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Golang / GoLang Production Patterns and Web Standards Interview Questions

How do you implement structured logging in Go using the slog package?

log/slog was added to the standard library in Go 1.21 as the official structured logging solution. It produces machine-readable output (JSON or key-value pairs) and supports levels, attributes, and handler customisation.

import "log/slog"

// Default logger — uses text format to stderr
slog.Info("server starting", "port", 8080)
slog.Warn("connection slow", "latency_ms", 450, "host", "db.internal")
slog.Error("request failed", "error", err, "path", r.URL.Path)

// JSON handler for production (machine-parseable)
logger := slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{
    Level: slog.LevelInfo, // minimum level to emit
    AddSource: true,       // include file:line in output
}))
slog.SetDefault(logger)

// Output: {"time":"2024-01-15T...","level":"INFO","msg":"user created","id":42}

// Grouping related attributes
logger.Info("request completed",
    slog.Group("request",
        slog.String("method", r.Method),
        slog.String("path", r.URL.Path),
        slog.Int("status", 200),
        slog.Duration("duration", time.Since(start)),
    ),
)

// Logger with pre-set attributes (child logger)
reqLogger := logger.With(
    slog.String("request_id", requestID),
    slog.String("user_id", userID),
)
reqLogger.Info("fetching user data")
reqLogger.Info("user data fetched", "count", len(users))

// Context-aware logging — attach logger to context
func withLogger(ctx context.Context, logger *slog.Logger) context.Context {
    return context.WithValue(ctx, loggerKey{}, logger)
}
func loggerFromCtx(ctx context.Context) *slog.Logger {
    if l, ok := ctx.Value(loggerKey{}).(*slog.Logger); ok { return l }
    return slog.Default()
}

What advantage does slog.NewJSONHandler provide over the standard log package?
What does logger.With(...) return in slog?

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