feat(logging): structured JSON logger with request_id context

internal/logging/logger.go

@@ -0,0 +1,63 @@
+// Package logging configures the structured JSON logger used across the service.
+package logging
+
+import (
+	"context"
+	"log/slog"
+	"os"
+)
+
+type ctxKey int
+
+const requestIDKey ctxKey = iota
+
+// New returns a slog.Logger that writes JSON to stdout at the given level.
+// Valid levels: "debug", "info", "warn", "error". Unknown levels default to info.
+func New(level string) *slog.Logger {
+	var lvl slog.Level
+	switch level {
+	case "debug":
+		lvl = slog.LevelDebug
+	case "warn":
+		lvl = slog.LevelWarn
+	case "error":
+		lvl = slog.LevelError
+	default:
+		lvl = slog.LevelInfo
+	}
+
+	handler := slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{
+		Level: lvl,
+		ReplaceAttr: func(_ []string, a slog.Attr) slog.Attr {
+			if a.Key == slog.TimeKey {
+				return slog.Attr{Key: "time", Value: a.Value}
+			}
+			if a.Key == slog.MessageKey {
+				return slog.Attr{Key: "event", Value: a.Value}
+			}
+			return a
+		},
+	})
+	return slog.New(handler)
+}
+
+// WithRequestID stores the request ID in the context for downstream loggers.
+func WithRequestID(ctx context.Context, id string) context.Context {
+	return context.WithValue(ctx, requestIDKey, id)
+}
+
+// RequestIDFrom returns the request ID stored in ctx, or empty string if absent.
+func RequestIDFrom(ctx context.Context) string {
+	if v, ok := ctx.Value(requestIDKey).(string); ok {
+		return v
+	}
+	return ""
+}
+
+// FromContext returns a logger pre-bound with the request_id from ctx (if any).
+func FromContext(ctx context.Context, base *slog.Logger) *slog.Logger {
+	if id := RequestIDFrom(ctx); id != "" {
+		return base.With("request_id", id)
+	}
+	return base
+}