rollbacklogs

Rollback logs are persistent data structures used in transactional and stateful systems to revert changes and restore a prior state. They record enough information to reverse an operation or undo a transaction, enabling rollback in response to errors, failures, or explicit undo requests.

In database and file-system contexts, rollback logs typically store the original values of data items before they are modified, or the necessary compensating operations to reverse those changes. They are related to, but distinct from, redo logs, which are designed to reapply committed changes after a crash. Rollback logs focus on undoing uncommitted or failed operations.

Mechanically, when a transaction intends to modify data, the system writes the relevant rollback data to the log, often before performing the modification (write-ahead). If the transaction commits, the rollback log entries may be discarded or deferred; if it aborts, the system uses the log to undo the changes and restore the previous state. Some architectures also support savepoints that allow partial rollbacks by applying a sequence of undo actions.

Usage: Rollback logs support crash recovery, ensuring atomicity and consistency, and enable explicit user-initiated or application-driven rollback of long-running operations. They are also used in distributed transactions where partial failures require compensating transactions.

Considerations: The presence of rollback logs adds I/O and storage overhead and may raise privacy or security concerns because logs can contain sensitive data. Proper retention policies and encryption are common mitigations.