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Structured logging

P37.logging.01 · Audience: guest, it-ml, language-pro

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Observability starts with logs — but not the print-a-sentence kind. A production service logs structured records: dicts of key/value fields a log system can filter, aggregate and alert on. This module has you build a structured logger, so the two mechanics that make it work — context binding and redaction — are yours.

Step 1 / 3 — Structured, not stringly
ⓘ Concept: Log fields, not sentences
A structured log line is a dict of key/value fields(rendered as JSON), not a formatted string. Instead of "handled request for alice, status 200" you emit {"event": "handled", "user": "alice", "status": 200} — now "all 500s for alice" is a query, not a grep.

Why it matters — You cannot filter or alert on a sentence; a log system works on fields, so a structured record is what makes logs useful at scale.

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🎓 Practice ladder

2 graded rungs · ~20 min

Build a structured logger with context binding and PII redaction, then design a service's logging in an interview-style prompt.

Rung 1 — build a structured logger (context binding + redaction)

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Rung 2 — logging in production (interview)

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Logs tell you about individual events. The next module adds the second and third pillars — metrics for aggregates and traces for where the time went.

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Structured logging — TransformerLab