About Propagations

Propagation is the process of sending messages from one queue to another. These queues can be in the same database or in different databases. Propagations are not a required part of a messaging system. Propagations enable applications to communicate with each other even if they are not connected to the same database or same queue. Also, using multiple propagations, you can send the same messages from one queue to several other queues.

Propagations use database links to send messages between queues at different databases. A database link is a pointer that defines a one-way communication path from an Oracle database to another database. Propagation is performed by an Oracle Scheduler job called a propagation job. A propagation schedule determines how often a propagation sends messages, and you control the schedule for each propagation job.

Oracle Streams Advanced Queuing (AQ) supports both queue-to-queue or queue-to-database link (queue-to-dblink) propagations. A queue-to-queue propagation always has its own exclusive propagation job to send messages from the source queue to the destination queue. Therefore, the propagation schedule of each queue-to-queue propagation can be managed separately. A single database link can be used by multiple queue-to-queue propagations. Use queue-to-queue propagation when you want fine-grained control over multiple propagations that send messages from a single queue, and each propagation should use a different schedule.

In contrast, a queue-to-dblink propagation shares a propagation job with other queue-to-dblink propagations from the same source queue that use the same database link. If several propagations send messages from a single source queue to several queues at the remote database, then these propagations share the same propagation schedule. Any change to the propagation schedule affects all of the queue-to-dblink propagations from the same source queue that use the database link. Use queue-to-dblink propagation when you want bulk control over multiple propagations that send messages from a single queue, and all of the propagations should use the same schedule.