Structured logging with log/slog (Go 1.21+)

The standard log package produces unstructured text lines that are hard to parse, query, and correlate in log aggregation systems. Third-party structured loggers (zap, zerolog) fragment the ecosystem.

Use the built-in log/slog package:

import "log/slog"

func main() {
    // JSON output for production
    logger := slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{
        Level: slog.LevelInfo,
    }))
    slog.SetDefault(logger)

    // Structured log calls
    slog.Info("server started", "addr", ":8080", "pid", os.Getpid())
    slog.Error("request failed", "error", err, "path", r.URL.Path)

    // With context (for trace ID propagation)
    slog.InfoContext(ctx, "processing job", "job_id", job.ID)

    // Group related attributes
    logger.With(slog.Group("request",
        "method", r.Method,
        "path", r.URL.Path,
    )).Info("handled")
}

slog provides a standard interface for structured logging. Handlers are pluggable — swap JSON for text or connect to OpenTelemetry without changing call sites. The default handler is the standard log package output.