Oracle Essentials [Electronic resources] : Oracle Database 10g, 3rd Edition نسخه متنی

12.5 Oracle Streams

Oracle9i Release 2 introduced
Oracle
Streams, a method of sharing data and events within a database or
between databases. Streams enables the propagation of changes via a
capture-and-apply process. Changes can be propagated between Oracle
instances, from Oracle instances to non-Oracle instances (via
Transparent Gateways), and from non-Oracle databases to Oracle (via
messaging gateways). Streams leverages log-based procedures during
change capture and queuing procedures as part of the staging.
User-supplied "apply" rules define
consumption.

Changes are captured from the source database redo log via a
background database process into a logical change record (LCR). LCR
and user message events are enqueued (staged) in a Streams queue.
Events are propagated from source to target queues and then, via a
background process, dequeued in the target database and applied.
Oracle Database 10g adds new capabilities for
the downstream capture of changes and for the enqueue /dequeue of
messages in batch.

Oracle Streams is typically managed through the Java version of
Oracle Enterprise Manager. In Oracle Database
10g, Streams can be configured to provide new
message notification via email, HTTP, and PL/SQL. For migration from
Advanced Replication to Streams, Oracle Database
10g provides a migration tool.

12.5.1 Oracle Streams and Grid Computing

Oracle Streams provides key functionality in
Oracle
Database 10g grid computing implementations. By
its nature, grid computing can consist of widely distributed data,
users, and platforms. Streams enables the movement of data when and
where it is needed, as well as message sharing, notification or
invocation of user procedures on events, message and database change
subscriptions, and interoperation with other platforms. Oracle
Database 10g streamed databases can offload
processing to replica databases by creating operational data stores,
or can create replicas and apply changes from replicas or data
transformations to the production database. (Grid computing is
described in more detail in Chapter 14.)

The use of Streams also plays a role in database migrations to grid
computing and Oracle Database 10g. A single
command begins the migration process. During this process, Streams
captures changes on the source database that are then applied to the
target as migration nears completion.