User Guide

SAP AG November 2002

History List

The main storage area required for history management [Page 48] is the history list.

The history list is an internal database list of all history files [Page 48]. The history list is used

to assign a suitable history file to the garbage collectors [Page 49] for processing.

The history list is written to the data area [Page

131] on each savepoint [Page 149]. In the

case of a restart [Page 146], the history list is rebuilt on the basis of the data stored in the

data area.

Garbage Collector

The garbage collectors are active components during history management [Page 48].

Garbage collectors are user kernel threads (UKT) [Page 14]. When the database system is

started, they are initialized using permanently stored, internal configuration information.

During history management, the history list [Page 49] is used to determine the history file

[Page 48] that is to be processed by the garbage collector. In the undo log files [Page 48] that

were determined using the history files, the system searches for delete operations for objects.

If it finds a delete operation, the garbage collector deletes the object.

Restart or Recovery

The log concept [Page 43] for the SAP DB database system includes the import of the

required log entries [Page

44], if the database instance is restarted [Page 146] or recovered.

The redo log manager [Page 49] ensures that the required log entries are made available and

processed in the correct, time-based sequence.

• For each transaction [Page

152], the available redo log entries [Page 44] are stored by

the log reader [Page

50] in a redo log file [Page 50].

• Transactions for which a redo is actually required are stored by the log reader in a redo

list [Page 51].

• The redo tasks [Page

51] evaluate the redo list, and import the required redo and undo

log entries [Page 44] for each transaction they find.