Salient Features

Each Capture process uses one LogMiner session. The LogMiner session is constantly scanning the redo log files for database changes and converting them into LCR events. The LogMiner process and methodology can be used independent of Streams to extract the transactions and know the SQL Statements executed during a particular period. Using the LogMiner session, the Capture process internally reads the same set of redo entries and converts them into the LCR objects needed for the Streams operation.

Buffered Queues

There are buffered queues that are extremely useful for the Capture process. A buffered queue includes the following storage areas:

• SGA memory associated with a SYS.AnyData queue that contains only captured events.
   

• Part of a queue table for a SYS.AnyData queue that stores captured events that have spilled from memory.

Recall that every queue has a buffer part and the table part. Oracle uses these queue buffers to stage the captured events. The buffers are part of the SGA, and as such they help to optimize and process at very high speeds. Buffering of captured events in a SYS.anyData queue is the default behavior. The buffering of captured events is not just confined to the source database.  Buffering also happens at destination databases and at intermediate databases when they are configured as Down Stream databases.

The user-enqueued LCR events and user enqueued non-LCR events are stored in the queue tables associated with the appropriate queue. Queue buffers are not used for storing these events. This may have a slight performance impact if there is a large quantity of user-enqueued events.