1. ARIES

Algorithms for Recovery and Isolation Exploiting Semantics

Developed at IBM Research in early 1990s for the DB2 DBMS.

Write-Ahead Logging:

• Any change is recorded in log on stable storage before the database change is written to disk.
• Must use STEAL + NO-FORCE buffer pool policies.

Repeating History During Redo:

• On restart, retrace actions and restore database to exact state before crash.

Logging Changes During Undo:

• Record undo actions to log to ensure action is not repeated in the event of repeated failures.

2. Log Sequence Numbers

Every WAL log record includes a globally unique log sequence number (LSN).

Various components in the system keep track of LSNs that pertain to them…

• Each data page contains a pageLSN.
  • The LSN of the most recent update to that page.
  • Update the pageLSN every time a txn modifies a record in the page.
• System keeps track of flushedLSN.
  • The max LSN flushed so far.
  • Update the flushedLSN in memory every time the DBMS writes out the WAL buffer to disk.
• Before page x can be written to disk, we must flush log at least to the point where:
  • pageLSNx ≤ flushedLSN

3. Normal Commit & Abort Operations

Each txn invokes a sequence of reads and writes, followed by commit or abort.