Golang Logging

---
name: golang-logging
description: Standards for structured logging and observability in Golang. Use when adding structured logging or tracing to Go services.
metadata:
  triggers:
    files:
    - 'go.mod'
    - 'pkg/logger/**'
    keywords:
    - logging
    - slog
    - structured logging
    - zap
---
# Golang Logging Standards

## **Priority: P1 (STANDARD)**

## Principles

- **Structured Logging**: Use JSON or structured text. Readable by machines and humans.
- **Leveled Logging**: Debug, Info, Warn, Error.
- **Contextual**: Include correlation IDs (TraceID, RequestID) in logs.
- **No `log.Fatal`**: Avoid terminating app inside libraries. Return error instead. Only `main` should exit.

## Libraries

- **`log/slog` (Recommended)**: Stdlib since Go 1.21. Fast, structured, zero-dep.
- **Zap (`uber-go/zap`)**: High performance, good if pre-1.21 or extreme throughput needed.
- **Zerolog**: Zero allocation, fast JSON logger.

## Workflow: Set Up Structured Logging with slog

1. Create JSON handler at startup in `main()`
2. Optionally wrap in middleware to inject request-scoped attributes
3. Use `slog.With()` to add correlation IDs per request
4. Pass logger via context or dependency injection

See [slog setup and usage examples](references/slog-patterns.md)

## References

- [Slog Patterns](references/slog-patterns.md)

## Anti-Patterns

- **No fmt.Println in production**: Use slog or zap for structured, leveled logging.
- **No log.Fatal in libraries**: Return errors; only main() should call os.Exit.
- **No unstructured log strings**: Include correlation IDs and structured fields via slog.Attr.