clog

A lightweight log/slog.JSONHandler wrapper that adapts the fields to the Google Cloud Logging structured log format.

The intended use case is Cloud Run, but it should work in similar environments where logs are emitted to stdout/stderr and automatically picked up by Cloud Logging (e.g. App Engine, Cloud Functions, GKE).

Features

• Lightweight. The handler merely reformats/renames the structured JSON log fields. It's still log/slog.JSONHandler under the hood. It does NOT send logs to Cloud Logging directly (e.g. using the Cloud SDK).

• Tracing. A tracing middleware is provided to automatically extract tracing information from traceparent or X-Cloud-Trace-Context HTTP request header, and attaches it to the request context. The Handler automatically includes any tracing information as log attributes.

• Custom levels as supported by Google Cloud Logging, e.g. CRITICAL and NOTICE.

Usage

import (
	"log/slog"

	"cloud.google.com/go/compute/metadata"
	"github.com/ronny/clog"
)

func main() {
	projectID, err := metadata.ProjectID()
	if err != nil {
		panic(err)
	}

	logger := slog.New(
		clog.NewHandler(os.Stderr, clog.HandlerOptions{
			Level: clog.LevelInfo,
			GoogleProjectID: projectID,
		}),
	)
	slog.SetDefault(logger)

	mux := http.NewServeMux()
	mux.Handle("POST /warn", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		ctx := r.Context()
		// ⚠️ The log will have tracing attrs since we're using
		// trace.Middleware below (assuming trace header is in the request):
		// "logging.googleapis.com/trace"
		// "logging.googleapis.com/spanId"
		// "logging.googleapis.com/traceSampled"
		slog.WarnContext(ctx, "flux capacitor is too warm",
			"mycount", 42,
		)
	}))
	mux.Handle("POST /critical", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		ctx := r.Context()
		// ⚠️ Custom level CRITICAL
		slog.Log(ctx, clog.LevelCritical, "flux capacitor is on fire")
	}))

	port := os.Getenv("PORT")
	if port == "" {
		port = "8080"
	}
	log.Printf("listening on port %s", port)

	// ⚠️ `trace.Middleware` to make tracing information available in ctx in mux
	// handlers.
	if err := http.ListenAndServe(":"+port, trace.Middleware(mux)); err != nil {
		log.Fatal(err)
	}
}

Credits and acknowledgements

Thank you to Remko Tronçon for doing most of the hard work in https://github.com/remko/cloudrun-slog which is the basis for this library.