Algorithms for Recovery and Isolation Exploiting Semantics: Difference between revisions

Revision as of 19:41, 9 December 2024

Latest revision as of 16:14, 2 August 2025

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Three main principles lie behind ARIES:

* [[Write-ahead logging]]: Any change to an object is first recorded in the [[database log|log]], and the log must be written to [[stable storage]] before changes to the object are written to disk.

* Repeating history during Redo: On restart after a crash, ARIES retraces the actions of a database before the crash and brings the system back to the exact state that it was in before the crash. Then it undoes the transactions still active at crash time.

* Logging changes during Undo: Changes made to the database while undoing transactions are logged to ensure such an action isn't repeated in the event of repeated restarts.

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The ARIES algorithm relies on logging of all database operations with ascending Sequence Numbers. Usually the resulting logfile is stored on so-called "stable storage", that is a storage medium that is assumed to survive crashes and hardware failures.

To gather the necessary information for the logs, two [[data structure]]s have to be maintained: the dirty [[page table]] (DPT) and the transaction table (TT).

The dirty page table keeps record of all the pages that have been modified, and not yet written to disk, and the first Sequence Number that caused that page to become dirty. The transaction table contains all currently running transactions and the Sequence Number of the last log entry they created.