what are needed are methods and systems that deliver only those notifications of specific interest to each client. Such methods and systems, optimally, should asynchronously publish and deliver the notifications of systems and data events of interest to the clients, the publication and delivery thereby closely matching the manner in which the events occur and are detected. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention, therefore, to provide methods and systems to efficiently notify users of database systems of interesting changes to the database system or data. It is a further object of the present invention to provide methods and systems to detect system and data events and to publish and deliver notifications thereof only to those clients having expressed an interest therein. It is also an object of the prevent invention to provide methods and systems to asynchronously publish and deliver notifications of system and data events of interest to the clients, the publication and delivery thereby closely matching the manner in which the events occur and are detected. 
     In accordance, with the above-described objects and those that will be mentioned and will become apparent below, a computer implemented method of asynchronously notifying an application client of an event of interest within a database may include steps of receiving a subscription to an event name from a logical agent, the event name corresponding to the vent of interest and receiving a registration from the application client, the registration including delivery information indicating at least where and how a notification concerning the event is to be delivered. The occurrence of the event may then be detected within the database and the associated notification may be published to a data structure referenced by the event name upon detecting the occurrence of the event. The delivery information may then be retrieved and the published notification may then be formatted according to the retrieved delivery information. The formatted notification may then be delivered asynchronously to the application client over a network, such as the Internet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a further understanding of the objects and advantages of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying figures, in which: 
         FIG. 1   a  is a flowchart of a first method to synchronously extract information relative to an asynchronous event of interest from a database. 
         FIG. 1   b  is a flowchart of a second method to synchronously extract information relative to an asynchronous event of interest from a database. 
         FIG. 2  shows an example of retrieving information from a database wherein the database “pushes” information from the database to a number of loosely-coupled users, according to an embodiment of the present invention. 
         FIG. 3  is a block diagram of a system for asynchronous event notification, according to an embodiment of the present invention. 
         FIG. 4  is a flowchart of a method for asynchronous notification of database events, according to an embodiment of the present invention. 
         FIG. 5  illustrates a block diagram of a computer with which an embodiment of the present invention may be implemented. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FUNCTIONAL OVERVIEW 
     The present invention is drawn to methods and systems for asynchronous notification of client specified data and system events. In the case of system events, for example, timely and asynchronous receipt of system event notifications allows client applications to smoothly transition on and off-line without having to repeatedly poll the database or handle abrupt errors. The detection and publication of system events such as logon and logoff may be used for auditing or other purposes. Data events, according to the present invention, may be delivered asynchronously without any substantial loss to transaction throughput. 
     To achieve these objectives, the present invention implements methods and systems for the registration, detection, publication and delivery of both system and data events. To insure that clients (be they physical users or applications) receive only the narrowly tailored information that is of interest to them, each client, according to an embodiment of the present invention, may register an interest in receiving notifications regarding one or more data and/or system events. Thereafter, whenever one of these system or data events occurs, only those clients having registered an interest in the event receive a corresponding notification. 
       FIG. 3  is a block diagram of a system for asynchronous event notification, according to an embodiment of the present invention. The major components of the asynchronous database event notification system according to the present invention include a database  310  (an instance thereof), a channel  315  and a plurality of notification clients  370 ,  372 ,  374  and  376 . The database  310  may be thought of as a supplier of event notifications and each of the plurality of clients  370 ,  372 ,  374  and  376  may be thought of as a consumer of such event notifications. Each of the clients  370 ,  372 ,  374  and  376  may be coupled to the database  310  through the channel  315  via a network  380  that may include, for example, the Internet, a leased line and/or some other public, private or hybrid network. 
     The present invention, according to one embodiment thereof, implements a push-push model of delivering event notifications from notification supplier (the database  310 ) to the notification consumers (clients  370 ,  372 ,  374  and/or  376 ). Indeed, according to an embodiment of the present invention, data or system event notifications are asynchronously pushed to the channel  315 , whereupon the channel  315  selectively pushes the notifications to the notification clients  370 ,  372 ,  374  and/or  376 . Other event notification models may also be implemented within the context of the present invention; namely, pull-pull, pull-push, or push-pull. The present invention will hereafter be described in terms of the push-push model, it being understood that the other aforementioned event notification models may also readily be implemented. 
     According to the present invention, the database  310  receives subscriptions  320  from the notification clients  370 ,  372 ,  374  and/or  376 . Each of the notification clients  370 ,  372 ,  374  and/or  376  may subscribe to be notified upon the occurrence of one or more data or system events. To promote the loosely coupled nature of the subscription process, the clients  370 ,  372 ,  374  and/or  376  themselves, according to an embodiment of the present invention, do not subscribe to receive particular notifications. Rather, a logical entity termed an agent expresses an interest in receiving notification upon the occurrence of a specified event by way of a subscription. More particularly still, an agent, acting on behalf of one of the clients  370 ,  372 ,  374  and/or  376  subscribes to one or more event names. Each event name identifies a particular data or system event within the database  310 . According to the present invention, all subscription  320  in the system for asynchronous event notification  300  “belong” to an agent and an agent may have multiple subscriptions  320 . The database  310 , therefore, has no knowledge of the clients  370 ,  372 ,  374  and/or  376 , as all the database  310  has received are subscriptions  320  to particular event names from agents acting on behalf of one or more clients  370 ,  372 ,  374  and/or  376 . 
     After receiving the subscriptions  320 , the database  310  updates a logical entity termed a names schema  330 . The names schema  330  stores the delivery information that specifies to which client(s)  370 ,  372 ,  374  and/or  376  the notification is to be delivered and how the notification is to be sent. The updating of the names schema  330 , according to the present invention, is termed the registration process. The delivery information stored in the names schema  330  includes information as to where the notification is to be sent and how the notification is to be sent. For example, the delivery information provided by a particular client  370 ,  372 ,  374  or  376  may include an identification of the client process&#39; host and port (where) and may include a callback indicating the manner in which delivery of the notification should be made (how). The registration process, therefore, associates a subscription  320  with a physical, potentially transient client  370 ,  372 ,  374  and/or  376 . Should the client (process) clients  370 ,  372 ,  374  and/or  376  die and thereafter be invoked again, the re-invoked client process clients  370 ,  372 ,  374  and/or  376  preferably must again register its delivery information in the names schema  330 . This is akin to a newspaper subscriber (the client  370 ,  372 ,  374  and/or  376 ) moving his or her residence (host/port) and calling the newspaper circulation department (names schema  330 ) to advise them of a new delivery address. 
     A data or system event, according to the present invention, may be detected by means of triggers, collectively shown in  FIG. 3  at reference  314 . The triggers  314  may include a stored procedure of (for example, SQL) triggering statements that are fired upon the occurrence of a data or system event. The triggers  314  are predefined database functions that are invoked on, database operations on a subset of database entities. Finally, a procedure or procedure (a PL/SQL block, for example) may be executed within the trigger body when the corresponding triggering statement is fired and the triggering restriction evaluates to TRUE. For example, a triggering data event may be related to an INSERT, DELETE, or UPDATE statement operating on one or more of the tables 312 within the database  310 . Alternatively, a triggering system event may be related to systems-level database server events, such as logons, logoffs, startup, shutdown, etc. Appropriate triggers  314  may be developed to trap most any data or system event that may be of interest to any of the clients  370 ,  372 ,  374  and/or  376 . 
     When an event (whether data or system-related) fires a trigger among the triggers  314 , a notification is generated. The notification is published to the channel  315 . The channel  315  may then utilize the registration information stored in the names schema  330  to propagate the notification to one or more of the notification clients  370 ,  372 ,  374  and/or  376 . To publish the notification of a data or system event to the channel  315 , the database  310  may push the notification to a memory space  340  that is shared between the database  310  and the event monitor  350 . The event monitor  350  (a single event monitor  350  may be instantiated with each instance of the database  310 ) reads new event notifications from the shared memory space  340 , and consults the names schema  330  to determine whether any of the subscriptions  320  have specified the event name corresponding to the published notification. If so, the channel  315  notifies all currently registered clients  370 ,  372 ,  374  and/or  376  that have subscribed to the event name corresponding to the published notification. To do so, the event monitor  350  retrieves the client registration information from the subset of the subscriptions  320  having specified the event name corresponding to the published notification from the names schema  330 . This enables the channel  315  to not only identify which of the clients  370 ,  372 ,  374  and/or  376  have, via their agents, expressed an interest in receiving notifications of the type of event just published to the shared memory space  340 , but also allows the channel  315  to retrieve the delivery information (e.g., host/port, for example) of those interested clients  370 ,  372 ,  374  and/or  376 . 
     Thereafter, the channel  315  may send the notification to each of the clients  370 ,  372 ,  374  and/or  376  whose registration information (i.e., delivery information) has been retrieved from the names schema  330 . This process may be termed “posting”. To post an event notification to a client  370 ,  372 ,  374  and/or  376 , the event monitor  350  may pass the delivery information for each client  370 ,  372 ,  374  and/or  376  whose subscription  320  specified the event name corresponding to the published event notification to a protocol engine  360 . The protocol engine  360 , according to an embodiment of the present invention, includes a process that sends a notification to appropriate clients  370 ,  372 ,  374  and/or  376  in the manner specified by the registration information in the names schema  330 , as specified by the client  370 ,  372 ,  374  or  376  when the client  370 ,  372 ,  374  or  376  registered to receive notifications to the event. For example, the protocol engine  360  may send the notification to one or more of the clients  370 ,  372 ,  374  and/or  376  via one or more of any number of public protocols such as, for example, Hyper Text Transfer Protocol (http), File Transfer Protocol (ftp), Internet Interoperability Protocol (iiop), Simple Mail Transfer Protocol (smtp), Open Financial Exchange (OFX) and/or private protocols, such as Oracle Corporation&#39;s net8, for example. 
     Therefore, clients  370 ,  372 ,  374  and/or  376  may register to receive notifications relating to data and/or system events that are of interest to them, and have those notifications asynchronously delivered in the manner that is best suited to them. For example, a client process clients  370 ,  372 ,  374  and/or  376  may receive notifications of database events via http directly into a Web sit administered by the clients  370 ,  372 ,  374  and/or  376 , or as a file transferred via ftp, for example, to a location specified by the client clients  370 ,  372 ,  374  and/or  376  upon registration. 
     The present invention, according to one embodiment thereof, may utilize queuing mechanisms wherein each event name maps onto a queue. Using queues, the clients  370 ,  372 ,  374  and/or  376  may subscribe and register to receive event notifications through their respective agents that, in this embodiment, may be thought of as logical persistent queue users. The agents, when subscribing to an event name, cause an entry to be made into the name schema. The agents, when publishing to an event name, insert a message into a queue referenced by the event name of interest to the client  370 ,  372 ,  374  or  376 . The queuing mechanism allows the clients, through their agents, to express an interest in receiving particular notifications and proceed with their work, without waiting for a reply from the database  310 , thereby implementing what may be characterized as a disconnected and deferred communication model. According to the present invention, when a trigger  314  fires, the database  310  for persistent queues, may enqueue the notification message into the queue referenced by the event name associated with the trigger that fired and write the notification message to shared memory  340 . The event monitor  350  may then read the notification from the shared memory  340  and selectively propagate the notification toward the appropriate clients  370 ,  372 ,  374  and/or  376  using the registration information stored in the names schema  330  and the protocol engine  360 . The clients  370 ,  372 ,  374  and/or  376  may thereafter dequeue the notification message at their convenience. For non persistent queues, the entire notification message may be written to shared memory  340  and thereafter propagated to the clients  370 ,  372 ,  374  and/or  376 . According to an embodiment of the present invention, the Advanced Queuing protocol of Oracle Server 8.1 (or later versions) (hereafter Oracle AQ) may be advantageously implemented. Oracle AQ is described, for example, in Chapter 15 of  Oracle 8  Server Concepts  incorporated herein above. 
     At the time of subscription and registration, the clients  370 ,  372 ,  374  and/or  376  may specify the quality of service desired for the database notifications. Within the context of the present invention, the phrase “quality of service” shall be deemed to include information as to whether the notification has been specified to be persistent or non-persistent. Persistent notifications include notifications that must be delivered to all interested clients  370 ,  372 ,  374  and/or  376  at or near the point in time (inasmuch as possible) at which the underlying event occurs, and the underlying client  370 ,  372 ,  374  and/or  376  performs an explicit dequeue operation to retrieve the message. Non persistent notifications deliver the message along with the notification. The clients  370 ,  372 ,  374  and/or  376 , during the subscription and registration process, may specify the desired quality of service for each event in which they are interested in receiving notifications. 
     For example, a client  370 ,  372 ,  374  or  376  may subscribe to receive notifications from a stock ticker application that sends out frequent notifications, as stock prices change. Such notifications may be specified as non persistent, as the client  370 ,  372 ,  374  or  376  likely may not care if a few notifications are dropped within a short time interval because, for example, the system  300  is loaded and/or the shared memory  340  is full. Moreover, such client  370 ,  372 ,  374  or  376  may not care to receive stock price notifications if they are more than a few minutes old, for example. Such notifications messages may then be enqueued in a non-persistent queue and delivered on a best efforts basis to the subscribed clients  370 ,  372 ,  374  and/or  376  who are currently registered and dropped if non-deliverable within a specified time frame. Non-persistent queues, according to the present invention, include queues that do not persist beyond the instance of the database  310 . The notification messages in non-persistent queues are preferably deleted after they have been delivered to the clients  370 ,  372 ,  374  and/or  376 . 
     Alternately, a client  370 ,  372 ,  374  and/or  376  may subscribe and register to receive notifications of selected data and/or system events and specify that such notifications are to be persistent. For such notifications to be persistent, the notifications must be delivered exactly once to each subscribed client  370 ,  372 ,  374  and/or  376  that is currently registered, even in the presence of network, machine and application failures. To do so, the database  310  may enqueue the notification messages in a persistent queue referenced by the event name corresponding to the event of interest to the client. A persistent queue, according to an embodiment of the present invention, is a queue that persists beyond the instance of the database  310 . If, for example, the shared memory  340  is full, persistent notifications may be written to the persistent storage  345  until such time as they may be rewritten to the shared memory  340  and delivered to the clients  370 ,  372 ,  374  and/or  376 . The persistent storage  345  may include, for example, hard disk storage, tape storage, optical storage and/or some other storage means. At some later point in time, the notifications written to the persistent storage  345  may be delivered to the clients  370 ,  372 ,  374  and/or  376  who have subscribed for the persistent notifications and are still currently registered. Several events may have occurred, however, during the period in which the system  300  was loaded and/or the shared memory  340  was full. In that case, the clients  370 ,  372 ,  374  and/or  376  may wish to receive only the latest persistent notification, for example, Such delivery options for such notifications may be included in the client-specified registration parameters set upon registration. 
     An example of a reason for a full shared memory  340  is the case wherein the link between the channel  315  and the clients  370 ,  372 ,  374  and/or  376  includes relatively low bandwidth communications channels, such as telephone lines (POTS), for example. In this case, the event notifications may accumulate faster in the shared memory  340  than they can be delivered to the clients  370 ,  372 ,  374  and/or  376 . Indeed, the link between the shared memory  340  and the database  310  may be several orders of magnitude faster than the link between the channel  315  and the clients  370 ,  372 ,  374  and/or  376 , resulting in a bottleneck at the shared memory  340 . In this case, it is particularly important for the clients  370 ,  372 ,  374  and/or  376  to specify the desired quality of service. Alternatively, the application itself may set the quality of service parameter. For example, the stock ticker application may itself set the quality of service parameter to unreliable to allow some flexibility in event notifications depending upon, for example, the current load on the system  300 . 
     Queues, whether persistent or non-persistent, may be stored in queue tables within the database  310 . A queue table is a database table and contains one or more queues. As such, the full functionality native to the database  310  is available to operate on the queue tables. Indeed, the clients  370 ,  372 ,  374  and/or  376  may take advantage of the full panoply of available database access expressions to evaluate rules based thereon over a single or multiple queue tables. For example, at the time of subscribing and registering to receive system and/or data notifications from the database  310 , the clients  370 ,  372 ,  374  and/or  376  may also specify a rule to be evaluated before delivery of the associated notification occurs. Such a rule may contain database access language expressions such as, for example, SQL and/or PL/SQL statements (or other ANSI/ISO-compliant language expressions, for example) to access the notification message&#39;s properties, history and content. Alternatively, the rule may include a procedure (written in PL/SQL, for example) that calls an external procedure or function written in another programming language, such as the C programming language, for example. In this manner, even though a client  370 ,  372 ,  374  or  376  may have subscribed (through an agent) and registered to receive notifications concerning an event, delivery of the associated notification may not occur until the even occurs and the rule specified by the client at the time of subscribing and registering evaluates to TRUE or satisfies some other selected condition. As the notification is tightly integrated with the database  310 , the full functionality native to the database  310  is brought to bear upon the generation and delivery of data and/or system event notifications to the clients  370 ,  372 ,  374  and/or  376 . For example, notifications may be specified, at the time of subscription and registration, as available for delivery only after a specified time elapses (a delay time) and as having to be delivered before a specified time limit elapses. The client applications  370 ,  372 ,  374  and/or  376  may also specify, for example, that the notifications be stored after delivery, thereby keeping a history of relevant messages that may be used for tracking, data warehouse and data mining operations, for example. 
     A functional example of a method for asynchronous notification of database event, according to the present invention, is shown in FIG.  4 . As shown in step S 1 , an agent, acting on behalf of one or more of the clients  370 ,  372 ,  374  and/or  376  (which may include application programs and/or other entities) subscribers to an event name. That is, one or more of the client applications  370 ,  372 ,  374  and/or  376 , acting through an agent, expresses an interest in receiving notifications when the even reference by the event name occurs in the database  310 . In step S 2 , the client application(s)  370 ,  372 ,  374  and/or  376  registers the delivery information in the names schema  330  shown in FIG.  3 . The delivery information may include the physical process where the client program runs (host and port information, for example) and may include a callback indicating the manner in which delivery of the notification should be made. Indeed, the delivery information registered in the names schema  330  includes sufficient information to allow the event monitor  350  and the protocol engine  360  to determine the place and manner of delivery of the event notifications. Quality of service information may also be specified at the time of registration, the client(s)  370 ,  372 ,  374  and/or  376  indicating whether the notifications are to be persistent or non persistent, as detailed above. Additionally, the clients  370 ,  372 ,  374  and/or  376  may, at the time of subscription, specify a rule that must evaluate to TRUE, for example, before an event notification may be delivered. 
     In step S 3 , it is determined whether a new data and/or system event has occurred for which one or more of the clients  370 ,  372 ,  374  and/or  376  has subscribed. According to the present invention, this determination may be made by the trigger(s)  314 . As long as one of the triggers  314  does not fire, it may be concluded that no new data or system event of interest has occurred, and the method remains at step S 3  until a new event of interest occurs. Upon the occurrence of a new event for which one or more of the clients  370 ,  372 ,  374  and/or  376  has subscribed, the rule (if any) specified by one or more of the clients  370 ,  372 ,  374  and/or  376  is evaluated, as shown in step S 4 . If the conditions coded in the rule specified in step S 2  do not evaluate to TRUE or satisfy some other selected condition in step S 5 , the method may return to step S 3 , to resume detection of new data and/or system events. If, however, the conditions coded in the rule specified in step S 2  evaluate to TRUE in step S 5 , the method proceeds to step S 6 , whereupon it is determined whether any of the client applications  370 ,  372 ,  374  and/or  376  who have expressed an interest in receiving notification through a subscription are currently registered, as shown in step S 6 . If non of the clients  370 ,  372 ,  374  and/or  376  who have (through an agent) subscribed to receive notifications of the event referenced by the event name are currently registered, the method of the present invention may revert back to step S 3 . If one or more of the clients  370 ,  372 ,  374  and/or  376  are currently registered, the delivery information specified upon registration is retrieved in step S 7  and the notification is formatted according to the protocol (iiop, ftp, http, etc.) specified by the client(s)  370 ,  372 ,  374  and/or  376  upon registration, as shown in step S 8 . The properly formatted notification, in step S 9 , is then pushed out, or transmitted, to the client(s)  370 ,  372 ,  374  and/or  376 . It is to be noted that a same notification may be sent to several different clients  370 ,  372 ,  374  and/or  376 , each having specified a different rule and delivery information, rule and/or quality of service. Each of these client applications  370 ,  372 ,  374  and/or  376 , in turn, may carry out different actions based upon the received notification. For example, should a notification of a system event such as a system shutdown be received by clients  370  and  372 , client  370  may opt to do nothing because it is not currently accessing the database  310 , whereas the client  372  process may save partial results, shut down in an orderly fashion and resume operations only after receiving notification that the database  310  is once again operational. By promoting inter-enterprise integration in this manner, notifications may be delivered to outside, disconnected client applications  370 ,  372 ,  374  and/or  376  over multiple protocols to allow the clients  370 ,  372 ,  374  and/or  376  to programmatically take some predetermined (e.g., corrective or preemptive) action. 
     If it is determined that the notification sent in step S 9  was successful (such as by the receipt of an acknowledgement from each client  370 ,  372 ,  374  and/or  376  having registered to receive the notification), the method according to the present invention may revert back to step S 3 , to detect later-occurring data and/or system events. If, however, the notification sent in step S 9  was not successfully sent, the method may proceed to step S 11 . The notification may not have been successfully sent if, for example, one of the communications lines between the protocol engine  360  and the clients  370 ,  372 ,  374  and/or  376  has been interrupted, such as may occur in the presence of a disruption in the network  380 . Alternatively, or as a consequence of the failure of the communications lines between the clients  370 ,  372 ,  374  and/or  376  and the protocol engine  360 , the shared memory buffer  340  may be full. Alternatively still, step S 9  may have failed for any number of reasons not specifically enumerated herein. In any event, if the notification was not successfully sent, it may be determined, in step S 11 , whether the quality of service was specified as reliable or unreliable at the time of registration. If the quality of service was not specified as reliable, the method may revert back to step S 3 , it not being critical that the notification is successfully delivered. Other steps (not shown) may be taken to insure that at least some notifications are actually delivered to the registered client applications  370 ,  372 ,  374  and/or  376 , inasmuch as is possible. If, however, the quality of service was specified as reliable, the notification may be written to the persistent storage  345  (such as a hard disk drive, optical memory, etc.). The undelivered notifications written to the persistent storage  345  may, at some later time, be written to the shared memory  340  and later retrieved therefrom by the event monitor  350 . The event monitor  350  may then, in concert with the protocol engine  360 , re-attempt delivery of the notification to the client(s) clients  370 ,  372 ,  374  and/or  376  having registered therefor. Additional filtering may occur prior to re-attempting delivery of the notifications stored in the persistent storage  345 . For example, the client(s)  370 ,  372 ,  374  and/or  376  may be interested in receiving only the latest of a series of notifications relating to a given event name. The method may thereafter revert back to step S 3 , to continue the detection of event names for which subscriptions exist. It should be noted that the steps shown in  FIG. 4  may be carried out in an order that is different than shown. For example, the determination of the quality of service may be made concurrently with step S 7 . Other sequences of steps are possible, and all such combinations are deemed to fall within the scope of the present invention. 
     HARDWARE OVERVIEW 
       FIG. 5  illustrated a block diagram of a computer  500  with which an embodiment of the present invention may be implemented. Computer system  500  includes a bus  501  or other communication mechanism for communicating information, and a processor  502  coupled with bus  501  for processing information. Computer system  500  further comprises a random access memory (RAM) or other dynamic storage device  504  (referred to as main memory), coupled to bus  501  for storing information and instructions to be executed by processor  502 . Main memory  504  also may be used for storing temporary variables or other intermediate information during execution of instructions by processor  502 . Computer system  500  also includes a read only memory (ROM) and/or other static storage device  506  coupled to bus  501  for storing static information and instructions for processor  502 . A data storage device  507 , such as a magnetic disk or optical disk, is coupled to bus  501  for storing information and instructions. 
     Computer system  500  may also be coupled via bus  501  to a display device  521 , such as a cathode ray tube (CRT), for displaying information to a computer uses. An alphanumeric input device  522 , including alphanumeric and other keys, is typically coupled to bus  501  for communicating information and command selections to processor  502 . Another type of user input device is cursor control  523 , such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor  502  and for controlling cursor movement on display  521 . This input device typically has two degrees of freedom in two axes, a first axes (e.g., x) and a second axis (e.g., y), which allows the device to specify positions in a plane. Alternately, a stylus or pen may be used to interact with the display. A displayed object on a computer screen may be selected by using a stylus or pen to touch the displayed object. The computer detects the selection by implementing a touch sensitive screen. Similarly, a light pen and a light sensitive screen may be used for selecting a displayed object. Such devices may thus detect selection position and the selection as a single operation instead of the “point and click,” as in a system incorporating a mouse or trackball. Stylus and pen based input devices as well as touch and light sensitive screens are well known in the art. Such a system may also lack a keyboard such as  522  wherein all interface is provided via the stylus as a writing instrument (like a pen) and the written text is interpreted using optical character recognition (OCR) techniques. 
     The present invention is related to the use of computer system  500  to asynchronously notify client applications  370 ,  372 ,  374  and/or  376  of data and/or system event within the database  310 . According to one embodiment, the methods according to the present invention are implemented by one or more computer systems  500  in response to processor(s)  502  executing sequences of instructions contained in memory  504 . Such instructions may be read into memory  504  from another computer-readable medium, such as data storage device  507 . Execution of the sequences of instructions contained in memory  504  causes processor(s)  502  to perform the process steps that are described above. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement all or selected portions of the present invention. Thus, the present invention is not limited to any specific combination of hardware circuitry and software. 
     While the foregoing detailed description has described preferred embodiments of the present invention, it is to be understood that the above description is illustrative only and not limiting of the disclosed invention. For example, notifications of events of interest may be disabled and later re-enabled by one or more application client clients  370 ,  372 ,  374  and/or  376 . Moreover, events may be triggered not by some significant change in the data or the system, but by a user of the database  310 . These events may also be trapped and notifications therefor may also be sent to one or more of the clients  370 ,  372 ,  374  and/or  376 . Those of skill in this art will recognize other alternative embodiments and all such embodiments are deemed to fall within the scope of the present invention. Thus, the present invention should be limited only by the claims as set forth below.