Transaktionslogfiler

Transaktionslogfiler (transaction log files) are specialized records used by database and file systems to capture all changes made within a transaction. They serve as a durable record that enables atomicity, durability, and crash recovery by detailing which transactions occurred and what data was modified.

A log entry typically includes a transaction identifier, a timestamp, an operation type (insert, update, delete), the affected data or page, and a before-and-after image or sufficient information to redo or undo the change. Logs are usually append-only and written to stable storage before the corresponding data pages are updated, following the write-ahead logging principle.

These logs play a central role in recovery and durability. If the system crashes, the log can be replayed to redo committed transactions and undo uncommitted ones, allowing a consistent database state up to a known point. Logs also enable point-in-time recovery, replication support, and robust backup strategies.

Examples across systems include PostgreSQL, which uses the Write-Ahead Log (WAL); SQL Server, which maintains a transaction log for each database; MySQL with the InnoDB engine, which uses a redo log (and undo log) for recovery; and Oracle, which relies on redo and archived logs. While terminology varies, the core function is the same: capturing the sequence of changes to guarantee data integrity.

Operationally, transaktionslogfiler require careful management. They must be stored reliably, backed up, archived when appropriate, and monitored for growth. Retention policies, log shipping, and sufficient disk space are typical considerations to ensure timely recovery and ongoing system availability.