What is Go and structured logging with slog?

Question

Accepted Answer

slog is a **built-in package** for structured logging (Go 1.21+).

```go
import "log/slog"

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

// Basic logging
    slog.Info("server started", "addr", ":8080")
    // {"time":"2024-01-15T10:30:00Z","level":"INFO","msg":"server started","addr":":8080"}

// Attribute grouping
    slog.Info("request handled",
        "method", "GET",
        "path", "/api/users",
        "status", 200,
        "duration", "123ms",
    )

// With context
    ctx := context.Background()
    slog.InfoContext(ctx, "processing", "item_id", 42)

// Complex attributes
    slog.Info("user created",
        "user", slog.GroupValue(
            slog.String("name", "Alice"),
            slog.Int("age", 25),
        ),
    )
    // {"msg":"user created","user":{"name":"Alice","age":25}}
}

// Custom handler
type CustomHandler struct {
    handler slog.Handler
}

func (h *CustomHandler) Handle(ctx context.Context, r slog.Record) error {
    r.Add("service", "my-app")
    return h.handler.Handle(ctx, r)
}

func (h *CustomHandler) Enabled(ctx context.Context, l slog.Level) bool {
    return h.handler.Enabled(ctx, l)
}

func (h *CustomHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
    return &CustomHandler{handler: h.handler.WithAttrs(attrs)}
}

func (h *CustomHandler) WithGroup(name string) slog.Handler {
    return &CustomHandler{handler: h.handler.WithGroup(name)}
}
```

**Advantages:** Structured data, JSON format, performance.

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