If the Capture process is disabled on the source database for an extended period of time and the redo log file is archived in the meanwhile, the redo log file is no longer available for the Capture process when it resumes the scan. In that case, the Capture process reads the archived redo log files. In this way, the disabled or deleted Capture process does not hold up the database activity and the redo log switching activity. By scanning the archived redo logs in addition to online redo logs, the Capture process is guaranteed to be able to extract all the transactions in a transactional order.  As a result, the process acts as a backup plan because it will search for any and all data that might have been missed by the Capture process while it was out of commission.

The Capture process, which extracts the database changes from redo log or archive log files, formats the information into events called LCR events. The term event denotes that something has occurred which could cause some action. A more detailed explanation of the LCR is presented in a later section of this chapter. 

After capturing the database changes and converting them into LCR events, they are inputted to a source queue. There may be more than one queue to receive the LCR events. The queues act as staging areas. A Capture process is associated with a single SYS.AnyData queue.  There may be more than one queue defined for the database.