Propagation Process

A single source queue may propagate events to multiple destination queues, and a single destination queue may receive events from multiple source queues. However, only one propagation is allowed between a particular source queue and a particular destination queue. A single queue may be a destination queue for some propagations and a source queue for other propagations.

Propagation Rules

The Propagation process also follows user defined rules. Rules control those events which need to be propagated and those which need to be discarded. These rules are defined at three levels: Table, Schema and Global:

• A Table rule propagates or discards either row changes resulting from DML changes or DDL changes to a particular table. Subset rules are table rules that include a subset of the row changes to a particular table.
   

• A Schema rule propagates or discards either row changes resulting from DML changes or DDL changes to the database objects in a particular schema.
   

• A Global rule propagates or discards either all row changes resulting from DML changes or all DDL changes in the source queue.

When a particular captured change is configured for propagation to multiple destinations with relevant Apply processes, the staged event remains at the source queue until all the configured destination queues receive the events. When all destination queues receive and acknowledge, the source event is dropped successfully.

Prior to propagation, all the events are buffered into buffered queues which reside partly in the SGA and partly in the queue table. The buffering of events helps optimize the Propagation process.