Go Logging

Core Principle

Logs are for operators, not developers. Every log line should help someone diagnose a production issue. If it doesn't serve that purpose, it's noise.

Choosing a Logger

Normative: Use log/slog for new Go code.

slog is structured, leveled, and in the standard library (Go 1.21+). It covers the vast majority of production logging needs.

Which logger?
├─ New production code      → log/slog
├─ Trivial CLI / one-off    → log (standard)
└─ Measured perf bottleneck → zerolog or zap (benchmark first)

Do not introduce a third-party logging library unless profiling shows slog is a bottleneck in your hot path. When you do, keep the same structured key-value style.

Read references/LOGGING-PATTERNS.md when setting up slog handlers, configuring JSON/text output, or migrating from log.Printf to slog.

Structured Logging

Normative: Always use key-value pairs. Never interpolate values into the message string.

The message is a static description of what happened. Dynamic data goes in key-value attributes:

// Good: static message, structured fields
slog.Info("order placed", "order_id", orderID, "total", total)

// Bad: dynamic data baked into the message string
slog.Info(fmt.Sprintf("order %d placed for $%.2f", orderID, total))

Key Naming

Advisory: Use snake_case for log attribute keys.

Keys should be lowercase, underscore-separated, and consistent across the codebase: user_id, request_id, elapsed_ms.

Typed Attributes

For performance-critical paths, use typed constructors to avoid allocations:

slog.LogAttrs(ctx, slog.LevelInfo, "request handled",
    slog.String("method", r.Method),
    slog.Int("status", code),
    slog.Duration("elapsed", elapsed),
)

Read references/LEVELS-AND-CONTEXT.md when optimizing log performance or pre-checking with Enabled().

Log Levels

Advisory: Follow these level semantics consistently.

Level	When to use	Production default
Debug	Developer-only diagnostics, tracing internal state	Disabled
Info	Notable lifecycle events: startup, shutdown, config loaded	Enabled
Warn	Unexpected but recoverable: deprecated feature used, retry succeeded	Enabled
Error	Operation failed, requires operator attention	Enabled

Rules of thumb:

If nobody should act on it, it's not Error — use Warn or Info
If it's only useful with a debugger attached, it's Debug
slog.Error should always include an "err" attribute

slog.Error("payment failed", "err", err, "order_id", id)
slog.Warn("retry succeeded", "attempt", n, "endpoint", url)
slog.Info("server started", "addr", addr)
slog.Debug("cache lookup", "key", key, "hit", hit)

Read references/LEVELS-AND-CONTEXT.md when choosing between Warn and Error or defining custom verbosity levels.

Request-Scoped Logging

Advisory: Derive loggers from context to carry request-scoped fields.

Use middleware to enrich a logger with request ID, user ID, or trace ID, then pass the enriched logger downstream via context or as an explicit parameter:

func middleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        logger := slog.With("request_id", requestID(r))
        ctx := context.WithValue(r.Context(), loggerKey, logger)
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

All subsequent log calls in that request carry request_id automatically.

Read references/LOGGING-PATTERNS.md when implementing logging middleware or passing loggers through context.

Log or Return, Not Both

Normative: Handle each error exactly once — either log it or return it.

Logging an error and then returning it causes duplicate noise as callers up the stack also handle the error.

// Bad: logged here AND by every caller up the stack
if err != nil {
    slog.Error("query failed", "err", err)
    return fmt.Errorf("query: %w", err)
}

// Good: wrap and return — let the caller decide
if err != nil {
    return fmt.Errorf("query: %w", err)
}

Exception: HTTP handlers and other top-of-stack boundaries may log detailed errors server-side while returning a sanitized message to the client:

if err != nil {
    slog.Error("checkout failed", "err", err, "user_id", uid)
    http.Error(w, "internal error", http.StatusInternalServerError)
    return
}

See go-error-handling for the full handle-once pattern and error wrapping guidance.

What NOT to Log

Normative: Never log secrets, credentials, PII, or high-cardinality unbounded data.

Passwords, API keys, tokens, session IDs
Full credit card numbers, SSNs
Request/response bodies that may contain user data
Entire slices or maps of unbounded size

Read references/LEVELS-AND-CONTEXT.md when deciding what data is safe to include in log attributes.

Quick Reference

Do	Don't
`slog.Info("msg", "key", val)`	`log.Printf("msg %v", val)`
Static message + structured fields	`fmt.Sprintf` in message
`snake_case` keys	camelCase or inconsistent keys
Log OR return errors	Log AND return the same error
Derive logger from context	Create a new logger per call
Use `slog.Error` with `"err"` attr	`slog.Info` for errors
Pre-check `Enabled()` on hot paths	Always allocate log args

Related Skills

Error handling: See go-error-handling when deciding whether to log or return an error, or for the handle-once pattern
Context propagation: See go-context when passing request-scoped values (including loggers) through context
Performance: See go-performance when optimizing hot-path logging or reducing allocations in log calls
Code review: See go-code-review when reviewing logging practices in Go PRs

Reference Files

references/LOGGING-PATTERNS.md — slog setup, handler configuration, HTTP middleware, migration from log
references/LEVELS-AND-CONTEXT.md — Level semantics, context-based logging, performance, what not to log

go-logging