User Guide

SAP AG November 2002
Log Entry
The log concept [Page 43] includes the writing of log entries. The SAP DB database system
differentiates between the following log entries:
• Redo log entry [Page
44]
• Undo log entry [Page
44]
Redo Log Entry
Redoing (rolling forward) a transaction [Page 152] means that the modifications of a
successful transaction are repeated, that is to say, the database is set to the consistent state
that it had after the transaction had ended. The redo measure means that additional,
redundant data management is required.
For each transaction, the values of the changed objects are therefore recorded. These logged
values are described as redo log entries [Page
44] (or after-image entries).
Redo log entries must first be stored permanently in the log area [Page
142] using the
COMMIT [Page 129] in the transaction (Redo Log Management [Page 45]).
The redo log entries are also required for the redo when the database system is restarted or
recovered [Page 49].
Undo Log Entry
Undoing (rolling back) a transaction [Page 152] means that the transaction is reset, that is to
say, the database is set to the consistent state that it had before the transaction was started.
The undo measure means that additional, redundant data management is required.
Therefore, for each transaction, the original values of the data objects that are to be changed
by the transaction, that is, the values that were available before the transaction was started,
are stored. These logged values are described as undo log entries [Page 44] (or before-
image entries).
Undo log entries are permanently stored in the stored in the data area [Page
131] before an
SQL statement is executed (Undo Log Management [Page 47]). Each transaction can access
its undo log entries without any locks being set.
In addition, the undo log entries are needed for the undo if the database is restarted or
recovered [Page 49] and are used for History Management [Page 48].
Online Logging
The log concept [Page 43] for the SAP DB database system includes online logging, which is
required for normal database operation.
User Manual: SAP DB 44