Recovery Manager (WAL)

The Recovery Manager is the component that ensures the Durability aspect of ACID. It guarantees that once a transaction is committed, its effects are permanent, even in the face of system crashes or power failures.

This is achieved through a Write-Ahead Log (WAL) and a recovery protocol inspired by ARIES.

Core Concepts

• Write-Ahead Logging: The fundamental principle is that any change to a data page must be recorded in a log file on durable storage before the data page itself is written back to disk.

• Log Records: The WAL is a sequence of records, each with a unique Log Sequence Number (LSN). Records describe changes (PageDelta, HeapInsert), transaction outcomes (Commit, Abort), or internal state (Checkpoint).

• ARIES Protocol: On startup, the database replays the log to restore a consistent state. This involves three phases: Analysis (to figure out what was happening during the crash), Redo (to re-apply all changes since the last checkpoint), and Undo (to roll back any transactions that did not commit).

This section contains:

• The ARIES Protocol in QuillSQL: A deep dive into the three phases of recovery and how they are implemented.