persistent service

[Former Member]

hi what is mean by persistent service and the importance of it and importance of persistent objects

[Former Member]

hi chaitu,

Concepts Used in Persistence

Transient and persistent data

In principle, ABAP programs work with local program data, which resides in the program’s internal session. This data lives only as long as its context – that is, as long as its associated procedure (for local procedure data); its object (for attributes of classes); or its program (for global program data). This data is known as transient. Data that can be preserved beyond the runtime of the program is known as persistent. In SAP Systems, persistent data usually occurs as the content of database tables, but also as the content of files on application and presentation servers.

To work with persistent data, the system has to load it into transient data objects belonging to the ABAP program while that program is being executed. Then, after processing has been completed, it stores the data in a persistent form again. During this time, the content of the data exists twice: once in the ABAP program (transiently), and once in the appropriate storage medium (persistently). A typical process would be reading data from a database table using the SELECT statement into a transient work area; modifying the work area; and then updating the database table (using UPDATE). In such cases, the contents of transient and persistent data are different in the interim during this process.

Data in object-oriented programming

In an ideal object-oriented application, data occurs only as the attributes of objects (if we ignore the local data in methods for the time being). Objects are an aggregation of functions (in methods) and data (in attributes). The description of an object – that is, the class – occurs persistently as a piece of source code, but its attributes exist only as long as the object. However, an object in ABAP Objects is transient in principle. It exists in the internal program session only from the time it is generated (using CREATE OBJECT) until it is deleted by the Garbage Collector. Therefore, to work with persistent data in objects, you must program access to where those objects are stored within the methods of the class.

However, in completely object-oriented business application programming, then it is pointless simply to transfer the classical separation of data and functions to the methods – that is, to work with objects, but use procedural programming within the objects themselves. Ideally you could save the encapsulation of data and functions persistently within the object instead. A program could then leave an object in a certain state and a second program could continue working on the object in that state. Classes of objects are already persistent anyway, but you need some way of saving the attributes of an object persistently and then make reference to the appropriate class. The Persistence Service allows you to do exactly that.

The Persistence Service for Persistent Objects

Technically speaking, ABAP Objects are always transient, just like the data objects in ABAP programs. There are no persistent objects in ABAP Objects. However, the Persistence Service within Object Services allows application developers to work with persistent objects. The Persistence Service can be thought of as a software layer between the ABAP program and the data repository (that is, the database), which allows you to save the attributes of objects with a unique identity, and then load them again when you need them.

Put simply, the Persistence Service ensures that an object is initialized in a specified state, and saves the state of that object when required. The relationship between the object and the description of its state in the database is similar to the relationship between transient and persistent data outlined above. The state of the object when it is instantiated reflects the state of the data in the database at that time. Changes to the object state in the ABAP program are not written to the database immediately, but only after the appropriate request has been made (that is, the COMMIT WORK statement has been executed). Thus, a persistent object exists as an original in the database and as a copy in one or more ABAP programs. If several programs use the Persistence Service to instantiate objects of the same class before one of these programs has changed the state using COMMIT WORK, all the objects will have the same initial state. At present, we have not implemented a Persistence Service lock concept, which would ensure that there was only one transient mapping for each persistent object. So ultimately, ABAP programmers are not really working with persistent objects as such; rather, the Persistence Service makes it appear as if they are.

Persistent Classes

To use the Persistence Service for objects, the classes of these objects must be created as persistent classes in the Class Builder. The term persistent class does not imply that a class is persistent. (As a template for objects, every class is persistent). Rather, it means that the objects of that class and their state are managed by the Persistence Service. For example, the objects of these classes are instantiated in the ABAP program with a method of the Persistence Service, which ensures that the initialization is correct (not with the usual CREATE OBJECT statement). When the Class Builder creates a persistent class, it automatically generates an associated class, known as the class actor or class agent, whose methods manage the objects of persistent classes. As well as their identity, persistent classes can contain key attributes, which allow the Persistence Service to ensure that the content of each persistent object is unique.

Managed Objects

The objects of persistent classes are managed by the Persistence Service. This means, among other things, that these objects are instantiated with a method of the class actor, not with the CREATE OBJECT statement. These objects are known as managed objects. Objects managed by the Persistence Service can be either persistent or transient.

Persistent objects must be managed by the Persistence Service. The Persistence Service connects the object and the database.

Transient objects of persistent classes are also managed by the Persistence Service. For example, the Persistence Service ensures that the object is unique within a program (by checking its key attributes), but not for a connection to the database.

Persistent Objects and COMMIT WORK

To apply the changes made to the runtime objects of persistent classes to the actual persistent objects in the database, execute the COMMIT WORK statement. (Alternatively, use COMMIT WORK AND WAIT or SET UPDATE TASK LOCAL). Unless you are executing an object-oriented transaction from within the Transaction Service, you must include the COMMIT WORK statement explicitly in the program. Otherwise, it is encapsulated in the Transaction Service. (If you explicitly include the COMMIT WORK statement as described here, the t op-level transaction runs in compatibility mode).

The function of the COMMIT WORK statement is extended when you use it in conjunction with Object Services. Before COMMIT WORK closes the SAP LUW and triggers an update, it calls internal methods of the Persistence Service. These methods bundle the changes made to managed objects of the Persistence Service and pass them to a special update function module using CALL FUNCTION ... IN UPDATE TASK. Thus the Persistence Service works with traditional update methods. The update module is usually registered after any update modules that have already been registered. The update is then triggered and the update task executes the update module in the order in which they were registered.

After the system executes the COMMIT WORK statement, it sets the attributes of each persistent object in the ABAP program to initial. (That is, it calls the IF_OS_STATE~INVALIDATE method).

Local Update

If you want to change managed objects directly, rather than using the update module, you must change the update mode of the implicitly used Transaction Service. That is, the following statements must be executed before the COMMIT WORK statement:

DATA TM type ref to IF_OS_TRANSACTION_MANAGER.

DATA T type ref to IF_OS_TRANSACTION.

...

TM = CL_OS_SYSTEM=>GET_TRANSACTION_MANAGER( ).

T = TM->GET_CURRENT_TRANSACTION( ).

T->SET_MODE_UPDATE( OSCON_DMODE_DIRECT ).

COMMIT WORK.

To ensure database consistency, the local update is activated internally using SET UPDATE TASK LOCAL. In such cases, the system raises an exception if there are already update modules registered in the update task. You can only register additional classical update modules after setting the mode. They will then also be updated locally.

thanks

karthik

reward me if usefull