Usage
Before a user receives a "Commit complete" message, the system must first successfully write the new or changed data to a redo log file.
All changes included in the transaction are first written into the log buffer. Using memory in this way for the initial capture aims to reduce disk IO. Of course, when a transaction commits, the redo log buffer must be flushed to disk, because otherwise the recovery for that commit could not be guaranteed. It is LGWR (Log Writer) that does that flushing.
The redo log makes it possible to replay SQL statements. Before Oracle changes data in a datafile it writes these changes to the redo log. If something happens to one of the datafiles, a backed up datafile can be restored and the redo, that was written since, replayed, which brings the datafile to the state it had before it became unavailable. The same technique is also used in a environment (standby databases): one database (the primary database) records all changes and sends them to the standby databases. These standby databases in turn apply (replay) the arrived redo which keeps the synchronized with the primary database.
If a database crashes, the recovery process has to apply all transactions, both uncommitted as well as committed, to the data-files on disk, using the information in the redo log files. Oracle must re-do all redo-log transactions that have both a begin and a commit entry, and it must undo all transactions that have a begin entry but no commit entry. (Re-doing a transaction in this context simply means applying the information in the redo log files to the database; the system does not re-run the transaction itself.) The system thus re-creates committed transactions by applying the “after image” records in the redo log files to the database, and undoes incomplete transactions by using the “before image” records in the undo tablespace.
Change data capture can read the redo logs.
Read more about this topic: Redo Log
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