Garbage collectors are active components of history management. They belong to the user kernel threads. The garbage collectors job is to delete undo log files that are no longer needed along with any database objects belonging to them. Garbage collectors enable data to be read consistently without locking.
Only SAP liveCache instances use garbage collectors.
When the database system starts, the garbage collectors are initialized using permanently stored, internal configuration information.
History management always starts the garbage collectors when a consistent view is completed. The required information is supplied by the lock management. A garbage collector processes sequentially the history file that history management assigned to it.
The garbage collector asks history management, which of the undo files in the history file can be deleted. History management passes the query on to the lock management, where it is checked whether the associated transactions still appear in any views.