One of the main internal functions of the SAP DB database system is logging.
In normal database
operation (operational state
ONLINE), the modifying SQL statements of every transaction must be logged,
meaning that log entries (redo
and undo log entries) are written. These log entries are required so that the database
system can ensure transaction consistency and so that individual SQL
statements can be reversed if required. In addition, the undo log entries are needed to
enable consistent reading when no locks are set.
The task of online logging is to store the log entries so that they are constantly available for normal database operation.
In the case of a restart or restore, the required log entries must be made available and processed in the correct time-based sequence.