Patent Publication Number: US-7716677-B2

Title: System and method for mapping container managed persistence persistent object states to java data object states

Description:
TECHNICAL FIELD 
   Embodiments of the invention generally relate to the field of data processing and, more particularly, to a system and method for mapping Container Managed Persistence (CMP) persistent object states to Java Data Object (JDO) states. 
   BACKGROUND 
   Java 2 Enterprise Edition (“J2EE”) is a specification for building and deploying distributed enterprise applications. Unlike traditional client-server systems, J2EE is based on a multi-tiered architecture in which server side program code is divided into several layers including a “presentation” layer and a “business logic” layer. 
     FIG. 1  illustrates an exemplary J2EE application server  100  in which the presentation layer is implemented as a Web container  111  and the business layer is implemented as an Enterprise JavaBeans (“EJB”) container  101 . Containers are runtime environments which provide standard common services  119 ,  109  to runtime components. For example, the Java Naming and Directory Interface (“JNDI”) is a service that provides application components with methods for performing standard naming and directory services. Containers also provide unified access to enterprise information systems  117  such as relational databases through the Java Database Connectivity (“JDBC”) service, and legacy computer systems through the J2EE Connector Architecture (“JCA”) service. In addition, containers provide a declarative mechanism for configuring application components at deployment time through the use of deployment descriptors (described in greater detail below). 
   As illustrated in  FIG. 1 , each layer of the J2EE architecture includes multiple containers. Web container  111 , for example, is itself comprised of a servlet container  115  for processing servlets and a Java Server Pages (“JSP”) container  116  for processing Java server pages. EJB container  101  includes three different containers for supporting three different types of enterprise Java beans: a session bean container  105  for session beans, an entity bean container  106  for entity beans, and a message driven bean container  107  for message driven beans. The EJB specification defines, among other things, aspects of the three types of enterprise Java beans. The EJB specification refers to any of the EJB specifications including, for example, the specification entitled, “Enterprise JavaBeans™ Specification Version 2.1,” (Nov. 12, 2003). A more detailed description of J2EE containers and J2EE services can be found in R AGAE  G HALY AND  K RISHNA  K OTHAPALLI,  SAMS T EACH  Y OURSELF  EJB  IN  21 D AYS  (2003) (see, e.g., pages 353-376). 
   The information systems of a modem day enterprise (such as a corporation or government institution) are often responsible for managing and performing automated tasks upon large amounts of data. Persistent data is that data that “exists” for extended periods of time (i.e., it “persists”). Persistent data is typically stored in a database so that it can be accessed as needed over the course of its existence. Here, complex database software (e.g., such as DB2, Oracle, and SQL Server) is often used to read the data and perhaps perform various intelligent functions with it. Frequently, persistent data can change over the course of its existence (e.g., by executing a series of reads and writes to the data over the course of its existence). Moreover, multiple items of different persistent data may change as part of a single large scale “distributed transaction.” 
     FIG. 2  illustrates the manner in which persistent data is managed in current J2EE environments. Session beans  255 - 257  comprise the high level workflow and business rules implemented by application server  100 . For example, in a customer relationship management (“CRM”) system, session beans define the business operations to be performed on the underlying customer data (e.g., calculate average customer invoice dollars, plot the number of customers over a given timeframe, . . . etc.). 
   Session beans typically execute a single task for a single client during a “session.” Two versions of session beans exist: “stateless” session beans and “stateful” session beans. As its name suggests, a stateless session bean interacts with a client without storing the current state of its interaction with the client. By contrast, a stateful session bean stores its state across multiple client interactions. 
   Entity beans are persistent objects which represent data (e.g., customers, products, orders, . . . etc.) stored within a database  223 . Typically, an entity bean  252  (or  253 ) is mapped to a table  260  in the relational database and, as indicated in  FIG. 2 , each “instance” of the entity bean is typically mapped to a row in the table (referred to generally as an “object/relational mapping”). Two different types of persistence may be defined for entity beans: “bean-managed persistence” and “container-managed persistence.” With bean-managed persistence, the entity bean designer must provide the code to access the underlying database (e.g., SQL Java and/or JDBC commands and/or Java Data Objects model). By contrast, with container-managed persistence, the EJB container  101  manages the underlying calls to the database. 
   The Java Data Objects (JDOs) specification defines a persistence mechanism in which the Java language is used to persist and retrieve data. The JDO specification refers to any of the JDO specifications including, for example, Java Specification Request (JSR)-000012 entitled, “Java Data Objects.” A JDO persistence capable class (or, for ease of reference, a JDO) is an object that can be stored in and retrieved from a persistent data store (e.g., database  223 ). More specifically, a JDO is a class that implements the javax.jdo.PersistenceCapable interface. Java Data Object (JDO)  254  represents data stored in database  223 . Typically, a JDO (e.g., JDO  254 ) is mapped to a database table (e.g., table  262 ) and each field of the JDO is mapped to a column of the database. 
   The term O/R mapping metadata refers to metadata that defines a mapping between an object and a relational database. The O/R mapping metadata for each application is declaratively described and is typically stored in an eXtensible Markup Language (XML) file. These files are commonly called O/R mapping metadata files. The structure of O/R mapping metadata files is not specified by either the EJB specification or the JDO specification. In conventional systems, Container-Managed Persistence (CMP) entity beans and JDOs have separate O/R mapping metadata files. Thus, in conventional systems, an O/R mapping metadata file can map either an entity bean or a JDO to a database, but not both. 
   SUMMARY OF THE INVENTION 
   Embodiments of the invention are generally directed to a system and method for mapping Container Managed Persistence (CMP) persistent object states to Java Data Object (JDO) states. In an embodiment, mapping logic identifies a state of a CMP persistent object. The identified CMP persistent object state is mapped to a JDO state. In an embodiment, the mapping is based, at least in part, on whether the CMP persistent object is associated with a transactional context. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which like reference numerals refer to similar elements. 
       FIG. 1  is a block diagram illustrating several layers within a prior art J2EE architecture. 
       FIG. 2  is a block diagram illustrating the conventional approach of using separate Object/Relational (O/R) mapping for an entity bean and a Java Data Object. 
       FIG. 3  is a block diagram illustrating selected elements of an O/R processing module, according to an embodiment of the invention. 
       FIG. 4  is a block diagram illustrating an example of O/R mapping metadata (Data Type Definition (DTD) used by the metadata XML) for a CMP entity bean abstract schema. 
       FIG. 5  is a block diagram illustrating an example of O/R mapping metadata (DTD used by the metadata XML) for a Java Data Object model. 
       FIG. 6  is a block diagram illustrating selected states of a Container Managed Persistence (CMP) persistent object. 
       FIG. 7  is a table illustrating selected state transitions for a CMP persistent object, according to an embodiment of the invention. 
       FIG. 8  is a flow diagram illustrating certain aspects of a method for mapping a CMP persistent object state to a JDO state, according to an embodiment of the invention. 
       FIG. 9  is a conceptual illustration of mapping a CMP persistent object state to a JDO state when the CMP persistent object is associated with a transactional context. 
   

   DETAILED DESCRIPTION 
   Embodiments of the invention are generally directed to a system and method for mapping Container Managed Persistence (CMP) persistent object states to Java Data Object (JDO) states. In an embodiment, mapping logic identifies a state of a CMP persistent object. As is further discussed below, the identified CMP persistent object state is mapped to a JDO state. In an embodiment, the mapping is based, at least in part, on whether the CMP persistent object is associated with a transactional context. 
     FIG. 3  is a block diagram illustrating selected elements of O/R processing module  300 , according to an embodiment of the invention. O/R processing module  300  includes O/R mapping logic  302  and common O/R mapping metadata file  304 . As indicated in  FIG. 3 , O/R mapping logic  302  receives deployment descriptor file  310 . A “deployment descriptor file” defines metadata for the component or application with which it is associated. In one embodiment, deployment descriptor file  310  defines, at least in part, the deployment of an entity bean or a JDO onto an application server (e.g., application server  100 , shown in  FIG. 1 ). For each entity bean or JDO, the deployment descriptor defines “persistent fields” which represent and store a single unit of data, and “relationship” fields which represent and store references to other persistent data. The deployment descriptor also includes O/R mapping metadata. In one embodiment, deployment descriptor file  310  is an XML file. Embodiments of the invention, however, are not limited to any particular file type. 
   O/R mapping logic  302  maps one or more elements of deployment descriptor  310  to database schema  320  based, at least in part, on common O/R mapping metadata file  304 . As is further described below, in an embodiment, common O/R mapping metadata file  304  provides a mapping for both an entity bean (or entity beans) and a JDO (or JDOs) to database schema  320 . Database schema  320  provides metadata defining the structure of (and constraints on) one or more tables of database  330 . In one embodiment, database schema  320  is an XML file. In an alternative embodiment, database schema  320  is structured according to the syntax and semantics of a different language (e.g., a different markup language). In an embodiment, O/R mapping metadata file  304  is part of deployment descriptors  310 . O/R mapping logic  302  maps persistent fields (at the object layer) of CMP or JDO to a database element specified by O/R mapping metadata file  304 . 
   In an embodiment, O/R processing module  300  may be part of a multi-tiered network. The multi-tiered network may be implemented using a variety of different application technologies at each of the layers of the multi-tier architecture, including those based on the Java 2 Enterprise Edition™ (“J2EE”) platform, the Microsoft .NET platform, the Websphere platform developed by IBM Corporation, and/or the Advanced Business Application Programming (“ABAP”) platform developed by SAP AG. 
     FIG. 4  is a block diagram illustrating mapping metadata for a CMP abstract schema (typically comprising many CMP entity beans, e.g., entity bean  252 , shown in  FIG. 2 ). Persistent-ejb-map element  402  is the root element and it contains information about the persistent fields of one or more entity beans and relationships among the entity beans. For example, entity-beans element  404  contains a list of the entity beans to which the metadata applies and relationship element  406  describes the relationships between the entity beans in the database tables. 
   Entity-bean element  408  specifies a mapping between an entity bean and a particular database table. EJB-name element  410  provides the name of the entity bean (e.g., as defined in the ejb-jar.xml of the entity bean) and table-name element  412  provides the name of the database table to which the entity bean is mapped. The entity bean described by entity-bean element  408  typically includes one or more fields of data to be persisted on the database table. Typically, each of the one or more fields is mapped to a particular column of the database table. Field-map element  414  defines the mapping between the fields of the entity bean and the columns of the table that is identified by element  412 . 
   Relational databases typically include columns having different column types. In one embodiment, the column that stores a value for a cmp-field (e.g., non-relationship field) may be of types: primary key, no key, or unique key. A primary key type column refers to a column that contains the primary key for the database table. A no key type column, as the name implies, refers to a column that does not contain a key for the database table. The unique key type column refers to a column that contains a unique key for the database table. In an embodiment, key-type element  416  specifies a column type for the columns referenced in field-map element  414 . Field-name element  418  specifies a field of the entity bean and column element  420  contains metadata for the column to which the field is mapped (e.g., a column name as provided by column-name element  422 ). 
   Relationships element  406  specifies relationships between entity beans. A relationship between entity beans is characterized by its multiplicity (e.g., 1:1, 1:M, M:N). A relationship is “one-to-one” if each entity bean is associated with a single instance of another entity bean and “one-to-many” if each entity bean is associated with many instances of another entity bean. A relationship is “many-to-many” if entity bean instances may be related to multiple instances of each other. 
   Table-relation element  424  defines a relationship between two entity beans using references between primary key column(s) and foreign key column(s), or logical foreign key column(s). A column is called “a logical foreign key” if it is a column that contains the values of the referenced table&#39;s primary key column, but is not declared as a foreign key. As used herein, the term “foreign key” refers to both a standard foreign key and a logical foreign key. 
   Help-table element  426  provides the name of a join table which is used in the case of an M:N relationship. In many-to-many bidirectional relationships, there are corresponding fields of set type on both sides of the relationship. In the relational model there is typically a database table that contains foreign keys to one table of both sides of the many-to-many bidirectional relationship. The term “join table” refers the database table that contains these foreign keys. The terms bidirectional relationship and unidirectional relationship refer to the object representation of a relationship. Typically relationships with multiplicity 1:1, 1:M, or M:1 are represented in the database by pair(s) (primary key column, foreign key column). In the case of a compound primary key (e.g., a primary key consisting of many columns) many pairs are used. Primary key columns are part of the “one side of the relationship” bean&#39;s table while foreign key columns are part of the “other side of the relationship” bean&#39;s table and they refer to the primary key columns. In the case of a M:M relationship a join table is used. Some of the join table columns are foreign key columns that refer to the primary key columns of the “one side of the relationship” bean&#39;s table. The remaining columns of the join table are foreign key columns that refer to the primary key columns of the “other side of the relationship” bean&#39;s table. 
   Table-relationship-role element  428  specifies a role in the database (primary key or foreign key side) for the relationship using key-type element  430 . Each relationship has two sides. Each of the two sides is described by a table-relationship-role element. The table relationship role refers to which side of the relationship (e.g., either referencing or referenced) element  428  is directed. For example, if key-type element  430  specifies “primary key,” then element  428  is directed to the referenced side of the relationship. Alternatively, if key-type element  430  specifies either “no key” or “foreign key,” then element  430  is directed to the referencing side of the relationship. In either case, ejb-name element  432  and cmr-field  434 , respectively, provide the name of the entity bean and field that is ether referencing or referenced by the relationship. If element  428  is directed to the referencing side of the relationship, Fk-column-name element  436  specifies a foreign key column and its reference to a corresponding primary key field. For example, column-name element  438  provides the name of the foreign key column and Pk-field-name element  440  provides the name of the referenced primary key field. Table-relationship-role element  442  defines the opposing role to the role defined by element  428 . That is, if element  428  specifies the “referencing side” of relationships, then element  442  specifies the “referenced” side of one or more relationships. At the object layer the ejb-name and cmr-field uniquely identify the described relationship. Thus, ejb-name field element  432  and cmr-field element  434  are used to specify the relationship. 
     FIG. 5  is a block diagram illustrating mapping metadata for a JDO (e.g., JDO  254 , shown in  FIG. 2 ). In general terms, the relation to the JDO object model is facilitated through the class and field elements. The relation to the database schema is facilitated through the column and foreign key elements. Map element  502  is the root element and includes version attribute  504  to specify the version of the mapping metadata. Package element  506  includes all classes of a particular package that is identified by name attribute  508 . 
   Class element  510  includes the relationship and non-relationship fields of the class identified by name attribute  512 . Class element  510  includes a field element  514  for each persistent non-relationship field of the class. Name attribute  516  is the field name as defined in the class. Field element  514  includes metadata specifying the column to which a corresponding field is mapped. For example, column element  518  contains a description of a column to which field element  514  is mapped. Column element  518  includes, for example, table attribute  520  and name attribute  522 . Name attribute  522  provides the name for the column. Table attribute  520  specifies the table to which the column belongs. 
   JDBC type element  524  describes the JDBC type of column  518 . The JDBC type is identified by the following attributes: name  526 , length  528 , and decimals  530 . In an embodiment, JDBC type element  524  is optional and is typically used when no catalog information is available for the column. JDBC type element  524  also includes is-primary-key attribute  532 . In an embodiment, is-primary-key attribute  532  specifies whether column  518  contains a primary key of the table. 
   Class element  510  includes a relationship-field element  534  for each persistent relationship field of the class. Relationship-field element  534  contains, for example, the foreign keys and generic persistent references for the relationship. Field name attribute  536  contains the name of the persistent relationship field as defined by the class. Multiplicity attribute  538  is “one” or “many,” depending on the relationship type (e.g., 1:1, 1:M, M:M, etc.). Join-table attribute  540  specifies whether a join table is used (e.g., with a true/false value). Update attribute  542  specifies whether the value of this relationship field is used for updating the database. Update attribute  542  is relevant for many-to-many and one-to-one bidirectional relationships. In such cases, one of the sides of the relationship is declared as non-updatable. 
   Foreign-key element  544  describes a foreign key in the database. Name attribute  546  provides the name of the foreign key. Foreign-key-table attribute  548  and primary-key-table attribute  550 , respectively, specify the foreign key and primary key tables in the database. Column-pair element  552  contains one or more column pairs (e.g., attributes  554 - 556 ) corresponding to the columns in the foreign key and primary key tables specified in foreign-key element  544 . 
     FIG. 6  is a block diagram illustrating selected states of a CMP persistent object  600 . In an embodiment, CMP persistent object  600  includes the following states: CREATED  610 , DEFAULT+Not Loaded  620 , DEFAULT+Loaded  630 , UPDATED  640 , REMOVED  650 , and V_REMOVED  660 . In an alternative embodiment, CMP persistent object  600  may have more states, fewer states, and/or different states. 
   CREATED state  610  applies to instances of a CMP persistent object that have been made persistent in the current transaction and have not yet been saved. An instance of a CMP persistent object is in DEFAULT+Not Loaded state  620  if it represents an object in the data store whose values have not yet been loaded from the data store into the instance. If the instance represents a specific instance in the data store whose values are loaded (e.g., via read operation  602 ) but whose values have not been changed in the current transaction, then the instance is in DEFAULT+Loaded state  630 . The instance of a CMP persistent object is in UPDATED state  640  if persistent data has been modified (e.g. via write operation  604 ) during the current transaction and the modifications have not yet been committed. A CMP persistent object that has been deleted in the current transaction (e.g., via delete operation  606 ) is in REMOVED state  650 . A CMP persistent object that has been created in the current transaction but does not represent a specific object on the data store and that has been deleted (e.g., via delete operation  608 ) is in V_REMOVED state  660 . 
   The term “state transition” refers to transitioning, either directly or indirectly, from a first state to a second state. A CMP persistent object may change state, for example. in response to certain method calls.  FIG. 7  shows table  700  to illustrate selected state transitions for a CMP persistent object according to an embodiment of the invention. Row  702  lists selected CMP persistent object states  720 - 729 . Column  704  lists selected method calls  730 - 740  that may invoke a state transition. The listed values show state transitions that occur in response to method calls  730 - 740 . For example, if write method  732  is called, when the CMP persistent object is in DEFAULT+Not Loaded state  720 , then the CMP persistent object transitions to UPDATED state  750 . In an alternative embodiment, state transitions other than those shown in table  700  are possible. 
   Turning now to  FIGS. 8-9 , the particular methods associated with embodiments of the invention are described in terms of computer software and hardware with reference to a flowchart. The methods to be performed by a computing device (e.g., an application server) may constitute state machines or computer programs made up of computer-executable instructions. The computer-executable instructions may be written in a computer programming language or may be embodied in firmware logic. If written in a programming language conforming to a recognized standard, such instructions can be executed on a variety of hardware platforms and for interface to a variety of operating systems. In addition, embodiments of the invention are not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement embodiments of the invention as described herein. Furthermore, it is common in the art to speak of software, in one form or another (e.g., program, procedure, process, application, etc.), as taking an action or causing a result. Such expressions are merely a shorthand way of saying that execution of the software by a computing device causes the device to perform an action or produce a result. 
     FIG. 8  is a flow diagram illustrating certain aspects of a method for mapping a CMP persistent object state to a JDO state, according to an embodiment of the invention. Referring to process block  810 , the state of a CMP persistent object is identified. In one embodiment, the state of the CMP persistent object is identified by querying an internal variable representing the current state of the CMP persistent object. The current state of the CMP persistent object may be one of a predetermined set of persistent object states. For example, in one embodiment, the current state of the CMP persistent object is one of the states illustrated in  FIG. 6 . 
   Related operations on persistent data are often grouped together into a unit of work that forms a logical operation from an application&#39;s perspective. The term “transaction” refers to such units of work and additionally refers to ensuring that concurrent units of work are executed in an atomic, consistent, isolated, and durable manner as if the concurrent units of work were executed serially one after the other. The term “transactional context” refers to operating on a persistent object within the context of a transaction. In an embodiment, operations may or may not be performed on a CMP persistent object within a transactional context. 
   Referring to process block  820 , the current state of the CMP persistent object is mapped to a Java Data Object (JDO) state. The term “JDO state” refers to one of the JDO life cycle states as described, for example, in the JDO specification. The JDO life cycle states may include: Transient-Clean, Transient-Dirty, Transient, Persistent-New, Persistent-New-Deleted, Hollow, Persistent-Clean, Persistent-Dirty, and Persistent-Deleted. The term “mapping” broadly refers to determining a JDO state that corresponds to the identified state of the CMP persistent object. As is further described below, the mapping of a CMP persistent object state to a JDO state is based, at least in part, on whether the CMP persistent object is associated with a transactional context. 
     FIG. 9  is a conceptual illustration of mapping the identified CMP persistent object state to a JDO state when the CMP persistent object is associated with a transactional context, according to an embodiment of the invention.  FIG. 9  includes selected aspects of CMP persistent object  900  and JDO  920 . CMP persistent object  900  includes states  902 - 912  and internal variable  914 . In an embodiment, internal variable  914  provides an indication of whether CMP persistent object  900  is associated with a transactional context. JDO  920  includes JDO states  922 - 940 . 
   As shown in  FIG. 9 , each CMP persistent object state can be mapped to a particular JDO state. Table 1 illustrates the mappings of CMP persistent object states to JDO states according to an embodiment of the invention. In an alternative embodiment, more mappings, fewer mappings, and/or different mappings may be possible. 
   
     
       
         
             
           
             
               TABLE 1 
             
           
          
             
                 
             
             
               TRANSACTIONAL CONTEXT 
             
          
         
         
             
             
             
          
             
                 
               CMP Persistent Object State 
               Corresponding JDO State 
             
             
                 
                 
             
             
                 
               DEFAULT + loaded 
               Persistent-Clean 
             
             
                 
               DEFAULT + not loaded 
               Hollow 
             
             
                 
               UPDATED 
               Persistent-Dirty 
             
             
                 
               CREATED 
               Persistent-New 
             
             
                 
               REMOVED 
               Persistent-Deleted 
             
             
                 
               V_REMOVED 
               Persistent-New-Deleted 
             
             
                 
                 
             
          
         
       
     
   
   If CMP persistent object  900  is not associated with a transactional context, then the mapping of CMP persistent object states to JDO states is different than the mapping shown in  FIG. 9 . Table 2 illustrates the mappings of CMP persistent object states to JDO states when CMP persistent object  900  is not associated with a transactional context. In an alternative embodiment, more mappings, fewer mappings, and/or different mappings may be possible. 
   
     
       
         
             
           
             
               TABLE 2 
             
           
          
             
                 
             
             
               NON-TRANSACTIONAL CONTEXT 
             
          
         
         
             
             
             
          
             
                 
               CMP Persistent Object State 
               Corresponding JDO State 
             
             
                 
                 
             
             
                 
               DEFAULT + loaded 
               Persistent-Non-Transactional 
             
             
                 
               DEFAULT + not loaded 
               Hollow 
             
             
                 
                 
             
          
         
       
     
   
   The solid lines having an arrow on the end that are shown in  FIG. 9  provide examples of state transitions. These state transitions merely provide an example of selected state transitions. A more complete example of possible state transitions for CMP persistent object  900  is shown in  FIG. 7 . In addition, the JDO specification provides a more complete example of state transitions for JDO  920 . 
   Referring again to  FIG. 8 , an operation to manipulate the CMP persistent object and/or the persistent data that it represents is invoked at  830 . Invoking an operation includes, for example, calling a method of a persistent object. In an embodiment, the invoked operation occurs within a transactional context. Examples of the invoked operation include: write, read, delete, create, flush, commit, and rollback. 
   The CMP persistent object may change state in response to the invoked operation. In an embodiment, the CMP persistent object state transition can be mapped to a JDO state transition. The mapping of CMP persistent object state transition to JDO state transition depends, at least in part, on which operation is invoked. Table 3 illustrates the mapping of state transitions if a read operation is invoked. In an alternative embodiment, more state transition mappings, fewer state transition mappings, and/or different state transition mappings may be possible. 
   
     
       
         
             
           
             
               TABLE 3 
             
           
          
             
                 
             
             
               READ OPERATION 
             
          
         
         
             
             
          
             
               CMP Persistent Object 
                 
             
             
               State Transitions 
               Corresponding JDO State Transitions 
             
             
                 
             
             
               DEFAULT + not loaded 
               Hollow to Persistent-Clean 
             
             
               to DEFAULT + loaded 
             
             
               DEFAULT + loaded to 
               Persistent-Clean to Persistent-Clean 
             
             
               DEFAULT + loaded 
             
             
               UPDATED to UPDATED 
               Persistent-Dirty to Persistent-Dirty 
             
             
               CREATED to CREATED 
               Persistent-New to Persistent-New 
             
             
               V-REMOVED to error 
               Persistent-New-Deleted to error 
             
             
               REMOVED to error 
               Persistent-Deleted to error 
             
             
                 
             
          
         
       
     
   
   Table 4 illustrates the mapping of state transitions if a write operation is invoked. In an alternative embodiment, more state transition mappings, fewer state transition mappings, and/or different state transition mappings may be possible. 
   
     
       
         
             
           
             
               TABLE 4 
             
           
          
             
                 
             
             
               WRITE OPERATION 
             
          
         
         
             
             
          
             
               CMP Persistent Object 
                 
             
             
               State Transitions 
               Corresponding JDO State Transitions 
             
             
                 
             
             
               DEFAULT + not loaded 
               Hollow to Persistent-Dirty 
             
             
               to UPDATED 
             
             
               DEFAULT + loaded 
               Persistent-Clean to Persistent-Dirty 
             
             
               to UPDATED 
             
             
               UPDATED to UPDATED 
               Persistent-Dirty to Persistent-Dirty 
             
             
               CREATED to CREATED 
               Persistent-New to Persistent-New 
             
             
               V_REMOVED to error 
               Persistent-New-Deleted to error 
             
             
               REMOVED to error 
               Persistent-Deleted to error 
             
             
                 
             
          
         
       
     
   
   Table 5 illustrates the mapping of state transitions if a delete operation is invoked. In an alternative embodiment, more state transition mappings, fewer state transition mappings, and/or different state transition mappings may be possible. 
   
     
       
         
             
           
             
               TABLE 5 
             
           
          
             
                 
             
             
               DELETE OPERATION 
             
          
         
         
             
             
          
             
               CMP Persistent Object 
                 
             
             
               State Transitions 
               Corresponding JDO State Transitions 
             
             
                 
             
             
               DEFAULT + not loaded 
               Hollow to Persistent-Deleted 
             
             
               to REMOVED 
             
             
               DEFAULT + loaded 
               Persistent-Clean to Persistent-Deleted 
             
             
               to REMOVED 
             
             
               UPDATED to REMOVED 
               Persistent-Dirty to Persistent-Deleted 
             
             
               CREATED to V_REMOVED 
               Persistent-New to Persistent-New-Deleted 
             
             
                 
             
          
         
       
     
   
   Table 6 illustrates the mapping of state transitions if a flush operation is invoked. In an alternative embodiment, more state transition mappings, fewer state transition mappings, and/or different state transition mappings may be possible. 
   
     
       
         
             
           
             
               TABLE 6 
             
           
          
             
                 
             
             
               FLUSH OPERATION 
             
          
         
         
             
             
          
             
                 
               Corresponding JDO 
             
             
               CMP Persistent Object State Transitions 
               State Transitions 
             
             
                 
             
             
               DEFAULT + not loaded to DEFAULT + not loaded 
               Hollow to Hollow 
             
             
               DEFAULT + loaded to DEFAULT + loaded 
               Persistent-Clean to 
             
             
                 
               Persistent-Clean 
             
             
                 
             
          
         
       
     
   
   Table 7 illustrates the mapping of state transitions if either a commit or a rollback operation is invoked. In an alternative embodiment, more state transition mappings, fewer state transition mappings, and/or different state transition mappings may be possible. 
   
     
       
         
             
           
             
               TABLE 7 
             
           
          
             
                 
             
             
               COMMIT/ROLLBACK OPERATION 
             
          
         
         
             
             
          
             
               CMP Persistent Object State 
               Corresponding JDO State 
             
             
               Transitions 
               Transitions 
             
             
                 
             
             
               DEFAULT + not loaded to garbage 
               Hollow to Hollow to Transient 
             
             
               collected 
             
             
               DEFAULT + loaded to garbage 
               Persistent-Clean to Hollow to 
             
             
               collected 
               Transient 
             
             
               UPDATED to garbage collected 
               Persistent-Dirty to Hollow to 
             
             
                 
               Transient 
             
             
               CREATED to garbage collected 
               Persistent-New to Hollow to 
             
             
                 
               Transient 
             
             
               V_REMOVED to garbage collected 
               Persistent-New-Deleted to 
             
             
                 
               Transient 
             
             
               REMOVED to garbage collected 
               Persistent-Deleted to Transient 
             
             
                 
             
          
         
       
     
   
   In an alternative embodiment more operations, fewer operations, and/or different operations may be invoked. In another alternative embodiment, only a selected subset of the mappings shown in Tables 3-7 is implemented. 
   Elements of embodiments of the present invention may also be provided as a machine-readable medium for storing the machine-executable instructions. The machine-readable medium may include, but is not limited to, flash memory, optical disks, compact disks-read only memory (CD-ROM), digital versatile/video disks (DVD) ROM, random access memory (RAM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), magnetic or optical cards, propagation media or other type of machine-readable media suitable for storing electronic instructions. For example, embodiments of the invention may be downloaded as a computer program which may be transferred from a remote computer (e.g., a server) to a requesting computer (e.g., a client) by way of data signals embodied in a carrier wave or other propagation medium via a communication link (e.g., a modem or network connection). 
   It should be appreciated that reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Therefore, it is emphasized and should be appreciated that two or more references to “an embodiment” or “one embodiment” or “an alternative embodiment” in various portions of this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined as suitable in one or more embodiments of the invention. 
   Similarly, it should be appreciated that in the foregoing description of embodiments of the invention, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed subject matter requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention. 
   It should be appreciated that the CMP persistent object states referred in this document and figures may be labeled as shown in Table 8. The labels in the right hand side column are alternative labels for clarification and do not change the meaning of the labels on the left hand side column as used in the description above and in figures. 
   
     
       
         
             
           
             
               TABLE 8 
             
           
          
             
                 
             
             
               CMP STATES AND EQUIVALENT RE-LABELS 
             
          
         
         
             
             
          
             
               CMP Persistent Object States 
               Relabeled CMP Persistent Object States 
             
             
                 
             
             
               DEFAULT + not loaded 
               DEFAULT-AND-NOT-LOADED 
             
             
               DEFAULT + Not Loaded 
               DEFAULT-AND-NOT-LOADED 
             
             
               DEFAULT + loaded 
               DEFAULT-AND-LOADED 
             
             
               DEFAULT + Loaded 
               DEFAULT-AND-LOADED 
             
             
               V_REMOVED 
               V-REMOVED