Queue Tables

In reality, the information on queues brings to light that there are two distinct parts. The first part included the queue buffers which are part of the SGA, and the second part included the associated physical tables, which are called queue tables. Every queue has its queue table and the queues make use of these queue tables for storing the events or messages when they spill over from the buffers.

Capture Process

The Capture process is an Oracle background process that scans the database redo log files to extract the DML and DDL changes that were done on the database objects. The database in which the database changes occur is called the source database.

A typical database transaction initially creates a redo entry in the log buffer, which is a part of the SGA. The background process log writer (LGWR) subsequently writes those redo entries into the online redo log files.  The LGWR also writes the commit records into the redo logs. The Streams Capture process can only read the redo logs or archived redo logs when extracting the transactional information.  It does not read the log buffer.

The Capture process usually runs on the source database system where the actual transactional activity takes place. In which case, the Capture process is called the local Capture process. However, the Capture process can also run on a remote database other than source database, in which case the remote database is called a downstream database. When the Capture process runs on a downstream database, it cannot read the redo log files of the source database; rather, it scans and extracts information from the archived log files of the source database copied to the downstream database.