Transaction Log File
Transaction Log Files
In the realm of databases, a transaction log file is an indispensable component that records all the changes made to the data. This log, also known as a transaction journal or audit trail, ensures the integrity and recoverability of a database by maintaining a detailed account of every action or transaction executed within the system.
Purpose and Functionality
The primary purpose of a transaction log file is to provide a reliable mechanism for data recovery and ensure atomicity of transactions. In the event of a system crash or failure, the transaction log can be used to revert the database to a consistent state. This is achieved through processes known as rollback and rollforward operations.
- Rolling Back: If a transaction is incomplete due to an error or explicit request, the log records are used to undo any modifications made by that transaction.
- Rolling Forward: Upon restarting a database server, any transactions that were committed but not yet written to the data files are re-applied to ensure all changes are preserved.
Key Concepts
- Write-Ahead Logging (WAL): This is a technique where changes are first recorded in the log before being applied to the data files, ensuring that no data is ever lost, even in the case of a failure.
- Log Shipping: This refers to the process of automating the backup of transaction log files from a primary server to a secondary server, ensuring data replication and availability.
Implementation in Database Systems
In SQL Server, the transaction log is crucial for database recovery processes. If there's hardware or disk failure, the database can be restored to its last consistent state using full database backups alongside differential and transaction log backups. Similarly, TimesTen databases utilize transaction logs alongside checkpoint files to ensure data integrity and durability.
Integration with File Systems
File systems, such as the Common Log File System (CLFS), provide the underlying infrastructure for transaction logs. CLFS marshals log records to in-memory buffers and subsequently writes them to log files on disk, supporting both Transactional NTFS (TxF) and Transactional Registry (TxR) within the Kernel Transaction Manager.