Apply Process

The Apply process is an optional Oracle background process that dequeues LCRs and user messages from a specific queue.  It then either applies each one directly or passes it as a parameter to a user-defined procedure, called a handler. The Apply process is basically a part of the consumer of or subscriber to the database changes. In other words, it is an end point in the data replication flow.

The Main Features of the Apply Process

The Apply process is implemented as an Oracle background process, whose process name is Annn, in which nnn is an Apply process number. The Apply process is the process which consumes the LCR events or user messages. 

In other words, it is a dequeuing process. When dequeued by the Apply process, captured events, such as the LCRs, contain the DML changes or DDL changes that need to be applied to the destination database objects. The Apply process directly dequeues and implements the database changes. The apply user applies all row changes resulting from DML operations and all DDL changes.

A single Apply process can apply either captured events or user-enqueued events, but it cannot do both. If a queue at a destination database contains both captured and user-enqueued LCRs, the destination database must have at least two Apply processes to process the events. The Apply process can be associated with a SYS.AnyData queue, but it cannot be associated with a typed queue. The Apply process applies the changes based on the rules that are defined by the DBA. When the rule condition evaluates to TRUE, the changes are applied. Based on the rules evaluation, changes are either applied or discarded.