Redoing (rolling
forward) a transaction 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 (or after-image entries).
Redo log entries do
not need to be saved permanently in the log area until
the
COMMIT is executed
for the transaction (Redo Log
Management).
The redo log entries are also required for the redo when the database system is restarted or recovered.
See also: Undo log entry