Algorithm for Recovery and Isolation Exploiting Semantics (ARIES)

ARIES is a robust recovery algorithm based on the Write-Ahead Logging (WAL) protocol, widely used in modern DBMS to ensure durability and consistency after system failures.

Types of Log Records in ARIES

Every update operation creates a log record, which can be of the following types:

• Undo-only Log Record: Stores only the before image (original data). Used to undo changes.
• Redo-only Log Record: Stores only the after image (new data). Used to redo changes.
• Undo-Redo Log Record: Stores both before and after images. Used for both undo and redo operations.

Log Sequence Numbers (LSN)

• Every log record is assigned a unique, monotonically increasing LSN.
• Every data page has a pageLSN indicating the LSN of the last update applied to it.

Write-Ahead Logging (WAL) Rules

• A log record must reach stable storage before the corresponding data page is written to disk.
• Log writes are buffered in a memory area called the log tail.
• The log tail is flushed to disk when full.
• A transaction is not considered committed until its commit record is flushed to disk.

Checkpoints

Periodically, the system writes a checkpoint record to the log.

A checkpoint contains:

• Transaction Table
• Dirty Page Table

A master log record, stored in stable storage, holds the LSN of the most recent checkpoint.

Recovery Phases in ARIES

When the system restarts after a crash, ARIES performs three recovery phases:

1. Analysis Phase

• Starts from the latest checkpoint.
• Reconstructs the state of active transactions and dirty pages at the time of the crash.

2. Redo Phase

• Replays operations forward from the smallest LSN of dirty pages.
• Reapplies necessary changes to bring the database to the state before crash.

3. Undo Phase

• Scans the log backward.
• Rolls back the effects of uncommitted (loser) transactions.