Patent Publication Number: US-9432240-B2

Title: Flexible failover configuration

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/117,851 filed Apr. 29, 2005, which application is incorporated in its entirety herein by reference. 
    
    
     FIELD OF INVENTION 
     The field of the invention relates to the software arts; and, more specifically, a Flexible Failover Configuration. 
     BACKGROUND 
     Sessions and Information 
     A “session” can be viewed as the back and forth communication over a network between a pair of computing systems. Referring to  FIG. 1 , in the case of a client/server architecture, basic back and forth communication involves a client  101  sending a server  100  a “request” that the server  100  interprets into some action to be performed by the server  100 . The server  100  then performs the action and if appropriate returns a “response” to the client  101  (e.g., a result of the action). Often, a session will involve multiple, perhaps many, requests and responses. A single session through one or more of its requests may invoke the use of different application software programs. 
     An aspect of session management is the use of session state information over the course of a session&#39;s end-to-end lifetime. Session state information is a record of the status and/or use of a session. For example, as part of a server&#39;s response process, the server may save in the session state information a time in the future at which the session is to be deemed “expired” if a next request is not received by the server for that session beforehand. Session state information may also include information used to “pick-up” a session from where it last “left-off” (such as the latest understood state of a client&#39;s web browser), and/or, data or other information sent to the user over the course of the session (such as one or more graphics or animation files whose content is presented to the client as part of the client&#39;s session experience) 
     Persistence 
     In the software arts, “persistence” is a term related to the saving of information. Generally, persisted information is saved in such a fashion such that, even if the entity that most recently used and saved the information suffers a crash, the information is not lost and can be retrieved at a later time despite the occurrence of the crash. For example, if a first virtual machine suffers a crash after using and saving information to persistent storage, a second virtual machine may, subsequent to the crash, gain access to and use the persistently saved information. 
       FIG. 2  provides a simple example of the concept of “persistence” as viewed from the perspective of a single computing system  200 . Note that the computing system  200  of  FIG. 2  includes DRAM based system memory  210  and a file system  220  (noting that a file system is typically implemented with one or more internal hard disk drives, external RAID system and/or internal or external tape drives). Traditionally, the system memory  210  is deemed “volatile” while the file system  220  is deemed “non-volatile”. A volatile storage medium is a storage medium that loses its stored data if it ceases to receive electrical power. A non volatile storage medium is a storage medium that is able to retain its stored data even if it ceases to receive electrical power. 
     Because a file system  220  is generally deemed non-volatile while a system memory  210  is deemed volatile, from the perspective of the data that is used by computing system and for those “crashes” of the computing system effected by a power outage, the file system  220  may be regarded as an acceptable form of persistent storage while the system memory  210  is not. Here, if the computing system saves a first item of data to the system memory  210  and a second item of data to the file system  220  and then subsequently crashes from a power outage, the second item of data can be recovered after the crash while the first item of data cannot. File systems can be internal or external (the later often being referred to as “file sharing” because more than one computing system may access an external file system). 
     Another form of acceptable persistence storage relative to computing system  200  is an external database  230 . A database  230  is most often implemented with a computing system having “database software”. Database software is a form of application software that not only permits data to be stored and retrieved but also assists in the organization of the stored data (typically in a tabular form from the perspective of a user of the database software). Traditionally, database software have been designed to respond to commands written in a structure query language (SQL) or SQL-like format. Here, referring back to  FIG. 2 , if the computing system stores an item of data in the external database and then subsequently crashes, the item of data can still be accessed from the external database. 
     External databases are particularly useful where information is to be made accessible to more than one computing system. For example, if the external database  230  is designed to hold the HTML file for a popular web page, and if the depicted computing system  200  is just one of many other computing systems (not shown in  FIG. 2 ) that are configured to engage in communicative sessions with various clients, each of these computing systems can easily engage is sessions utilizing the popular web page simply by being communicatively coupled to the database. External file systems also exist. 
     Persistence of Session State Information 
       FIG. 3  shows that session management  301  and persistence  302  functions may overlap. Notably, the persistence of a session&#39;s session state information permits the possibility of a session to be successfully continued even if an entity that was handling the session crashes. For example, if a first virtual machine assigned to handle a session crashes after the virtual machine both responds to the session&#39;s most recent client request and persists the corresponding state information, upon the reception of the next client request for the session, a second virtual machine may seamlessly handle the new request (from the perspective of the client) by accessing the persisted session state information. That is, the second virtual machine is able to continue the session “mid-stream” because the session&#39;s state information was persisted. 
    
    
     
       FIGURES 
       The present invention is illustrated by way of example, and not limitation, in the figures of the accompanying drawings in which like references indicate similar elements and in which: 
         FIG. 1  (prior art) shows a network between a client and a server; 
         FIG. 2  (prior art) shows a computing system having internal memory and internal file system coupled to an external database; 
         FIG. 3  shows that session management and persistence functions may overlap; 
         FIG. 4  shows a hierarchical organization of session domains; 
         FIG. 5  shows file system persistent storage interface and an external database persistent storage plug-in; 
         FIG. 6  shows an embodiment of the organization of a file system accessed through a file system persistent storage interface; 
         FIG. 7  shows an embodiment of the organization of a database accessed through a database persistent storage interface; 
         FIG. 8  (prior art) shows a prior art computing system; 
         FIG. 9  shows an improved computing system that employs a shared memory that stores shared closures; 
         FIG. 10  shows a shared session context within a shared memory; 
         FIG. 11 a    shows an embodiment of a session state shared closure; 
         FIG. 11 b    shows an embodiment of a session management criteria shared closure; 
         FIG. 12  shows a process for accessing session state attributes from shared memory; 
         FIG. 13  shows a process for adding a session to a session management table stored in shared memory; 
         FIG. 14  shows a process for deleting a session from a session management table stored in shared memory; 
         FIG. 15  shows a process for deleting expired sessions from a session management table stored in shared memory; 
         FIG. 16  shows a deployment descriptor for specifying a particular session management persistence strategy; 
         FIG. 17  (prior art) shows a cluster of computing systems; 
         FIG. 18  shows a process for deploying applications according to a particular session management persistence strategy; 
         FIG. 19  shows an embodiment of a computing system&#39;s hardware design. 
     
    
    
     SUMMARY 
     A method is described that involves offering a user different persistent scope choice including: a) internal to a computing system that the deployment descriptor is to be sent to; and, b) external to the computing system that the deployment descriptor is to be sent to. The method also involves offering a user different persistence frequency choices including: a) persisting per request; and, b) persisting per session state information attribute change. The method also involves generating a deployment descriptor that reflects the user&#39;s choice of the persistence scope and persistence frequency. 
     DETAILED DESCRIPTION 
     1.0 In-Memory Session Domains 
     According to an object oriented implementation, state information for a particular session may be stored in a “session” object. In the context of a request/response cycle (e.g., an HTTP request/response cycle performed by a server in a client/server session), the receipt of a new request for a particular session causes the session&#39;s session object to be: 1) retrieved from some form of storage; 2) updated to reflect the processing of the response to the request; and, 3) re-stored so that it can be retrieved for the processing of the next request for the session. Here, a session object may be created by the server for each session that the server recognizes itself as being actively engaged in. Thus, for example, upon the receipt of a first request for a new session, the server will create a new session object for that session and, over the course of the session&#39;s lifetime, the server can retrieve, update and re-store the session object (e.g., for reach request/response cycle if necessary). 
     The server may be a Java 2 Enterprise Edition (“J2EE”) server node which support Enterprise Java Bean (“EJB”) components and EJB containers (at the business layer) and Servlets and Java Server Pages (“JSP”) (at the presentation layer). Of course, other embodiments may be implemented in the context of various different software platforms including, by way of example, Microsoft .NET, Microsoft Transaction Server (MTS), the Advanced Business Application Programming (“ABAP”) platforms developed by SAP AG and comparable platforms. For simplicity, because a server is a specific type of computing system, the term “computing system” will be largely used throughout the present discussion. It is expected however that many practical applications of the teachings provided herein are especially applicable to servers. 
     At any given time, a computing system may be engaged in a large number of active sessions. As such, in the simple case where a session object exists for each session that the computing system is actively engaged in, the computing system will have to manage and oversee the storage of a large number of session objects.  FIG. 4  shows a session management layer  410  that is designed to store session state data according to a hierarchical scheme  402 . According to a basic approach, session objects that are to be treated according to a same set of session management criteria are stored within a same region of the hierarchy. 
     Thus, for example, as depicted in  FIG. 4 , each of the S sessions associated with session objects  407   1  through  407   S  will be treated according to a first set of session management criteria because their corresponding sessions objects  407   1  through  407   S  are stored in session domain  1   404   1 ; and, each of the T sessions associated with session objects  408   1  through  408   T  will be treated according to a second set of session management criteria because their corresponding sessions objects  407   1  through  408   T  are stored in a session domain  2   404   Y . In an implementation, each session domain is accessed through reference to a specific region of memory (e.g., through a hashtable that maps keys to values). 
     Here, again according to a basic approach, the treatment applied to the S sessions will be different than the treatment applied to the T sessions because their respective session objects are stored in different storage domains  404   1 ,  404   Y  (because different session domains correspond to unique combinations of session management criteria). Session management criteria generally includes one or more parameters that define (or at least help to define) how a session is to be managed such as: 1) the session&#39;s maximum inactive time interval (e.g., the maximum amount of time that may elapse between the arrival of a request and the arrival of a next request for a particular session without that session being declared “expired” for lack of activity); 2) the maximum number of outstanding requests that may be entertained by the computing system for the session at any one time; 3) the session&#39;s access policy (which indicates whether the session can be accessed by multiple threads (i.e., is multi-threaded) or can only be accessed by a single thread); 4) the session&#39;s invalidation policy (e.g., whether or not the session object for an inactivated session is persisted or not); and, 5) the type of persistent storage to be used. 
     According to one approach, one or more session management criteria items for a particular session domain (such as any combination of those described just above) is stored within the session domain separately from the one or more session objects that are stored within the same session domain. Such an approach may improve efficiency because, at an extreme, the session objects need not carry any session management criteria information and may instead carry only pure session state information (e.g., the expiration time of the session, the most recent understood state of the client&#39;s web browser, etc.). 
     In the case of an application server computing system, clients engage in sessions with the computing system so that they can use application software located on the computing system. The application software (including any “pages” such as those from which the execution of a specific software routine may be triggered) may be run on the computing system and/or downloaded to and run on the client. In an approach that may be alternate to or combined with the basic embodiment described just above in which session domains are reserved for unique combinations of session management criteria, session domains may be established on a “per application” basis. That is, a first session domain may be established in the hierarchy for a first application, and, a second session domain may be established in the hierarchy for a second application. In an alternate or combined implementation, entire hierarchy trees (each having its own root node  403 ) are instantiated on a “per application” basis. 
     Depending on implementation preference, different applications having identical session management treatment policies may have their session objects stored in the same session domain or in different session domains. Moreover, again according to implementation preference, a single session domain may be created for the session objects of sessions that deserve “related” rather than “identical” session management treatment (e.g., for a particular session domain, some but not all session management criteria characteristics are identical; and/or, a range of possible criteria values is established for a particular session domain such as sessions having a maximum inactive time interval within a particular time range). 
     The storage hierarchy may also include the notion of parent and children nodes. In one configuration, a child node has the same lifecycle as its parent node. Thus, if the parent domain is destroyed all of its children nodes are destroyed. The domain hierarchy provides different space for the sessions (session domains) grouping it in logical structure similar to the grouping files to the folders. According to an implementation, the root  403  is used to provide high-level management of the entire hierarchy tree rather than to store session information. For example, if an application consists of many Enterprise Java Bean (EJB) sub applications you should be able to manage the sessions of different EJB components separately. As such, isolation between different EJB sub-applications is achieved by instantiating different session domains for different EJB components, while at the same time, there should exist the ability to manage the entire application as a whole. 
     For example, to remove all sessions of an application after removal of the whole application. Grouping the sessions in tree structure (where the root presents the application and the various children present the different ejb&#39;s) you can easily destroy all the sessions after removing the application simply by destroying the root. 
     The nomenclature of session domains  404   1  through  404   Y  is meant to convey that a wide range of different session domains (at least Y of them) may be established for a particular “context”. Different hierarchy “contexts”  402   1 ,  402   2 , . . .  402   X  are depicted in  FIG. 4 . The processing of a complete request/response often involves the processing of different active segments of code within the computing system. For example, a first segment of code may setup and teardown the specific “ports” through which a session&#39;s data flows in and out of the computing system, a second segment of code may handle the protocol between the client and computing system (e.g., an HTTP or HTTPS protocol), a third segment of code may be the actual application software that is invoked by the client (e.g., specific objects or components in an object oriented architecture (such as Enterprise Java Beans (EJBs) in a Java environment)). 
     Each of these different segments of code may depend on their own session state information in order to support the session over the course of its lifetime. As such, in an embodiment, different contexts are established, each containing its own hierarchy of session domains, for the different segments of code. For example, continuing with the example provided just above, context  402   1  may be used to store client/server protocol session state information, context  402   2  may be used to store computing system port session state information, and context  402   X  may be used to store EJB session state information. For simplicity the remainder of the present application will focus largely on client/server communication protocol session state information (e.g., HTTP session state information) that is kept in object(s) within an object oriented environment. 
     Referring to  FIG. 4 , a session management layer  410  is observed interfacing with the session domain storage hierarchy  402  to support sessions between one or more clients and software applications  411   1  through  411   K . According to one implementation, the session management layer  410  accesses  406   1  . . . ,  406   Y  the information stored in their respective session domains on behalf of the applications  411   1  through  411   K . Alternatively or in combination, the applications  411   1  through  411   K  may access session domain information directly. 
     Typically, the session contexts  402   1  through  402   X  and their associated session domains are essentially “cached” in the computing system&#39;s volatile semiconductor random access memory (e.g., within the computing system&#39;s system memory and/or the computing system&#39;s processors&#39; caches) so that rapid accesses  406   1  through  406   Y  to the session state and/or session management criteria information can be made. As alluded to in the background, the cached (“in-memory”) session state information may be persisted to a file system or database to prevent service disruption in case there is some operational failure that causes the cached session state information to no longer be accessible. Moreover, consistent with classical caching schemes, session state information that has not been recently used may be “evicted” to persistent storage to make room in the computing system memory for other, more frequently used objects. Thus, access to persistent storage for session information not only provides a “failover” mechanism for recovering from some type of operational crash, but also, provides a deeper storage reserve for relatively infrequently used session information so as to permit the computing system as a whole to operate more efficiently. 
     2.0 File System and Database Persistent Storage Interfaces 
     The session management layer  410  is responsible for managing the persistence of session objects consistently with the session management criteria that is defined for their respective session domains. Because of the different types of file systems that may exist, the syntax and/or command structure used to store and/or retrieve information to/from a particular file system may differ from that of another file system. Moreover, the types of activities that are performed on a file system with persisted information (most notably the storing and retrieving of persisted information) tend to be the same regardless of the actual type of file system that is implemented. That is, the high level operations performed on a file system with persisted information generally are independent of any particular file system. 
     In order to enable the straightforward configuration of a particular session domain whose content is to be persisted into a specific type of file system, some kind of easily configurable translation layer is needed whose functional role, after its instantiation and integration into the deployed software platform as a whole, is to interface between a set of high level persistence related commands and a specific type of file system. By so doing, source code level developers can develop the session management layer  410  and/or applications  411   1  through  411   K  that invoke persisted information be referring only to a high level command set. 
     Upon actual deployment of the executable versions of the source code, which approximately corresponds to the time at which the actual file system to be used for persistence is actually identifiable, the translation layer is instantiated to translate between these high level commands and the specific syntax and/or command set of the particular type of file system that is to be used to persist the data. According to one implementation, a unique block of translation layer code is instantiated for each session domain (i.e., each session domain is configured to have “its own” translation layer code). According to another implementation, a unique block of translation layer code is instantiated for each file system (i.e., each file system is configured to have “its own” translation layer code). 
     During actual runtime, in order to store/retrieve persisted information, the session management layer  410  and/or certain applications essentially “call on” a translation layer (through the high level command set) and use it as an interface to the translation layer&#39;s corresponding file system. For clarity, a translation layer as described just above will be referred to by the term “session persistence storage interface” or simply “persistence storage interface” or “persistence interface”. 
       FIG. 5  shows persistence storage interface models for a file system and for an external database. Specifically, persistence storage interface  510   1  is depicted as being an interface to a file system  513 ; and, persistence storage interface  510   Y  is depicted as being an interface to external database  514 . Persistence storage interface  510   1  is depicted as being the persistence interface between cached session domain  504   1  and file system  513 . Persistence storage interface  510   Y  is depicted as being the persistence interface between cached session domain  504   Y  and external database  514 . Here, note the similarity in nomenclature between  FIGS. 4 and 5  with regard to session domains  404   1,Y  and  504   1,Y  suggesting that session domain  404   1  of  FIG. 4  could be configured to use file system  510   1  as its persistent storage medium, and, that session domain  404   Y  of  FIG. 4  could be configured to use database  514  as its persistence storage medium. 
     Activities  506   1  and  506   Y  are meant to depict any activities stemming from the session management layer and/or any applications that are imparted upon session objects  507   1  through  507   S  and  508   1  through  508   T , respectively, and/or upon their respective session domains  504   1 ,  504   Y  as a whole. As described above, these activities  506   1 ,  506   Y  may involve method calls to the respective persistence storage interfaces  510   1 ,  510   Y . Here, activities  509   1  and  509   Y  are meant to depict the transfer of session state information between the cached session domain  504   1 ,  504   Y  and the corresponding persistence storage medium  513 ,  514 . 
     Notably, the depicted persistence interface models  510   1 ,  510   Y  use a “plug-in” architecture. A “plug-in” is a pre-existing segment of code that is not integrated into a larger software system, ultimately as a working component of the larger software system, unless an affirmative command to integrate the plug-in into the larger system is made. According to an implementation, a separate plug-in exists for different types of persistence storage implementations that may exist. For example, currently, there are different types of file systems and databases available on the open market that a “user” of the software may choose to use for persistence purposes. 
     In the case of databases, different database software vendors presently exist such as Oracle, IBM and Microsoft. Each of these different vendors tend to have an SQL based command language (e.g., insert). Often times, different command statements are needed to perform identical operations across different database software implementations. Therefore, according to the approach of  FIG. 5 , a separate database plug-in  512  is written for the various types of “supported” persistence database solutions that an ultimate end user of the computing system&#39;s application software may choose to implement. For example, a first plug-in may exist for database software offered by Oracle (e.g., Oracle Database based products), a second plug-in may exist for database software offered by IBM (e.g., IBM DB2), and, a third plug-in may exist for database software offered by Microsoft (e.g., Microsoft SQL Server based products). 
     According to an approach, upon deployment of software to a particular computing system and surrounding infrastructure (i.e., when a particular type of database software being used for persistence is actually known), the plug-in  512  for a particular type of database software is integrated into the computing system&#39;s software. According to a further embodiment, a separate database software plug-in is “plugged in” (i.e., integrated into the computing system&#39;s software) for each session domain having persistence to a database. Here, if different session domains are configured to persistent to a same database, some queuing space may need to be implemented between the session domains&#39; plug-ins and the database in order to handle quasi-simultaneous persistence operations made to the database from different session domains. 
     According to an alternate embodiment, a separate database plug-in is “plugged in” for each different database that is used for persistence (i.e., the persistence functions for different session domains that use the same database flow through the same plug-in). According to this implementation, some queuing space may need to be implemented between the plug-in for a particular database and the different session domains that persist to that database in order to handle quasi-simultaneous persistence operations made to the database. 
     According to another alternate embodiment, a separate database plug-in is “plugged in” for each different type of database that is used for persistence (i.e., the persistence functions for different session domains that use the same database command language (but perhaps different actual database instances) flow through the same plug-in). According to this implementation, some queuing space may need to be implemented between the plug-in for a particular database type and the different session domains that persist to that database type in order to handle quasi-simultaneous persistence operations made to the same type database. Moreover, additional queuing space may need to be implemented between the plug-in for a particular database type and the different actual instances of that database type that the session data is actually persisted to. 
     According to the approach of  FIG. 5 , a separate database plug-in  511  is written for the various types of “supported” file systems that a customer of the software may choose to persist to. Upon deployment of the software to a particular platform (again, when the particular type(s) of file system(s) being used for persistence is actually known), the plug-in  511  for a particular file system is integrated into the software. Consistent with the principles outline just above, file system plug-ins may be “plugged in” on a per session domain basis, a per file system basis, or, a per file system type basis. Separate plug-ins may be written for each of various vendor-specific or open sourced file systems. 
     According to an embodiment, each plug-in essentially serves as a translator that translates between a generic command set and the command set particular to a specific type of persistence. According to one implementation, the generic command set is not specific to any particular persistence type. Referring briefly back to  FIG. 4 , by designing the persistence commands called out by the session management layer  410  and/or applications  411   1  through  411   K  with the generic command set, after deployment and during runtime, the plug-in for a particular type of persistence will translate the generic commands from the session management and/or an applications into commands that are specific to the particular type of persistence that has been implemented (e.g., a particular type of file system or database). As such, ideally, the source code designers of the session management function and/or applications need not concern themselves with particular types of persistence or their corresponding command languages. 
     2.1 Embodiment of File System Persistent Storage Interface 
       FIGS. 6 and 7  depict additional details about a persistent storage interface for a file system and database, respectively. Referring firstly to  FIG. 6 , activity  616  represents the activity that may be imposed upon the memory based session domain  604  by a session management layer and/or one or more applications; and, activity  626  represents the activity that may be imposed upon a specific session object (in particular, session object  607   1 ) by a session management layer and one/or more applications. Here, because session domain  604  is configured to be “backed up” by persistent storage  613 , note that, from a communicative flow perspective, a session persistent storage interface  610  containing plug-in code  611  resides between the session objects  607   1  through  607   S  and the file system persistence storage  613 . 
     As described above with respect to  FIG. 5 , the persistence interface  610  comprehends a generic command set that the plug-in  611  is able to convert into commands that are specific to the particular type of file system that persistence storage  613  corresponds to (e.g., Linux based file systems (e.g., Ext, Ext2, Ext3), Unix based file systems (e.g., Unix Filing System (UFS), IBM based file systems (e.g., IBM Journaled File Systems (JFS), IBM General Parallel File System (GPFS)), Oracle based file systems (e.g., Oracle Clustered File System (OCFS), Oracle Real Application Clusters (RAC), Oracle Automatic Storage Management (OASM)), ReiserFS based files systems, Microsoft based file systems (e.g., FAT16, FAT32, NTFS, WINFS), etc.). 
     The application of the generic commands to the persistence storage interface  610  is represented as activity  631 . Here, certain activity  616  applied to the session domain as a whole, as well as certain activity  626  applied to a specific session object, will trigger activity  631  at the persistent storage interface  610  so that the information and/or structure of the memory based session domain  604  is effectively duplicated with the persistent storage  613 . 
     According to the perspective of  FIG. 6 , a “model” of a session object is persisted within the persistence storage  613 . Because the particular persistent storage  613  of  FIG. 6  is a file system, and because file systems tend to be organized through a hierarchy of directories that contain files having data, the persisted session object “model” of  FIG. 6  corresponds to a set of files stored within a directory. As an illustrative example,  FIG. 6  shows an exploded view of the specific model  650  that has been persisted for session object  607   1 . Here, a directory  627   1  has been created for session object  607   1 , and, the contents of this directory  627   1  include separate files  651   1  through  651   7  that, when taken together, correspond to the session information contained by session object  607   1  that is persisted by file system  613 . Note that separate directories  627   2  through  627   S  have been created for each of session objects  607   2  through  607   S , respectively. 
     The plug-in  611  is primarily responsible for, in response to the command activity  631  presented at the persistent storage interface  610 , creating and deleting directories in the file system  613  and creating, deleting, reading and writing to files in the file system  613 . According to one embodiment, the persistent storage interface  610  with its plug-in  611  causes the hierarchical structure of the directory that is persisted in the file system  613  to approximately mirror the hierarchical structure of the “in-memory” session domain  604 . For example, referring to  FIGS. 4 and 6 , in the simplest case where the file system  613  is the sole persistence resource for session contexts  402   1  through  402   X , the persistence interface  610  with its plug-in  611  creates a failover directory  625  in the file system where all persisted session information is kept. Here, a separate directory is then established within the failover directory  625  for each session context  402   1  through  402   X  in the session domain ( FIG. 6  only shows one such directory  624  which has been created for session context  624 ). 
     Within the directory for a particular context, directories are created for each session domain within the context.  FIG. 6  only shows one such directory  623  created for session domain  404   1 , at least Y such directories would be created within context directory  624 . The directory for a particular session domain, such as directory  623 , contains a directory for each session object that is associated with the session domain. Thus, as observed in  FIG. 6 , a separate directory  627   1  through  627   S  exists for each of session objects  607   1  through  607   S , respectively. If session domain  404   1  has any children session domains (not shown in  FIG. 4 ), directory  627   1  would contain additional directories for these children session domains (that, in turn, would contain directories for their corresponding session objects and children session domains). 
     Notably, the various contents of session object  607   1  are broken down into separate “attributes”, and, a separate file is created and stored in directory  627   1  for each attribute of session object  607   1  (or at least those attributes deemed worthy of persisting). According to the exemplary depiction of  FIG. 6 , session object  607   1  has “J” different attributes that are persisted through files  651   1  through  651   J , respectively. An attribute generally corresponds to a specific item of data that is a component of a session&#39;s session state information. Generic examples of attributes include and the session&#39;s sessionID and expiration time. However, in practice, attributes tend to be specific to the particular information that is persisted for a particular session. For instance, in the case of a “session” Enterprise Java Bean (EJB) various “attributes” of session state information that are specific or unique to the EJB will be persisted. 
     According to one embodiment, the following set of generic commands may be presented at the session persistent storage interface  610  for purposes of managing the persisted session domain as a whole. Communicative flow  632  is meant to depict this high level management view. Input arguments for the command methods are presented in parenthesis. 
     1. Create_Model (sessionID). The “Create_Model” command creates a model for a specific directory in the file system  613  for a specific session. As described above, according to one implementation, a unique session object is created for each unique session. When the computing system recognizes a new session, a new session object is created for that session, a session domain for that session object is identified or created, and, the Create Model command is called at the persistent storage interface  610  for the session domain  604 . In response to the Create Model command, the plug-in  611  creates a directory in the file system for the new session (e.g., directory  627   1 ) within the file system directory established for the session domain (e.g., directory  623 ). In an implementation, the plug-in  611  creates the directory though a “model” object that represents the new directory and contains “handlers” to the file system. For example, in the case of file system persistence, the model object keeps a reference to the java.ioFileOutputStream which is the object that provides the physical access to the file system. In one embodiment, the persistence storage interface  610  creates a mapping between the session object, the persistence model object and a specific directory in the file system  613 . That is, in this embodiment a one-to-one correspondence exists between sessions within the session domain  604  and model objects managed by the persistent storage interface. As session data is modified, the mapping ensures that the session data stored within the file system remains consistent with the session object (i.e., via the file system handlers). 
     When a new session is created it is assigned an identification code—referred to as the “SessionID”. Here, if per-session domain persistent storage interfaces are instantiated, the interface  610  need only be given the SessionID as the input argument in order to perform the appropriate operations upon the file system (assuming the interface  610  is configured to “know” as background information the identity of the file system it interfaces to as well as the session domain it services). The remainder of the commands below are described so as to apply to per-session domain persistent storage interfaces, but, the arguments given with the commands could be extended to include the identity of the session domain if persistence storage interfaces are instantiated per file system, or the identity of the session domain and a specific file system if per-file system type interfaces are instantiated. 
     2. Get_Model (sessionID). The “Get Model” command retrieves the entire persisted content for a session. Thus, for example, if the GetModel command where executed for the session for which directory  627   1  was created, the session model object of the plug-in  611  would read each of attribute files  651   1  through  651   J  from the file system. 
     3. Remove_Model (model). The “Remove Model” command essentially deletes the persisted information for a particular session. For example, if the Remove Model command were executed for the session for which directory  627   1  was created, the session model object of the plug-in  611  would delete directory  627   1  and all its contents from the (i.e., attribute files  651   1  through  651   J ) from the file system. A Remove Model command is often executed for a session after the session has been deemed no longer functioning (e.g., “expired”, “corrupted”, etc.). 
     4. Iterator. The “Iterator” command is called so that specific attribute files mapped to model objects can be fetched from each session directory within the session domain directory. According to one implementation, in response to the Iterator command, the interface  610  creates and returns to the caller of the Iterator command an “Iterator” object. An iterator object, such as Java&#39;s java.util.lterator interface object, is an object having methods to fetch some or all elements within a collection. Thus, for example, if the session management layer wanted to view the first attribute within each of session directories  627   1  through  62 ′ 7   S , it would first call the Iterator command at the persistence interface  610 . 
     The interface  610  would then return an Iterator object to the session management layer in response. The session management layer could then use the Iterator object to fetch the first attribute within each session directory. According to one embodiment, the interface  610  creates the iterator object so that is it executes a sequence of “Get Attribute(s)” commands at the interface  610  (specifically, one “Get Attribute(s)” command for each session directory within the session domain directory), where, the desired attribute(s) are specified by the caller of the “Iterator” command. The Get Attribute(s) command is described in more detail further below. 
     5. Iterator_All_Expired. The “Iterator_All_Expired” command operates similarly to the Iterator command, except that the created Iterator object only has visibility into the session directories of sessions that are deemed expired. According to an implementation, execution of the Iterate_All_Expired function involves the interface  610  having to first identify those sessions that are deemed expired and then having to create an Iterator object whose sequence of Get Attribute(s) commands only read into those session directories identified as being expired. In order for the interface  610  to determine which sessions have expired, the session domain directory  623  can be configured to include information sufficient for the determination to be made. 
     For example, in one embodiment, one of the attributes within each session object and its corresponding persisted directory is the time at which the corresponding session is deemed to be expired. If the plug-in  611  reads this attribute and the present time is beyond the time recorded in the attribute, the session is deemed “expired” (accordingly, note that the plug-in  611  should write the expiration time for a session into the appropriate attribute of the session&#39;s corresponding session directory each time a new request is received for the session). 
     6. Remove_All_Expired. The “Remove_All_Expired” command is used to delete all session directories from a session domain directory whose corresponding sessions are deemed expired. Here, consistent with the discussion provided just above with respect to the Interator_All_Expired command, session directories can be identified as being expired if they are designed to contain an attribute that identifies them as being expired; or, if the session domain directory contains information sufficient for the interface  610  to determine which sessions are expired. 
     Whereas the above commands provide session domain-wide management functions for a persisted session domain, in a further embodiment, the interface  610  and model object of its plug-in  611  are also written to support the following command set for managing the information that is persisted for a particular session (e.g., for managing a particular model&#39;s information). Each of the commands below can be assumed to be identified to the interface  610 , in some way, as pertaining to a particular session within the session domain. 
     1. Get_Session_ID. Execution of the Get_Session_ID command causes the plug-in  611  to read the sessionID for the particular session that the command is called on behalf of Note that, accordingly, the sessionID corresponds to information contained in one of the attributes associated with a session&#39;s persisted session state information. 
     2. Get_Expiration_Time. Execution of the Get_Expiration_Time command causes a model object of the plug-in  611  to read the expiration time for the particular session that the command is called on behalf of Note that, accordingly, the expiration time corresponds to information contained in one of the attributes associated with a session&#39;s persisted session state information. 
     3. Update_Expiration_Time (maximum inactive time interval). Execution of the Get_Expiration_Time command causes the model object within the plug-in  611  to write a new expiration time for the particular session that the command is called on behalf of According to one implementation, as observed above, the maximum inactive time interval is presented as an input argument for the Update_Expiration_Time command. Here, the expiration time is calculated by the interface  610  (or its plug-in  611 ) simply by adding the maximum inactive time interval to the present time. 
     In an embodiment, even though the maximum inactive time interval is more properly viewed as session management criteria information, the inactive time interval is “tagged along with” the expiration time or is recognized as its own separate attribute within the client&#39;s session state information. Typically, a new expiration time is calculated with each newly arriving request for a particular session (or, with each completed request/response cycle for a particular session). 
     Note that, according to an implementation, the “present time” used for calculating the expiration time is taken from a clock within the computing system. Although not entirely relevant for internal file systems, using a clock from an external persistence resource (such as an external database or RAID system) could cause unequal expiration treatment across different persistence resources if the clocks from the different persistence resources do not have identical core frequencies. Calculating the expiration time from the persistence interface  610  or plug-in  611  keeps the core frequency the same (i.e., a clock within the computing system is used) across all session irregardless of each session&#39;s particular persistence resource. 
     4. Get_Attribute(s) (attribute(s)). Execution of the Get_Attribute(s) command causes a model object within the plug-in  611  to read one or more specific attributes identified by the caller of the command (e.g., the session management layer or an application). The “attribute(s)” argument identifies the specific attribute(s) (i.e., specific file(s)) that are to be retrieved. 
     In an implementation each one of these attributes (as well as the sessionID and expiration time) can be obtained by explicitly calling for it in the attribute(s) argument of the Get_Attribute command. Moreover, as part of managing the visual presentation that is rendered on the client over the course of the session, the attributes of a session&#39;s session state information may also include fairly large graphics files. In this case, the session object is used to implement a kind of caching scheme for certain visual images that are to be displayed on the client over the course of the session. 
     It is in this respect that size management of a session object and its persisted information may become an issue. If a session object were to contain a number of such large graphics files, reading/writing all of its contents to/from persistence storage  613  as standard persistence accessing procedure would be inefficient. By granularizing a session object&#39;s content into smaller attributes, and by making these attributes separately accessible to/from persistent storage  613 , a single large graphics file can be individually read from persistence storage only if the file is actually needed—for instance (i.e., large graphics files that are not needed are not identified in the attribute(s) input argument and remain in persisted storage). 
     Perhaps more importantly, if only a relatively small piece of session state information is actually needed (e.g., just the expiration time), only that small piece of session state information can be read from persistent storage (i.e., the retrieval of un-desired large graphics file is avoided). Hence, the ability to specifically target only certain portions of a persisted session object results in more efficient operation as compared to an environment where only the entire content of a session object&#39;s contents can be read from or written to persistence. 
     Note that, according to an implementation, all attributes are individually accessible—not just those used for the storage of graphics files. Here, communicative flows  634   1  through  634   J  are meant to convey the individual accessibility of each of persisted attributes  651   1  through  651   S . In an implementation alternative to that described above, only a singleton “attribute” is presented as an input argument and separate Get Attribute(attribute) commands have to be called to retrieve more than one attribute. 
     5. Put_Attribute(s)(attribute(s)). Execution of the Put_Attribute(s) command enables the writing of individual attributes into persistent storage via the model object consistent with the same principles outlined above for the Get_Attribute(s) command. Notably, in an implementation, execution of a Get_Attribute(s) or Put_Attribute(s) command does not involve serialization of the attribute data. Traditionally, persisted objects have been serialized (into a “byte array”) prior to their being persisted so as to enable their transport across networks and/or enable their reconstruction (through a process referred to as deserialization) upon being recalled from persistent storage. Serialization and deserialization can be an inefficient, however, and accessing the attributes(s) in a non serialized format should eliminate inefficiencies associated with serialization/deserialization processes. 
     6. Get_Attribute(s)_Serialized(attributes(s)). Execution of the Get_Attribute(s)_Serialized command is essentially the same as the Get_Atribute(s) command described above, except that the persistence storage interface  610  (or model object within the plug-in  611 ) performs deserialization on attribute data read from persistent storage. 
     7. Put_Attribute(s)_Serialized(attributes(s)). Execution of the Put_Attribute(s)_Serialized command is essentially the same as the Put_Atribute(s) command described above, except that the persistence storage interface  610  (or model object within the plug-in  611 ) performs serialization on attribute data written to persistent storage. 
     According to one implementation, session management criteria information for a particular session domain (e.g., maximum inactive time interval, maximum number of outstanding requests, access policy, etc.) is kept in the in memory session domain  604  separately from the session domain&#39;s session objects  607   1  through  607   S  (e.g., in another object not shown in  FIG. 6 ); and, is persisted to a file in the session domain&#39;s directory  623  (also not shown in  FIG. 6 ) along with the individual session directories  627   1  through  627   S . Recalling the discussion provided above in Section 1.0 near the onset of the discussion of  FIG. 4 , separating session management criteria information from session state information keeps the size of the session objects  607   1  through  607   S  (and the size of their corresponding persisted directories  627   1  through  627   S ) beneath the size they would otherwise be if they were designed to include the session management criteria information themselves. 
     2.2 Embodiment of Database Persistent Storage Interface 
       FIG. 7  depicts additional details about a persistent storage interface for a database. Similar to  FIG. 6 , activity  716  of  FIG. 7  represents the activity that may be imposed upon the memory based session domain  704  by a session management layer and/or one or more applications; and, activity  726  represents the activity that may be imposed upon a specific session object (in particular, session object  708   1 ) by a session management layer and one/or more applications. Here, because session domain  704  is configured to be “backed up” by database  714 , note that, from a communicative flow perspective, the persistent storage interface  710  containing plug-in code  711  resides between the session objects  708   1  through  708   T  and the persistence storage  714 . 
     As described above with respect to  FIGS. 5 and 6 , the persistence interface  710  comprehends a generic command set that the plug-in  711  is able to convert into commands that are specific to the particular type of database that persistence storage  714  corresponds to. The application of the generic commands to the persistence storage interface  710  is represented as activity  731 . Here, certain activity  716  applied to the session domain as a whole, as well as certain activity  726  applied to a specific session object, will trigger activity  731  at the persistent storage interface  610  so that the information and/or structure of the memory based session domain  704  is effectively duplicated with the persistent storage  714 . 
     According to the perspective of  FIG. 7 , a “model” of a session object is persisted within the persistence storage  714 . Because the particular persistent storage  714  of  FIG. 7  is a database, and because database data tends to be organized with a table, the persisted session object “model” of  FIG. 7  corresponds to a row within a table  723  that has been established for the session domain  704 . As an illustrative example,  FIG. 7  shows tables rows  751 ,  752 , . . .  75 S as being the persisted models for session objects  607   1 ,  607   2 , . . .  607   S , respectively. The different session object attributes correspond to different columns within the table  723 . 
     For simplicity, the particular table  723  of  FIG. 7  is drawn so as to only apply to session domain  704 . In an implementation, table  723  is a segment of a larger table in database  714  whose organization reflects the hierarchy of an entire session domain context as observed in  FIG. 4 . For example, the first table column may be reserved to identify the context, the second table column may be reserved to identify the particular session domain within the session context (e.g., root/session_domain_ 1 ), and, the third table column may include the sessionID (which has been depicted as the first table column in  FIG. 7 ). A row for a particular session is therefore accessible by matching on the first three columns in the table. 
     The plug-in  711  is primarily responsible for, in response to the command activity  731  presented at the persistent storage interface  710 , creating and deleting rows in the table  723  as well as reading from and writing to the rows in the table  723 . According to a further implementation, a similar table is created in the database for each in-memory session domain that is persisted to the database  714 . 
     According to one embodiment, the following set of generic commands may be presented at the session storage interface  610  for purposes of managing the persisted session domain as a whole. Communicative flow  732  is meant to depict this high level management view. Input arguments for the command methods are presented in parenthesis. Note that, consistent with the discussion provided above with respect to  FIG. 5 , in order to provide different types of persistent storage tansparently to higher layers of software within the computing system (e.g., a session management layer and/or one or more applications), the command set is identical to the command set discussed above in  FIG. 6  with respect to file systems. The only difference is the operations performed by the corresponding plug-ins  611 ,  711 . 
     1. Create_Model (sessionID). The “Create_Model” command creates a row in the database table  723  for a specific session. As described above, according to one implementation, a unique session object is created for each unique session. When the computing system recognizes a new session, a new session object is created for that session, a session domain for that session object is identified or created, and, the Create Model command is called at the persistent storage interface  710  for the session domain  704 . In response to the Create Model command, the plug-in  711  (e.g., through a “model” object) creates a row in the table for the new session. When a new session is created it is assigned an identification code—referred to as the “SessionID”. In one embodiment, the persistence storage interface  610  creates a mapping between the session object, the persistence model object for that session and a specific row in the database table  723 . That is, in this embodiment a one-to-one correspondence exists between sessions within the session domain  604  and model objects managed by the persistent storage interface  610 . As session data is modified, the mapping ensures that the session data stored within the database remains consistent with the session object. 
     Here, if per-session domain persistent storage interfaces are instantiated, the interface  710  need only be given the SessionID as the input argument in order to perform the appropriate operations upon the file system (assuming the interface  710  is configured to “know” as background information the identity of the database it interfaces to as well as the session domain it services). The remainder of the commands below are described so as to apply to per-session domain persistent storage interfaces, but, the arguments given with the commands could be extended to include the identity of the session domain if persistence storage interfaces are instantiated per database, or the identity of the session domain and a specific database if per-database type interfaces are instantiated. 
     2. Get_Model (sessionID). The “Get Model” command retrieves the entire persisted content for a session. Thus, for example, if the GetModel command where executed for the session for which row  751  was created, the model object within the plug-in  711  would read each of the J attribute files in row  751  from the database table  723 . 
     Remove_Model(model). The “Remove Model” command essentially deletes the persisted information for a particular session. For example, if the Remove Model command were executed for the session for which row  751  was created, the model object within the plug-in  711  would delete row  751  from the database table  723 . A Remove Model command is often executed for a session after the session has been deemed no longer functioning (e.g., “expired”, “corrupted”, etc.). 
     Iterator. The “Iterator” command returns an “Iterator” object having visibility into all rows in the database table  723 . According to one embodiment, the interface  710  creates the iterator object so that it executes a sequence of “Get Attribute(s)” commands at the interface  710  (specifically, one “Get Attribute(s)” command for each row within the table  723 ), where, the desired attribute(s) are specified by the caller of the “Iterator” command. In this manner, the same one or more attributes can be retrieved from each row in the database table  723 . 
     Iterator_All_Expired. The “Iterator_All_Expired” command operates similarly to the Iterator command, except that the created Iterator object only has visibility into the session directories of sessions that are deemed expired. According to an implementation, the interface  710  first identifies those sessions that are deemed expired and then creates an Iterator object whose sequence of Get Attribute(s) commands only read into those session directories identified as being expired. In order for the interface  710  to determine which sessions have expired, the session attributes can be configured to include information sufficient for the determination to be made. 
     For example, one of the attributes within each session object (and corresponding database table column) is the time at which the corresponding session is deemed to be expired. If the model object within the plug-in  711  reads this attribute and the present time is beyond the time recorded in the attribute, the session is deemed “expired” (accordingly, note that the model object within the plug-in  711  should write the expiration time for a session into the appropriate column of the session&#39;s corresponding session database table row each time a new request is received for the session). 
     Remove_All_Expired. The “Remove_All_Expired” command is used to delete all rows from a session domain&#39;s database table whose corresponding sessions are deemed expired. Here, consistent with the discussion provided just above with respect to the Interator_All_Expired command, session rows can be identified as being expired if they are designed to contain an attribute that identifies them as being expired; or, if the row attributes contains information sufficient for the interface  710  to determine which sessions are expired. 
     Whereas the above commands provide session domain-wide management functions for a persisted session domain, in a further embodiment, the interface  710  and the relevant model object within its plug-in  711  are also written to support the following command set for managing the information that is persisted for a particular session (i.e., for managing a particular row&#39;s information). Each of the commands below can be assumed to be identified to the interface  710 , in some way, as pertaining to a particular session within the session domain. 
     Get_Session_ID. Execution of the Get_Session_ID command causes the model object within the plug-in  711  to read the sessionID for the particular session that the command is called on behalf of. 
     Get_Expiration_Time. Execution of the Get_Expiration_Time command causes the model object within the plug-in  711  to read the expiration time for the particular session that the command is called on behalf of 
     3. Update_Expiration_Time (maximum inactive time interval). Execution of the Get_Expiration_Time command causes the plug-in  711  to write a new expiration time for the particular session that the command is called on behalf of. According to one implementation, as observed above, the maximum inactive time interval is presented as an input argument for the Update_Expiration_Time command. Here, the expiration time is calculated by the interface  710  (or the model object within its plug-in  711 ) simply by adding the maximum inactive time interval to the present time. 
     In an embodiment, even though the maximum inactive time interval is more properly viewed as session management criteria information, the inactive time interval is “tagged along with” the expiration time or is recognized as its own separate attribute within the client&#39;s session state information. Typically, a new expiration time is calculated with each newly arriving request for a particular session (or, with each completed request/response cycle for a particular session). 
     Get_Attribute(s) (attribute(s)). Execution of the Get_Attribute(s) command causes the model object within the plug-in  711  to read one or more specific attributes identified by the caller of the command (e.g., the session management layer or an application). The “attribute(s)” argument identifies the specific attribute(s) (i.e., specific table column(s)) that are to be retrieved. Typically, the attributes that are recorded should be largely if not completely independent of the type of persistent storage employed. Hence, the same set of attributes discussed above with respect to the Get_Attribute(s) command for file systems can be used for database persistence. For substantially the same reasons described above with respect to file system&#39;s, the ability to specifically target only certain portions of a persisted session object results in more efficient operation as compared to an environment where only the entire content of a session object&#39;s contents can be read from or written to persistence. 
     Note that, according to an implementation, all attributes are individually accessible—not just those used for the storage of graphics files. Here, communicative flows  734   1  through  734   S  are meant to convey the individual accessibility of each of the persisted attributes across the database table&#39;s row structure. In an implementation alternative to that described above, only a singleton “attribute” is presented as an input argument and separate Get Attribute(attribute) commands have to be called to retrieve more than one attribute. 
     Put_Attribute(s)(attribute(s)). Execution of the Put_Attribute(s) command enables the writing of individual attributes into persistent storage consistent with the same principles outlined above for the Get_Attribute(s) command. Notably, in an implementation, execution of a Get_Attribute(s) or Put_Attribute(s) command does not involve serialization of the attribute data. 
     Get_Attribute(s)_Serialized(attributes(s)). Execution of the Get_Attribute(s)_Serialized command is essentially the same as the Get_Atribute(s) command described above, except that the persistence storage interface  710  (or the model object within the plug-in  711 ) performs deserialization on attribute data read from persistent storage. 
     Put_Attribute(s)_Serialized(attributes(s)). Execution of the Put_Attribute(s)_Serialized command is essentially the same as the Put_Atribute(s) command described above, except that the persistence storage interface  710  (or the model object within the plug-in  711 ) performs serialization on attribute data written to persistent storage. 
     According to one implementation, session management criteria information for a particular session domain (e.g., maximum inactive time interval, maximum number of outstanding requests, access policy, etc.) is kept in the in-memory session domain  704  separately from the session domain&#39;s session objects  708   1  through  708   S  (e.g., in another object not shown in  FIG. 7 ); and, is persisted to another table (also not shown in  FIG. 7 ) in the session domain&#39;s persistent storage  723 . Here, keeping the session management criteria separate from the session state attributes in table  723  should result in efficiencies and isolation from clocking issues similar to those described above with respect to the Update_Expiration_Time command for file system persistent storage. 
     3.0 Shared Closures 
       FIG. 8  shows a prior art computing system  800  having N virtual machines  113 ,  213 , . . . N 13 . The prior art computing system  800  can be viewed as an application server that runs and/or provides web applications and/or business logic applications for an enterprise (e.g., a corporation, partnership or government agency) to assist the enterprise in performing specific operations in an automated fashion (e.g., automated billing, automated sales, etc.). 
     The prior art computing system  800  runs are extensive amount of concurrent application threads per virtual machine. Specifically, there are X concurrent application threads ( 112   1  through  112   X ) running on virtual machine  113 ; there are Y concurrent application threads ( 212   1  through  212   Y ) running on virtual machine  213 ; . . . and, there are Z concurrent application threads (N 12   1  through N 12   Z ) running on virtual machine N 13 ; where, each of X, Y and Z are a large number. 
     A virtual machine, as is well understood in the art, is an abstract machine that converts (or “interprets”) abstract code into code that is understandable to a particular type of a hardware platform. For example, if the processing core of computing system  800  included PowerPC microprocessors, each of virtual machines  113 ,  213  through N 13  would respectively convert the abstract code of threads  112   1  through  112   X ,  212   1  through  212   Y , and N 12   1  through N 12   Z  into instructions sequences that a PowerPC microprocessor can execute. 
     Because virtual machines operate at the instruction level they tend to have processor-like characteristics, and, therefore, can be viewed as having their own associated memory. The memory used by a functioning virtual machine is typically modeled as being local (or “private”) to the virtual machine. Hence,  FIG. 1  shows local memory  115 ,  215 , N 15  allocated for each of virtual machines  113 ,  213 , . . . N 13  respectively. 
     A portion of a virtual machine&#39;s local memory may be implemented as the virtual machine&#39;s cache. As such,  FIG. 1  shows respective regions  116 ,  216 , . . . N 16  of each virtual machine&#39;s local memory space  115 ,  215 , . . . N 15  being allocated as local cache for the corresponding virtual machine  113 ,  213 , . . . N 13 . A cache is a region where frequently used items are kept in order to enhance operational efficiency. Traditionally, the access time associated with fetching/writing an item to/from a cache is less than the access time associated with other place(s) where the item can be kept (such as a disk file or external database (not shown in  FIG. 8 )). 
     For example, in an object-oriented environment, an object that is subjected to frequent use by a virtual machine (for whatever reason) may be stored in the virtual machine&#39;s cache. The combination of the cache&#39;s low latency and the frequent use of the particular object by the virtual machine corresponds to a disproportionate share of the virtual machine&#39;s fetches being that of the lower latency cache; which, in turn, effectively improves the overall productivity of the virtual machine. 
     A problem with the prior art implementation of  FIG. 8 , is that, a virtual machine can be under the load of a large number of concurrent application threads; and, furthermore, the “crash” of a virtual machine is not an uncommon event. If a virtual machine crashes, generally, all of the concurrent application threads that the virtual machine is actively processing will crash. Thus, if any one of virtual machines  113 ,  213 , N 13  were to crash, X, Y or Z application threads would crash along with the crashed virtual machine. With X, Y and Z each being a large number, a large number of applications would crash as a result of the virtual machine crash. 
     Given that the application threads running on an application server  100  typically have “mission critical” importance, the wholesale crash of scores of such threads is a significant problem for the enterprise. 
       FIG. 9  shows a computing system  200  that is configured with less application threads per virtual machine than the prior art system of  FIG. 8 . Less application threads per virtual machine results in less application thread crashes per virtual machine crash; which, in turn, should result in the new system  200  of  FIG. 9  exhibiting better reliability than the prior art system  800  of  FIG. 8 . 
     According to the depiction of  FIG. 9 , which is an extreme representation of the improved approach, only one application thread exists per virtual machine (specifically, thread  122  is being executed by virtual machine  123 ; thread  222  is being executed by virtual machine  223 ; . . . and, thread M 22  is being executed by virtual machine M 23 ). In practice, the computing system  200  of  FIG. 9  may permit a limited number of threads to be concurrently processed by a single virtual machine rather than only one. 
     In order to concurrently execute a comparable number of application threads as the prior art system  800  of  FIG. 8 , the improved system  900  of  FIG. 9  instantiates more virtual machines than the prior art system  800  of  FIG. 8 . That is, M&gt;N. 
     Thus, for example, if the prior art system  800  of  FIG. 8  has 10 application threads per virtual machine and 4 virtual machines (e.g., one virtual machine per CPU in a computing system having four CPUs) for a total of 4×10=40 concurrently executed application threads for the system  800  as a whole, the improved system  900  of  FIG. 9  may only permit a maximum of 5 concurrent application threads per virtual machine and 6 virtual machines (e.g., 1.5 virtual machines per CPU in a four CPU system) to implement a comparable number (5×6 =30) of concurrently executed threads as the prior art system  100  in  FIG. 9 . 
     Here, the prior art system  800  instantiates one virtual machine per CPU while the improved system  900  of  FIG. 9  can instantiate multiple virtual machines per CPU. For example, in order to achieve 1.5 virtual machines per CPU, a first CPU will be configured to run a single virtual machine while a second CPU in the same system will be configured to run a pair of virtual machines. By repeating this pattern for every pair of CPUs, such CPU pairs will instantiate  3  virtual machines per CPU pair (which corresponds to 1.5 virtual machines per CPU). 
     Recall from the discussion of  FIG. 8  that a virtual machine can be associated with its own local memory. Because the improved computing system of  FIG. 9  instantiates more virtual machines than the prior art computing system of  FIG. 8 , in order to conserve memory resources, the virtual machines  123 ,  223 , . . . M 23  of the system  900  of  FIG. 9  are configured with less local memory space  125 ,  225 , . . . M 25  than the local memory  115 ,  215 , . . . N 15  of virtual machines  113 ,  213 , . . . N 13  of  FIG. 8 . Moreover, the virtual machines  123 ,  223 , . . . M 23  of the system  900  of  FIG. 9  are configured to use a shared memory  230 . Shared memory  230  is memory space that contains items that can be accessed by more than one virtual machine (and, typically, any virtual machine configured to execute “like” application threads that is coupled to the shared memory  230 ). 
     Thus, whereas the prior art computing system  800  of  FIG. 8  uses fewer virtual machines with larger local memory resources containing objects that are “private” to the virtual machine; the computing system  900  of  FIG. 9 , by contrast, uses more virtual machines with less local memory resources. The less local memory resources allocated per virtual machine is compensated for by allowing each virtual machine to access additional memory resources. However, owing to limits in the amount of available memory space, this additional memory space  230  is made “shareable” amongst the virtual machines  123 ,  223 , . . . M 23 . 
     According to an object oriented approach where each of virtual machines  123 ,  223 , . . . N 23  does not have visibility into the local memories of the other virtual machines, specific rules are applied that mandate whether or not information is permitted to be stored in shared memory  230 . Specifically, to first order, according to an embodiment, an object residing in shared memory  230  should not contain a reference to an object located in a virtual machine&#39;s local memory because an object with a reference to an unreachable object is generally deemed “non useable”. 
     That is, if an object in shared memory  230  were to have a reference into the local memory of a particular virtual machine, the object is essentially non useable to all other virtual machines; and, if shared memory  230  were to contain an object that was useable to only a single virtual machine, the purpose of the shared memory  230  would essentially be defeated. 
     In order to uphold the above rule, and in light of the fact that objects frequently contain references to other objects (e.g., to effect a large process by stringing together the processes of individual objects; and/or, to effect relational data structures), “shareable closures” are employed. A “closure” is a group of one or more objects where every reference stemming from an object in the group that references another object does not reference an object outside the group. That is, all the object-to-object references of the group can be viewed as closing upon and/or staying within the confines of the group itself. Note that a single object without any references stemming from it can be viewed as meeting the definition of a closure. 
     If a closure with a non shareable object were to be stored in shared memory  230 , the closure itself would not be shareable with other virtual machines, which, again, defeats the purpose of the shared memory  230 . Thus, in an implementation, in order to keep only shareable objects in shared memory  230  and to prevent a reference from an object in shared memory  230  to an object in a local memory, only “shareable” (or “shared”) closures are stored in shared memory  230 . A “shared closure” is a closure in which each of the closure&#39;s objects are “shareable”. 
     A shareable object is an object that can be used by other virtual machines that store and retrieve objects from the shared memory  230 . As discussed above, in an embodiment, one aspect of a shareable object is that it does not possess a reference to another object that is located in a virtual machine&#39;s local memory. Other conditions that an object must meet in order to be deemed shareable may also be effected. For example, according to a particular Java embodiment, a shareable object must also posses the following characteristics: 1) it is an instance of a class that is serializable; 2) it is an instance of a class that does not execute any custom serializing or deserializing code; 3) it is an instance of a class whose base classes are all serializable; 4) it is an instance of a class whose member fields are all serializable; 5) it is an instance of a class that does not interfere with proper operation of a garbage collection algorithm; 6) it has no transient fields; and, 7) its finalize ( ) method is not overwritten. 
     Exceptions to the above criteria are possible if a copy operation used to copy a closure into shared memory  230  (or from shared memory  230  into a local memory) can be shown to be semantically equivalent to serialization and deserialization of the objects in the closure. Examples include instances of the Java 2 Platform, Standard Edition 1.3 java.lang.String class and java.util.Hashtable class. 
     A container is used to confine/define the operating environment for the application thread(s) that are executed within the container. In the context of J2EE, containers also provide a family of services that applications executed within the container may use (e.g., (e.g., Java Naming and Directory Interface (JNDI), Java Database Connectivity (JDBC), Java Messaging Service (JMS) among others). 
     Different types of containers may exist. For example, a first type of container may contain instances of pages and servlets for executing a web based “presentation” for one or more applications. A second type of container may contain granules of functionality (generically referred to as “components” and, in the context of Java, referred to as “beans”) that reference one another in sequence so that, when executed according to the sequence, a more comprehensive overall “business logic” application is realized (e.g., stringing revenue calculation, expense calculation and tax calculation components together to implement a profit calculation application). 
     It should be understood that the number of threads that a virtual machine in the improved system of  FIG. 9  can concurrently entertain should be limited (e.g., to some fixed number) to reduce the exposure to a virtual machine crash. For example, according to one implementation, the default number of concurrently executed threads is 5. In a further implementation, the number of concurrently executed threads is a configurable parameter so that, conceivably, for example, in a first system deployment there are 10 concurrent threads per virtual machine, in a second system deployment there are 5 concurrent threads per virtual machine, in a third system deployment there is 1 concurrent thread per virtual machine. It is expected that a number of practical system deployments would choose less than 10 concurrent threads per virtual machine. 
     3.1 Shared Memory “Persistence” with Shared Closures 
     With respect to the improved computing system  900  of  FIG. 9 , note that the shared memory  230  can be used as a persistence layer for computing system  900  that provides for failover protection against a virtual machine crash. That is, if session domain information for a particular session is “persisted” into shared memory  230  as a shared closure and a virtual machine within system  900  that has been assigned to handle the session crashes, another virtual machine within system  900  can “pick-up” the session because of its access to the session information in shared memory  230 . Aspects of session handling and failover protection in shared closure/shared memory environments have already been described in U.S. patent application Ser. No. 11/024,924, filed, Dec. 28, 2004, entitled, “Failover Protection From A Failed Worker Node In A Shared Memory System,” by Christian Fleischer; Galin Galchev; Frank Kilian; Oliver Luik; and Georgi Stanev; U.S. patent application Ser. No. 11/025,525, filed Dec. 28, 2004, entitled, “Connection Manager That Supports Failover Protection,” by Christian Fleischer and Oliver Luik; U.S. patent application Ser. No. 11/025,514, filed Dec. 28, 2004, entitled, “API For Worker Node Retrieval Of Session Request,” by Galin Galchev; U.S. patent application Ser. No. 11/024,552, filed Dec. 28, 2004, entitled, “System And Method For Serializing Java Objects Over Shared Closures,” by Georgi Stanev; and U.S. patent application Ser. No. 11/025,316, filed Dec. 28, 2004, entitled, “System And Method For Managing Memory Of Java Objects,” by Georgi Stanev. All of which are assigned to the assignee of the present application. 
     Storing session information in shared memory as its primary storage area results in an overlap between both the “in memory” and “persistence” storage concepts. That is, if session information is stored in shared memory  230  as its primary storage area, the computing system  900  should enjoy both the speed of “in memory” storage and internal failover protection offered by internal “persistent” storage. As described in more detail below, in case failover protection is desired outside the computing system  900  (so that another computing system can “pick up” a session dropped by system  900 ), the session information can also be persisted to an external persistent storage resource (e.g., database, RAID system, tape drive, etc.). 
       FIG. 10  shows an approach in which the “in-memory” session domain hierarchy scheme described with respect to  FIG. 4  is essentially implemented within a shared closure based shared memory  1030 . Here, the features of the shared memory context  1002 , root  1003  and session domains  1004   1  through  1004   Y  may be the same as those described with respect to the context  402 , root  403  and session domains  404   1  through  404   Y  described with respect to  FIG. 4 . It is worth noting that the hierarchical session domain approach described with respect to  FIG. 4  may not only be implemented within the shared memory of a computing system that embraces shared closure/shared memory technology (as described with respect to  FIG. 9 ), but also may be implemented within the local memory of a virtual machine in a prior art computing system that does not embrace shared closure/shared memory technology (as described with respect to  FIG. 8 ). 
       FIG. 10  shows each of items  1007   1  through  1007   S  as containing session state information. Because each of the session domains  1003  and  1004   1  through  1004   Y  observed in  FIG. 10  are accessible to multiple virtual machines (making their contents shareable to the virtual machines),  FIG. 10  likewise refers to these session domains as “shared” session domains. 
     According to one approach, a persistent storage interface is not needed for access to the contents of a shared session domain in shared memory  1030  because the contents essentially reside “in-memory” (i.e., are accessible with a proper memory address). Similar to the Get_Attribute(s) command available for file system and database persistent storages, individual attributes of a particular session are individually accessible from shared closure shared memory as well (e.g., transfer  1025  shows a single attribute of user data  1007   1  being read into local memory). According to one embodiment, a hash-mapping function is used to directly access a specific attribute from a specific session&#39;s session state information. Here, the hash-mapping function employs a namespace in which the name of the session and the name of the desired attribute are used to uniquely identify the particular attribute in shared memory. 
     The ability to fetch attributes individually from shared memory  1030  should result in efficiency gains like those described above with respect to the Get_Attribute(s) command for file systems and databases. For example, if the session management layer (or an application) needs just the expiration time, only the expiration time is read from shared memory  1030  into local memory  1015 . Undesired large graphics files (for instance) within the session state information are not transferred (i.e., remain in shared memory  1030 ) which corresponds to conservation of computing system resources. 
     Note that a virtual machine typically “runs” off of local memory. A running session management or application routine therefore runs off of local memory as well (through a virtual machine). When a certain object existing in shared memory as its own shared closure (i.e., the object does not contain a reference to another object in shared memory nor is referenced by another object in shared memory) is needed by a running process, it is called into the local memory of the virtual machine running that process. Here, activity  1028  is meant to depict the use of a session state attribute read into local memory  1015  from shared memory  1030 . 
       FIG. 11 a    shows a depiction of session state information  1107  as stored in shared closure shared memory for a single session. For instance, session state information  1107  of  FIG. 11  can be viewed as a deeper view of the contents of session state information  1007   1  of  FIG. 10 . Consistent with the file system and database persistence strategies,  FIG. 11  shows the session state information for a particular session as being broken down into J separately accessible attributes  1110   1  through  1110   J . In an implementation, in order to make the attributes separately accessible and to be consistent with shared closure semantics, each attribute corresponds to its own shared closure (e.g., one object per attribute where no attribute object contains a reference to another attribute object). Note that a session domain  1027  may be established in local memory  1015  for session data associated with shared session domain  1004   1  (i.e., session domains may be setup in local memory  1015  having a one-to-one correspondence with shared session domains that reside in shared memory). 
     Discussed at length above was the notion that more efficient operations may be realized if session management criteria information is persisted separately from session state information (see the end of sections 2.1 and 2.2 above, respectively).  FIG. 10  shows a shared closure  1009  within session domain  1004   1  that contains session management criteria information for the shared session domain  1004   1 . Here, transfer  1026  is meant to show that the session management criteria (through shared closure  1009 ) can be transferred separately from session state information between shared memory  1030  and local memory  1015  so that, for instance, implementation of session management policies can be run from local memory  1015  without requiring session state information to be transferred between local and shared memory. Activity  1029  is meant to depict the use of session management criteria information read into local memory from shared memory  1030 . 
       FIG. 11 b    shows a detailed embodiment of a session management criteria shared closure  1109  (such as session management criteria shared closure  1009  of  FIG. 10 ). As observed in  FIG. 11 b   , the shared closure  1109  includes both an expiration management table  1120  and the session domain&#39;s session management criteria  1124  (e.g., maximum inactive time interval, maximum number of outstanding requests, access policy, etc.). Because of unique functional opportunities that exist as an artifact of having a shared memory approach, as described in more detail further below, the expiration management table  1120  allows any modification to one of a session domain&#39;s sessions (e.g., attribute modification to an existing session, addition of a new session, deletion of a completed session) to be used as a trigger to identify and drop all expired sessions within that session domain. 
     In one embodiment, the table  1120  is contained by a single object. In further embodiments, the table&#39;s object does not refer to nor is referenced by any objects associated with the session management criteria  1124  (or other object outside the table) and hence does not form part of a shared closure with the session management criteria (or other object). As such, the object containing table  1120  can be read in and out of shared memory as its own shared closure. In another embodiment, the table  1120  is implemented as a collection of objects in the form of a shared closure. 
     As seen in  FIG. 11 b   , the session management table may be configured to include the expiration time  1122  for each session in the session management table&#39;s corresponding session domain (where each session is identified by its sessionID  1121 ). Here, a table can be viewed as a data structure having an ordered design in which an item of a certain type of data belongs in a certain region within the data structure.  FIGS. 12, 13, 14, and 15   a,b,c  demonstrate different operational flows employing a session management table (such as table  1120  of  FIG. 11 b   ) that may be used to perform session management tasks for the sessions associated with a session domain. Each of these flows are discussed in secession immediately below. 
       FIG. 12  shows an operational flow for the processing of a request for an already established, existing session. According to the flow diagram of  FIG. 12 , when a process being run by a virtual machine processes a request for a session, the session management table from that session&#39;s session domain is first copied  1201  into the local memory of the virtual machine. Note that, referring back to the session management table embodiment  1120  of  FIG. 11 b   , the table may also include a column  1123  that identifies, for each session in the session domain, whether or not the session is “available”. According to one implementation, certain types of sessions are “distributed” which means that more than one virtual machine is able to process a request. If different requests from a same session are distributed to different virtual machines it is possible at least in some circumstances that a first virtual machine may be actively processing a first request while a second virtual machine attempts to process a second request from the same session. 
     The available column  1123  is used to specify whether or not a session is already being “dealt with” elsewhere (e.g., by another virtual machine). Thus, according to  FIG. 12 , after the table is copied into local memory  1201 , the available column for the session that the virtual machine running off of the local memory (“the local virtual machine”) is attempting to process a request for is checked to see if the session is available  1202 . If a session is currently being handled by another virtual machine (i.e., the session is unavailable), the local virtual machine will find some kind of affirmative indication in the availability column and will delay its attempt to process the request at a later time (in which case an updated session management table having an updated expiration time will be copied into local memory). 
     If the session is not currently being handled by another virtual machine (i.e., the session is available), the local virtual machine will find some affirmative indication in the availability column, and then update shared memory to include a session management table showing that the session is now unavailable (i.e., the local virtual machine causes the available column for the session in shared memory to be marked as being unavailable). The virtual machine will then process the session&#39;s request which may involve the modification  1203  of various attributes associated with the session (such as the addition, deletion or modification of large graphics files, updating the state of the client&#39;s web browser, etc.). 
     Here, as described with respect to  FIG. 10 , any specific already existing attributes that need to be used or modified may be directly copied into local memory from shared memory  1025  without copying over any unwanted/unneeded attributes. Upon completion of the processing of the session&#39;s request one or more of the attributes that were copied into local memory may have been modified (or a new attribute may have been created). If so, the new attribute information is written into the session&#39;s session state shared closure; and, the session management table that resides in local memory is modified to reflect the new expiration time for the session (by adding the maximum inactive time interval to the present time) and that the session is now “available”, and then, is written into the shared memory  1204 . 
       FIGS. 13 and 14  show processes for adding and deleting sessions, respectively. For both processes, again, the session management table in shared memory is copied into the local memory of the virtual machine that intends to add or delete a session to/from the session domain. Note that copying the session management table leaves a version in shared memory so that other sessions in the session domain can be concurrently processed. This property is also true with respect to the request processing described above in  FIG. 12  (i.e., an available session other than the session being referred to in  FIG. 12  can be concurrently processed because the session management table in shared memory properly is accurate with respect to that other session). 
     In the case of the addition of a new session, the local virtual machine adds a new session entry to the local copy of the session management table  1302 . In the case of the deletion of a session (e.g., because the session has been completed), the local virtual machine deletes an existing session entry from the local copy of the session management table  1402 . In the case of the addition of a new session, any attributes that are to be written for the new session are written into the new session&#39;s session state shared closure in shared memory; and, the updated session management table in local memory is written into shared memory  1303 . In the case of the deletion of an existing session, the session state shared closure for the session is deleted from shared memory; and, the updated session management table in local memory is written into shared memory  1403 . 
       FIG. 15  demonstrates a process that identifies and removes all sessions in a session domain that have expired. Notably, the process of  FIG. 15  may be combined with any of the processes described just above with respect to  FIGS. 12, 13 and 14 . According to the process of  FIG. 15 , the session management table is copied into local memory  1501 . Here, the copy operation  1501  of  FIG. 15  may be the same copy operation  1201 ,  1301 ,  1401  of  FIGS. 12, 13 and 14 , respectively. Once the table has been copied into local memory, the table is iterated through to see if any of the sessions within the session domain have expired (by comparing their expiration time against the present time)  1502 . 
     Those sessions that are deemed expired are then deleted from the session management table  1503 . Then, the updated session management table is written into shared memory  1504 . Here, the writing  1504  of the updated session management table into shared memory of  FIG. 15  may be the same write operation  1204 ,  1303 ,  1403  of  FIGS. 12, 13, and 14  respectively. Likewise, processes  1502 ,  1503  may be performed concurrently and/or in series, alone or in combination with any of processes  1202 ,  1203 ,  1302  and  1402  of  FIGS. 12, 13 and 14  respectively. 
       FIGS. 16 through 18  relate to the deployment of applications that are easily configured for a particular type of computing system, and, a particular persistent storage strategy defined by those who are deploying the applications. Importantly, low level details such as whether the targeted computing system is a “shared memory/shared closure” system (such as described above with respect to  FIG. 9 ), or, is a not a “shared memory/shared closure” system (such as described above with respect to  FIG. 8 ) are transparent to the deployer. A “scope” of persistence is merely defined at deployment time, and, the deployment process automatically configures the targeted computing system in light of the targeted computing system&#39;s capabilities. 
       FIG. 16  shows a deployment descriptor  1600  for defining the persistence strategy for one or more session domains. As is know in the art, a deployment descriptor is (often a text file or document) used to define particular variables that typically, are left as being “configurable” for the end-user who is deploying the software; and/or, depend on or are specific to the underlying platform (hardware and/or software) that the software is being deployed onto/into. Here, depending on implementation, a single deployment descriptor  1600  may define the persistence strategy for an entire computing system, groups of applications, a single application, groups of session domains or a single session domain. 
     The basic deployment descriptor embodiment  1600  of  FIG. 16  includes a disable parameter  1601 , frequency of persistence parameters  1602 ,  1603  and scope of persistence parameters  1604 ,  1605 . The DISABLE parameter  1601  defines whether or not persistence is to be used. If the DISABLE parameter  1601  is affirmatively marked, in the case of computing systems that do not use failover protection through a shared memory feature (such as the prior art computing system  800  of  FIG. 8 ), whatever session domain(s) that the deployment descriptor  1600  defines the strategy for will not instantiate a persistence storage interface (such as those described with respect to  FIGS. 5, 6 and 7 ). In the case of computing systems that implement failover protection through a shared memory feature, the code that actually performs the failover protection is not activated, in some fashion, for the session domain(s) that the deployment descriptor  1600  defines the persistence storage strategy for. 
     The frequency parameters  1602 ,  1603  define the frequency at which session state information is persisted. If the ON_REQUEST parameter  1602  is affirmatively marked, session state information is persisted each time a request is processed (e.g., generally, the process of generating a response for the request) for a session whose session domain persistence strategy is defined by the deployment descriptor. If the ON_ATTRIBUTE parameter  1603  is affirmatively marked, session state information is persisted only if a session state attribute is changed as a consequence of processing a request. In an implementation, the expiration time is always persisted upon the generation of a new request irrespective of the ON_REQUEST or ON_ATTRIBUTE setting. 
     The scope parameters  1604 ,  1605  define what “level” or “depth” of persistence is to be implemented for the session domain(s) whose persistence strategy is defined by the deployment descriptor  1600 . The term “instance”, according to  FIGS. 16, 17 and 18 , refers to a single computing system. If the INSTANCE_WIDE parameter  1604  is affirmatively marked, the persistent storage for the session(s) that the deployment descriptor corresponds to is implemented within the computing system that software being deployed is deployed onto. Referring back to  FIGS. 8 and 9 , recall that virtual machines occasionally crash, and that, multiple virtual machines are instantiated within a single computing system. Here, instance wide persistence can be used for a computing system&#39;s own “internal” session failover protection. For example, a session that is handled by a first virtual machine—which crashes during the session—may be saved by second virtual machine that takes over the handling of the session, where, both virtual machines are instantiated within the same computing system. 
     The term “cluster” refers to a group of computing systems.  FIG. 17  depicts a simple cluster of P computing systems  1702   1  through  1702   P  that are coupled together through a dispatcher  1701  at the cluster&#39;s “front-end” and a database  1703  at the cluster&#39;s “back-end”. Typically, particularly in high performance data processing centers, many if not all of the computing systems  1702   1  through  1702   P  contain one or more of the same software applications. 
     Requests from clients are received by the dispatcher  1701 , and the dispatcher  1701  determines, for each received request, which computing system is most fit to handle the request. In many cases, in the case of already existing sessions, the dispatcher will send the request to the computing system that processed the immediately previous request. In the case of requests that correspond to the first request of a new session, the dispatcher  1701  will determine which computing system (amongst those having the software capable of processing the client&#39;s request) should receive the request (e.g., based on a load balancing algorithm). 
     Because the computing systems are each coupled to a database  1703 , it is possible to have inter-system session failover. That is, if a session is being handled by a first computing system (e.g., computing system  1702   1 ) that persists the session&#39;s session domain information into the database  1703 , and if that computing system suffers a complete failure, another computing system (e.g., computing system  1702   P ) will be able to read the persisted session domain information from the database  1703  and carry the session forward to completion. Thus, referring back to  FIG. 16 , if the CLUSTER_WIDE parameter  1605  is affirmatively marked, the persistent storage for the session(s) that the deployment descriptor corresponds to is to the computing system that the deployed software is being deployed on. The external persistent storage is presumably accessible to other computing systems (not necessarily all computing systems) within the cluster. 
     Importantly, certain application software may be deployable on both: 1) a computing system that does not embrace a shared memory structure that can be used for instance wide session failover protection (such as the computing system  800  of  FIG. 8 ); and, 2) a computing system that does embrace a shared memory structure that does embrace a shared memory structure that can be used for instance wide session failover protection (such as computing system  900  of  FIG. 9 ). In the former case instance wide failover protection should be implemented on an internal file system, and, in the later case, instance wide failover protection should be implemented with shared memory. 
     Moreover, preferably, the end user who is attempting to deploy the software should not have to comprehend whether file system persistence or shared memory persistence is appropriate. From the end-user&#39;s perspective, all that should need to be defined is whether the persistence is instance-wide or cluster-wide. The deployment descriptor embodiment of  FIG. 16  is consistent with this perspective. 
       FIG. 18  shows a deployment process that can properly implement instance wide persistence on non-shared memory and shared memory systems alike even though the deployment descriptor does not articulate the proper type of persistent storage technology (file system or shared memory). According to the deployment process of  FIG. 18 , as a basic example, it is assumed that the session domain for which a deployment descriptor is prepared and deployed pertains only to a single application (i.e., the deployment descriptor defines the persistence strategy for a single application. Of course, consistent with statements made earlier above, the session domain range of a deployment descriptor can be made to vary from embodiment to embodiment. 
     Referring to  FIG. 18 , a deployment descriptor that defines an application&#39;s persistent storage strategy is first created and deployed to a computing system  1801 . The deployment descriptor is read by the computing system that the descriptor was deployed to, and, a session domain is created for the descriptor&#39;s corresponding application  1802 . Importantly, the deployed to computing system “knows” whether its instance wide persistence is a file system or shared memory. That is, a computing system that does not include shared memory technology will interpret a setting of “instance wide” in the deployment descriptor as a directive to implement file system persistent storage  1804 ,  1806 . As such a file system persistent storage interface will be instantiated for the application&#39;s session domain  1806 . 
     By contrast, a computing system that does include shared memory technology will interpret a setting of “instance wide” in the deployment descriptor as a directive to use shared memory as the persistent storage  1805 ,  1808 . As such, a persistent storage interface will not be instantiated for the application&#39;s session domain  1808 . Irrespective of the type of computing system being deployed to, a setting of “cluster wide” in the deployment descriptor will be interpreted by both systems as a directive to employ an external database (or external file system, external RAID system or external tape drive if that happens to be the external persistence solution)  1804 ,  1807  and  1805 ,  1809 . In either of these cases a database persistent storage interface will be instantiated for the application&#39;s session domain. 
     Moreover, conceivably, more than one persistent storage solution may be specified for a particular session domain. For example, in the case of a file system without shared memory technology, if both “instance wide” and “cluster wide” persistence is affirmatively marked in the deployment descriptor, an internal file system persistent storage interface and an external database persistent storage interface will be instantiated for the application&#39;s session domain. In more elaborate embodiments the deployment descriptor may be made to contain more specific information. For example, in the case where multiple internal or external persistent storage solutions are available, the deployment descriptor may particularly specify which persistent storage solution is to be used for the descriptor&#39;s corresponding session domain(s). 
     Processes taught by the discussion above may be performed with program code such as machine-executable instructions which cause a machine (such as a “virtual machine”, a general-purpose processor disposed on a semiconductor chip or special-purpose processor disposed on a semiconductor chip) to perform certain functions. Alternatively, these functions may be performed by specific hardware components that contain hardwired logic for performing the functions, or by any combination of programmed computer components and custom hardware components. 
     An article of manufacture may be used to store program code. An article of manufacture that stores program code may be embodied as, but is not limited to, one or more memories (e.g., one or more flash memories, random access memories (static, dynamic or other)), optical disks, CD-ROMs, DVD ROMs, EPROMs, EEPROMs, magnetic or optical cards or other type of machine-readable media suitable for storing electronic instructions. Program code may also be downloaded from a remote computer (e.g., a server) to a requesting computer (e.g., a client) by way of data signals embodied in a propagation medium (e.g., via a communication link (e.g., a network connection)). 
       FIG. 19  shows an embodiment of a computing system (e.g., “a computer”). The exemplary computing system of  FIG. 19  includes: 1) one or more processors  1901  having an “on-chip” DC-DC converter  1910 ; 2) a memory control hub (MCH)  1902 ; 3) a system memory  1903  (of which different types exist such as DDR RAM, EDO RAM, etc.); 4) a cache  1904 ; 5) an I/O control hub (ICH)  1905 ; 19) a graphics processor  1906 ; 19) a display/screen  1907  (of which different types exist such as Cathode Ray Tube (CRT), Thin Film Transistor (TFT), Liquid Crystal Display (LCD), DPL, etc.; 8) one or more I/O devices  1908 . 
     The one or more processors  1901  execute instructions in order to perform whatever software routines the computing system implements. The instructions frequently involve some sort of operation performed upon data. Both data and instructions are stored in system memory  1903  and cache  1904 . Cache  1904  is typically designed to have shorter latency times than system memory  1903 . For example, cache  1904  might be integrated onto the same silicon chip(s) as the processor(s) and/or constructed with faster SRAM cells whilst system memory  1903  might be constructed with slower DRAM cells. By tending to store more frequently used instructions and data in the cache  1904  as opposed to the system memory  1903 , the overall performance efficiency of the computing system improves. 
     System memory  1903  is deliberately made available to other components within the computing system. For example, the data received from various interfaces to the computing system (e.g., keyboard and mouse, printer port, LAN port, modem port, etc.) or retrieved from an internal storage element of the computing system (e.g., hard disk drive) are often temporarily queued into system memory  1903  prior to their being operated upon by the one or more processor(s)  1901  in the implementation of a software program. Similarly, data that a software program determines should be sent from the computing system to an outside entity through one of the computing system interfaces, or stored into an internal storage element, is often temporarily queued in system memory  1903  prior to its being transmitted or stored. 
     The ICH  1905  is responsible for ensuring that such data is properly passed between the system memory  1903  and its appropriate corresponding computing system interface (and internal storage device if the computing system is so designed). The MCH  1902  is responsible for managing the various contending requests for system memory  1903  access amongst the processor(s)  1901 , interfaces and internal storage elements that may proximately arise in time with respect to one another. 
     One or more I/O devices  1908  are also implemented in a typical computing system. I/O devices generally are responsible for transferring data to and/or from the computing system (e.g., a networking adapter); or, for large scale non-volatile storage within the computing system (e.g., hard disk drive). ICH  1905  has bi-directional point-to-point links between itself and the observed I/O devices  1908 . 
     It is believed that processes taught by the discussion above can be practiced within various software environments such as, for example, object-oriented and non-object-oriented programming environments, Java based environments (such as a Java 2 Enterprise Edition (J2EE) environment or environments defined by other releases of the Java standard), or other environments (e.g., a .NET environment, a Windows/NT environment each provided by Microsoft Corporation). 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.