Cookbook recipe

What the WAL is and why it makes you durable

Applies to PostgreSQL 13–17 Last reviewed May 2026 Grounded in source

What This Is

A production experience capsule based on real incident patterns.

Why It Matters

Builds production judgment under pressure.

Next Step

Run the decision point → · Enter full simulator →

Estimated investigation4 min

Scenario

How does PostgreSQL survive a crash mid-write without corruption? The write-ahead log records every change before the data files are touched. Diagnose it See the current WAL position: SELECT pg_current_wal_lsn(); Why it happens Before modifying a…

Investigation Path

How does PostgreSQL survive a crash mid-write without corruption? The write-ahead log records every change before the data files are touched.

Diagnose it

See the current WAL position:

SELECT pg_current_wal_lsn();

Why it happens

Before modifying a data page, PostgreSQL writes the change to the WAL and flushes it. On crash recovery it replays the WAL from the last checkpoint, redoing committed changes — guaranteeing durability and consistency.

This is a Pro lesson

Get every Learning Pathway and cookbook recipe — grounded in PostgreSQL source code, with diagnostics, fixes, and prevention for each topic.

Continue this lesson to learn:

  • How to fix it
  • Prevent it next time
  • Related & next steps
  • All 36 Learning Pathway lessons
  • 170+ cookbook recipes
  • Source-grounded diagnostics & fixes
Unlock everything — $24.99/month $199/year — save 35%

Secure checkout Cancel anytime Source-grounded

Career Impact

This scenario builds production judgment and operational confidence under pressure.

Open Career Dashboard →

Keep going

Related & next steps

More like this

Logical Replication: Publication Missing Column Changes Memory Contexts: Why PostgreSQL Almost Never Calls free() Diagnosing LWLock wait events Interpreting wait events in pg_stat_activity Index bloat vs table bloat: telling them apart

Concepts on this page

Replication Checkpoint Tuple
Browse the cookbook Step-by-step recipes Follow a pathway Learn it in order Open the glossary Every concept, plainly
Was this helpful?

← All cookbook recipes