lsn

LSN stands for Log Sequence Number, a identifier used in database systems to mark the position of a log record within a transaction log. An LSN provides a stable, monotonically increasing ordering reference for changes recorded in the log, independent of wall-clock time. It is a core concept for durability, recovery, and replication, enabling systems to locate exact points in the log for operations such as point-in-time recovery, incremental backups, and synchronization between primary and replica databases.

In PostgreSQL, LSN refers to a location in the write-ahead log (WAL). Each WAL record is assigned an LSN as it is written, and the LSN is used to track replication progress, perform streaming replication, and implement point-in-time recovery. The LSN also serves as a reference point when archiving WAL segments for restore scenarios.

In Microsoft SQL Server, LSNs identify specific entries in the transaction log. They are important for processes such as log backups, log shipping, and recovery planning. Tools and processes that monitor or control data replication and failover often rely on LSN values to determine what has been applied or needs to be rolled forward or back.

LSNs are typically local to a database or cluster and are not globally comparable across different systems without explicit metadata bridging. They are not timestamps but are ordered as logs are written, which makes them well-suited for ensuring correct replay and recovery semantics. The concept underpins many database features, including replication, backup strategies, and disaster recovery planning.