Loggsekvensnummer

Loggsekvensnummer, often abbreviated as LSN, is a concept primarily used in database systems, particularly in transactional databases that employ log-based replication or recovery mechanisms. An LSN is a unique, monotonically increasing identifier assigned to each record in a transaction log. The transaction log, also known as a write-ahead log (WAL), records all changes made to the database. Each log record represents an atomic unit of work, such as an insert, update, or delete operation.

The LSN serves as a precise marker for the position within the transaction log. This allows database systems to perform crucial operations efficiently. For instance, during recovery after a system crash, the LSN indicates the point up to which transactions were successfully committed and need to be reapplied. In replication scenarios, the LSN is used to track which log records have been sent to and applied by replica databases, ensuring data consistency between the primary and replica.

The monotonically increasing nature of LSNs is fundamental. It guarantees that a higher LSN always represents a later event in the log. This property is essential for ordering operations and determining the state of the database at a specific point in time. Without LSNs, managing the sequence of operations and ensuring data integrity during failures or replication would be significantly more complex and error-prone. LSNs are a core component for maintaining atomicity, consistency, isolation, and durability (ACID properties) in transactional database systems.