logging

Golang Logging Standards

Priority: P1 (HIGH)

Principles

• Structured Logging: Use JSON or structured key-value attributes. Machine-readable and filterable.
• Strict Single-Log Boundary: Log errors once where they are handled or translated at the boundary (service or handler level). Never double-log in repository, utility, or deep helper functions—wrap and return errors instead.
• Typed Context Keys: Always use private typed keys (type ctxKey string) for storing and retrieving request-scoped logger attributes.
• Business Traceability vs PII Redaction: Include non-sensitive business identifiers (e.g. tenant ID, organization code, order number) for traceability. Strictly redact credentials, API tokens, passwords, and sensitive PII.
• Leveled Logging: Debug, Info, Warn, Error.
• No log.Fatal: Avoid terminating apps inside libraries. Return errors instead; only main() should handle process exit.

Libraries

• log/slog (Recommended Stdlib): Stdlib since Go 1.21. Fast, structured, zero-dep.
• Zap (uber-go/zap): Production-grade structured logger for high-throughput microservices.
• Zerolog: Zero allocation, fast JSON logger.

Workflow: Set Up Structured Logging

1. Initialize structured logger at startup in main() with standard JSON output.
2. Inject request correlation IDs (TraceID, RequestID) via middleware using typed context keys.
3. Pass logger via context (logger.FromContext(ctx)) or constructor injection.
4. Log at the application boundary on error; wrap errors upstream without intermediate logging.

See slog setup and usage examples

Anti-Patterns

• No Double-Logging: Do not log an error in the repository/database helper AND log it again in the service/handler.
• No fmt.Println in production: Use structured, leveled loggers.
• No Untyped Context Keys: Avoid context.WithValue(ctx, "logger", l) with raw string keys.
• No Secrets in Logs: Redact bearer tokens, passwords, and sensitive personal info.
• No log.Fatal in libraries: Return errors; only main() should call os.Exit.

References

• Slog Patterns