Apply Process

The Apply process is an optional Oracle background process. It dequeues the LCR(s) and user messages from a destination queue. When the Apply process dequeues, the events may be applied to database objects directly as transactions or they may be passed on as parameters to user defined procedures, which are also called user-defined Apply handlers. The database changes may include the DML or DDL changes.

Figure 5.1 Apply Process and Apply Handlers
 

As shown in Figure 5.1, Apply handlers typically include Message handler, DML Handler, DDL Handler, and Pre-Commit handlers. The following is a brief description of the handlers:

Message Handler Procedure: This is a user-defined procedure that can process non-LCR user messages in a customized way. A message handler for an Apply process can be specified using the message_handler parameter in the create_apply or the alter_apply procedure in the dbms_apply_adm package.

DML and DDL Handlers: This is a user procedure that processes row LCRs resulting from DML statements.  It is called a DML handler. A user procedure that processes DDL LCRs resulting from DDL statements is called a DDL handler. An Apply process can have many DML handlers but only one DDL handler.  The DDL handler processes all DDL LCRs dequeued by the Apply process. These handlers provide a mechanism to use the LCR information and execute custom operations on the database tables.

Pre-Commit Handler procedure: - This is a user-defined PL/SQL procedure that can receive the commit information for a transaction and process it in a customized way. It can deal with LCRs and Messages. It helps track the transactional history and helps in audit process.

More details and examples about the Apply handlers will be presented in the next Chapter.