PostgreSQL’s transaction log is also known as the WAL (Write-Ahead Logging) system. It is responsible for ensuring data durability, high availability, and crash recovery. The WAL system relies on a sequential log of all changes made to the database, which can be used to recreate the database state in case of a crash or a failure.
The WAL system works by persistently writing all changes to the database to the log before applying them to the actual database. This ensures that even in the event of a crash or a system failure, the data changes are still intact and can be recovered.
The WAL system has three main components:
1. WAL buffer
2. WAL segments
3. Checkpoints
1. WAL buffer: The WAL buffer is a memory buffer used to store the latest changes to the database. When a transaction is committed, its changes are first written to the WAL buffer, and then to the WAL segments. The WAL buffer has a fixed size, and it is flushed to disk when it becomes full, or when a checkpoint occurs.
2. WAL segments: The WAL segments are individual files that make up the complete WAL. Each segment has a fixed size (usually 16 MB) and is written sequentially. Once a segment is filled, the WAL writer switches to a new segment file. The WAL segment files can be archived and shipped to other servers for replication and disaster recovery purposes.
3. Checkpoints: Checkpoints are a mechanism to ensure data consistency and limit the recovery time in the case of a crash or failure. The checkpoint process writes a special record to the WAL, indicating that all changes up to that point have been fully written to disk. This record also contains metadata that helps to speed up the crash recovery process. Additionally, checkpoints allow for the WAL segments to be safely truncated, ensuring that the disk space used by the WAL does not grow uncontrollably.
Log shipping is one way to replicate the WAL to another server. This ensures that the WAL is available to another server if the primary server fails or goes offline. Log shipping works by copying the WAL segments from the primary server to the standby server. The standby server applies the changes in the WAL segments, and it can take over as the primary server in case of a failure. PostgreSQL’s pg_receivexlog utility is used to receive and write the WAL data received from the primary server.
Truncating the WAL is the process of removing older WAL segments, which are no longer needed for recovery purposes. The WAL segments can be truncated once all changes up to the checkpoint before those segments have been persisted to disk on all replication targets. Once the WAL segments have been safely replicated, they can be safely removed from the primary server to make space for newly created WAL segments. The ‘pg_archivecleanup‘ utility can be used to purge archived WAL segments from a pg_xlog directory while preserving keep these files that are necessary for use with streaming replication.
In summary, PostgreSQL’s WAL system ensures the durability of all database changes by writing them to a sequential log. The WAL segments can be archived and shipped to other servers for replication and disaster recovery purposes, while checkpoints ensure that the WAL segments can be safely truncated, and WAL data are available for recovery purposes if needed.