How would you implement structured logging with log/slog in a production service?

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Accepted Answer

package main

import (
    "log/slog"
    "os"
)

func newLogger(env string) *slog.Logger {
    var handler slog.Handler
    opts := &slog.HandlerOptions{
        Level: slog.LevelInfo,
        AddSource: true, // include file:line in output
    }
    if env == "production" {
        handler = slog.NewJSONHandler(os.Stdout, opts)
    } else {
        handler = slog.NewTextHandler(os.Stdout, opts)
    }
    return slog.New(handler)
}

// Request-scoped logger via middleware
func loggingMiddleware(logger *slog.Logger, next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        reqLogger := logger.With(
            slog.String("request_id", r.Header.Get("X-Request-ID")),
            slog.String("method", r.Method),
            slog.String("path", r.URL.Path),
        )
        ctx := context.WithValue(r.Context(), loggerKey, reqLogger)
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

// Usage in handler
func loggerFromCtx(ctx context.Context) *slog.Logger {
    if l, ok := ctx.Value(loggerKey).(*slog.Logger); ok {
        return l
    }
    return slog.Default()
}

// Structured log entry
logger.Info("user created",
    slog.String("user_id", user.ID),
    slog.String("email", user.Email),
    slog.Duration("elapsed", time.Since(start)),
) JSON output for log aggregators (Datadog, Loki, CloudWatch) — text for local development Attach traceID, spanID, userID, requestID to every log entry for correlation Avoid logging sensitive data: passwords, tokens, PII — mask or omit Log at appropriate levels: Debug (local only), Info (normal ops), Warn (recoverable issues), Error (failures) Inject logger via dependency injection — global loggers make testing and context propagation harder

How would you implement structured logging with log/slog in a production service?

Use slog with a JSON handler for machine-parseable output, attach request-scoped fields via slog.With, and inject the logger via dependency injection rather than using a global.

How would you implement structured logging with log/slog in a production service?

Use slog with a JSON handler for machine-parseable output, attach request-scoped fields via slog.With, and inject the logger via dependency injection rather than using a global.

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