Redo Log

Redo Log is the mechanism by which Oracle records every data modification (INSERT, UPDATE, DELETE, DDL) before it is permanently written to the datafiles. It is the fundamental guarantee of transaction durability.

How it works #

Oracle writes changes to the online redo logs sequentially and continuously. Redo logs are organized in circular groups: when one group fills up, Oracle switches to the next. When all groups have been used, Oracle returns to the first (log switch).

Online vs Archived #

• Online redo log: the active files where Oracle writes in real time. They are circular and get overwritten
• Archived redo log: copies of online redo logs saved before overwriting. Required for point-in-time recovery and for Data Guard

The database’s ARCHIVELOG mode enables automatic creation of archived logs. Without it, redo logs are overwritten and recovery is limited to the last full backup.

Why they matter #

Redo logs are the heart of Oracle recovery and replication. Without redo:

• Instance recovery after a crash is not possible
• Point-in-time recovery (media recovery) is not possible
• Data Guard cannot function (replication relies entirely on redo)
• Flashback database is not possible