Glossary · Monitoring

Logs

Logs are timestamped records of discrete events a system produces, such as requests, errors and state changes, used to investigate what happened.

Updated October 6, 2026

Logs are append-only records that software writes when something happens: a request arrives, a query fails, a user signs in, a process restarts. Each entry normally carries a timestamp, a severity level and a message, and often extra fields such as a request ID.

Why it matters

When a metric shows that errors spiked, logs carry the detail: which endpoint, which user, which exception. They are also the primary evidence in security reviews and postmortems. Structured logs (for example JSON with named fields) can be filtered and counted by machines, while free-text logs force you to rely on fragile pattern matching.

Example

A structured web server log line:

{"time":"2026-10-06T09:14:03Z","level":"error","status":502,"path":"/checkout","upstream":"10.0.2.15:8080","request_id":"a91f3c","msg":"upstream timed out"}

Searching for request_id=a91f3c across services reconstructs the path of that one request.

Common mistakes

  • Logging secrets, passwords, tokens or personal data.
  • Keeping logs only on the server that crashed, so they vanish with it. Ship them elsewhere.
  • Running at debug level in production and paying for noise.
  • Omitting timestamps with time zones, which makes cross-server comparison unreliable.

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