What is a database transaction log?

A database transaction log is a history of all actions executed by a database management system to ensure data integrity.

In more detail, a database transaction log is a critical component of a database management system (DBMS). It is a file that keeps a record of all changes made to the database, including data modifications (insertions, updates, and deletions) and the specific transactions that enact these changes. The primary purpose of a transaction log is to ensure data integrity and facilitate recovery in the event of a system failure.

When a transaction is executed, the DBMS writes a log entry before any changes are made to the database. This log entry contains enough information to either redo the transaction or undo it, depending on the situation. This process is known as write-ahead logging and is crucial for maintaining the atomicity and durability properties of transactions.

In the event of a system failure, the DBMS can use the transaction log to recover the database to a consistent state. It does this by replaying the log entries to redo completed transactions and undoing any incomplete transactions. This process is known as recovery and is a crucial aspect of a DBMS's ability to handle failures gracefully.

Transaction logs also play a vital role in database replication and mirroring. In these scenarios, the transaction log of the primary database is used to replicate changes onto a secondary database. This ensures that the secondary database is an exact copy of the primary database, which is crucial for high availability and disaster recovery scenarios.

In summary, a database transaction log is an essential tool for maintaining data integrity, handling system failures, and facilitating database replication. It provides a reliable history of all transactions and changes made to a database, enabling a DBMS to recover from failures and ensure the consistency and durability of its data.