🔸 TL;DR
Log levels are a contract: TRACE/DEBUG for developers, INFO for business flow, WARN for “unexpected but handled”, ERROR/FATAL for “something is broken”. Keep logs actionable, not noisy. 🎯
🔸 WHY IT MATTERS
Bad log levels = either silence during incidents 😬 or 10GB/day of noise 🧨.
Good log levels = faster debugging, cheaper storage, better alerting. 📉🚨
🔸 HOW TO USE EACH LEVEL
▪️ TRACE 🧬
Ultra-detailed step-by-step (method entry/exit, loop iterations, protocol chatter).
✅ Use: short-lived troubleshooting / local reproduction
🚫 Avoid: production by default (too verbose)
▪️ DEBUG 🛠️
Developer-relevant details (computed values, decisions, branch chosen).
✅ Use: diagnosing issues without drowning in logs
Tip: prefer structured fields (e.g., orderId, customerId) over long strings
▪️ INFO 🧭
High-level lifecycle milestones (service started, request accepted, job completed).
✅ Use: “What happened?” timeline
🚫 Avoid: logging every single request in high-traffic systems (unless sampled)
▪️ WARN ⚠️
Unexpected situation, but the app can continue (fallback used, retry triggered, partial degradation).
✅ Use: “Pay attention soon” signals
Tip: make WARNs rare and meaningful, or nobody will care.
▪️ ERROR 💥
Operation failed (request failed, transaction rolled back, dependency down).
✅ Use: includes context + exception + next-step hint if possible
Tip: 1 error = 1 actionable event. Don’t log the same failure 5 times up the stack.
▪️ FATAL ☠️
The app cannot continue (startup failure, corrupted critical config, unrecoverable state).
✅ Use: right before exit/crash
Note: many stacks don’t treat FATAL differently—still useful as a “we’re going down” marker.
🔸 TAKEAWAYS
▪️ TRACE/DEBUG = diagnosis, INFO = narrative, WARN = degraded, ERROR/FATAL = broken 🧠
▪️ Write logs for future you during an incident 🧯
▪️ Prefer structured logs + IDs (traceId, userId, orderId) over long paragraphs 🧾
▪️ Avoid logging secrets 🔐 (tokens, passwords, full PII)
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