from_logs_to_trace.md

title	date
From logs to traces	2024-05-01

At SumUp we heavily rely on Opentelemetry tracing and Honeycomb for observability. Traces are, in my opinion, far superior tool for debugging and analysis than logs. Yet, there's time and place for everything and sometimes you need to log (e.g. gracfully ignored errors). In such cases, it's helpful to be able to move between logs and traces. With slog (and probably any other structured logging library that supports context.Context) this becomes metter of 20 lines of code:

type otelLogHandler struct {
	slog.Handler
}

// WithOTEL wraps the slog handler with OTEL handler that extracts and populates trace and span information
// on the log.
func WithOTEL(h slog.Handler) slog.Handler {
	return &otelLogHandler{h}
}

func (h *otelLogHandler) Handle(ctx context.Context, r slog.Record) error {
	spanCtx := trace.SpanContextFromContext(ctx)

	if spanCtx.IsValid() {
		r.AddAttrs(
			slog.String("trace.trace_id", spanCtx.TraceID().String()),
			slog.String("trace.span_id", spanCtx.SpanID().String()),
		)
	}

	return h.Handler.Handle(ctx, r)
}

Then when initializing your slog.Handler:

logger := slog.New(WithOTEL(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{
	AddSource: true,
	Level:     logLevel,
})))

and you are good to go as long as you use slog.ErrorContext (and alternatives for other verbosity levels).