The Main Features of the Apply Process

Apply process rules for the LCR events can be defined at various levels such as table level, schema level, and database level. For the non-LCR events, the DBA can create rules to control the consumption process.

Any typical Apply process can either handle captured events or user enqueued events. When the need exists to handle both of these types, two separate Apply processes must be created.

Direct Apply

Direct apply means that an Apply process applies an LCR without running a user procedure. This is likely the main method of applying changes to the destination database. It is also the simplest form of consuming the events at destination side.

The Apply process applies the change in the LCR to a database object unless a conflict or apply error is encountered.  In the case of a conflict or apply error, the Apply process tries to resolve the error with a conflict handler or a user-specified procedure called an error handler.

Custom Apply and Apply Handlers

When an Apply process passes the LCR, as a parameter, to a user procedure for processing, it is known as Custom Apply. The user procedure can then process the LCR in a customized way by using a suitable stored procedure. These user-developed procedures are known as apply handlers. Each apply process can have multiple apply handlers.

Figure 2.3 illustrates the four types of apply handlers that are based on the type of events it deals with.