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Understanding Database Recovery Mechanisms: WAL, Logging, and Commit Strategies

This document explores the critical concepts of database recovery, focusing on "Write-Ahead Logging" (WAL) and its implications for data integrity during transactions. It delves into the processes following commit and abort actions in transaction management, detailing how recovery from logs ensures consistency in both committed and uncommitted transactions. By examining the mechanisms for writing logs, auditing recovery strategies, and understanding outcomes, this guide provides a roadmap for handling failures and maintaining data stability within larger systems.

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Understanding Database Recovery Mechanisms: WAL, Logging, and Commit Strategies

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  1. Recoverability Recoverable sector D0 D1 chooser Larger actions? Commit point

  2. id  begin_ra() ABORT() COMMIT() end_ra()

  3. Version histories Cell storage Install Journal storage x WriteJournal (item x, v, id)

  4. ReadJournal (item x, v, id) Commit record table Bootstrap Slow

  5. Logging: Cell st.: read/write Log: non-volatile+sequential

  6. Plan: 1) Fail  Recover from log • uncommitted  back out (undo) • committed  install (rddo) 2) ABORT ()  undo cur. action (redo)

  7. Append-only: type: UPDATE id: 172 undo: x  old redo: x  new type: OUTCOME id: 174 status: COMMITTED 1) When to write log? 2) How to recover?

  8. Disk-bound DB: write x • WAL protocol • Write ahead logging • Log COMMIT record • before returning from • commit() log update record

  9. Recovery: • Scan log backwards • Winners: COMMITTED or ABORTED • Losers: Everything else • Redo COMM. Winners • Undo Losers

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