What Is Archive Log?
As a DBA, you are responsible to recover any failures to a point of failure and also to perform your backup while online. Your organization is a 24×7 shop and you are not able to shutdown the database. Therefore, the database has to be in an archive mode.
For example you have severe corruption or a database crash that required you to restore some datafiles from several hours ago. If you have all the archived logs from that pint in time up until the moment of the crash, you can roll forward all the changes contained in those archived logs against the restored datafiles.
This process will replays all the changes in the database over the past hours. After recovering the last archived log, Oracle will then look to roll forward even more by using the online redo logs. If those online redo logs contain changes necessary, Oracle will apply those changes also.
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