Different Logs, Different Purposes
The word “log” can mean a recovery mechanism inside a database or a record written by an application. These are not interchangeable.
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Write-ahead log (WAL): records database changes for crash recovery and related features such as replication.
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Application log: helps explain what a request or background job did and why it failed.
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Audit trail: records important actions with an actor, time, and outcome for later review.
Why Write Ahead?
In PostgreSQL, WAL records describing a change are flushed before the corresponding data-file changes. After a crash, recovery can replay changes that have not yet reached the data pages. WAL is managed by the database; an application’s “payment completed” message cannot replace a transaction.
Useful Application Logs
Record enough context to connect related events: timestamp, severity, operation, request ID, and outcome. A failed request should be traceable without dumping the entire request body.
Do not log passwords, access tokens, or unnecessary personal data. Restrict access, define retention, and make sure a logging failure does not expose information or exhaust disk space. Structured fields are easier to search than inconsistent free-form messages.
