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Supreme Being
Group: Administrators
Last Login: 1/26/2010 1:58:14 PM
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Overview of Streams Replication
Replication is the process of sharing database objects and data at multiple databases. To maintain replicated database objects and data at multiple databases, a change to one of these database objects at a database is shared with the other databases. In this way, the database objects and data are kept synchronized at all of the databases in the replication environment. In a Streams replication environment, the database where a change originates is called the source database, and a database where a change is shared is called a destination database.
When you use Streams, replication of a DML or DDL change typically includes three steps:
1. A capture process or an application creates one or more logical change records (LCRs) and enqueues them into a queue. An LCR is a message with a specific format that describes a database change. A capture process reformats changes captured from the redo log into LCRs, and applications can construct LCRs. If the change was a data manipulation language (DML) operation, then each LCR encapsulates a row change resulting from the DML operation to a shared table at the source database. If the change was a data definition language (DDL) operation, then an LCR encapsulates the DDL change that was made to a shared database object at a source database.
2. A propagation propagates the staged LCR to another queue, which usually resides in a database that is separate from the database where the LCR was captured. An LCR can be propagated to a number of queues before it arrives at a destination database.
3. At a destination database, an apply process consumes the change by applying the LCR to the shared database object. An apply process can dequeue the LCR and apply it directly, or an apply process can dequeue the LCR and send it to an apply handler. In a Streams replication environment, an apply handler performs customized processing of the LCR and then applies the LCR to the shared database object.
Step 1 and Step 3 are required, but Step 2 is optional because, in some cases, an application can enqueue an LCR directly into a queue at a destination database. In addition, in a heterogeneous replication environment in which an Oracle database shares information with a non-Oracle database, an apply process can apply changes directly to a non-Oracle database without propagating LCRs.
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