Redo log

A redo log is a crucial component of database management systems (DBMS) used to ensure data integrity and durability in the face of system failures, such as power outages or software crashes. It is a persistent, on-disk record of every change made to the database.

The primary function of the redo log is to allow the DBMS to recover committed transactions after a failure. When a transaction modifies data, the changes are first written to the redo log before being applied to the actual database files. This ensures that even if the system crashes before the data pages are written to disk, the changes can be replayed from the redo log.

The redo log typically consists of a sequence of redo records. Each redo record describes a single data modification, including information such as the table, row, and column that were modified, as well as the new value of the modified data.

During recovery, the DBMS scans the redo log for completed transactions that may not have been fully written to the database before the crash. It then applies the changes described in the redo log to the database, effectively "redoing" the committed transactions and restoring the database to a consistent state.

Several key characteristics define the operation of a redo log:

• Write-ahead logging (WAL): Changes are always written to the redo log before being applied to the database files. This is fundamental to ensuring data durability.
• Sequential writes: Redo logs are typically written sequentially to disk, optimizing write performance.
• Circular buffer: Redo logs often operate as a circular buffer. As the log fills up, older records are overwritten, provided that the corresponding transactions have been checkpointed and written to the database.
• Checkpoints: Periodically, the DBMS performs a checkpoint operation, which involves writing all dirty data pages (modified pages not yet written to disk) from the buffer pool to the database files. This reduces the amount of redo log data that needs to be processed during recovery.

Redo logs are essential for providing ACID (Atomicity, Consistency, Isolation, Durability) properties in a database system, particularly the Durability aspect. They ensure that committed changes survive system failures and that the database can be restored to a consistent state.