Go slog, example with context handler

main.go

package main

import (
	"context"
	"log/slog"
	"os"
)

type contextKey string

const transactionIDKey = contextKey("transactionID")

func contextWithTransactionID(ctx context.Context, transactionID string) context.Context {
	return context.WithValue(ctx, transactionIDKey, transactionID)
}

type ContextHandler struct {
	slog.Handler
}

func (h *ContextHandler) Handle(ctx context.Context, r slog.Record) error {
	if transactionID, ok := ctx.Value(transactionIDKey).(string); ok {
		r.AddAttrs(slog.String("tx_id", transactionID))
	}

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

func deepProcess(ctx context.Context) {
	slog.DebugContext(ctx, "inside the deep process")
}

func main() {
	var logLevel = new(slog.LevelVar)

	logger := slog.New(&ContextHandler{slog.NewTextHandler(
		os.Stdout, &slog.HandlerOptions{Level: logLevel})})

	slog.SetDefault(logger)

	slog.Debug("Debug 00 ignored")
	slog.Info("Info 00")

	logLevel.Set(slog.LevelDebug)

	slog.Debug("Debug 01")
	slog.Info("Info 01")

	ctx := context.Background()

	slog.DebugContext(ctx, "debug 02")

	// get or create transaction ID or event ID, and add it to the context
	ctx = contextWithTransactionID(ctx, "tx_or_event_or_alert_id_12345")
	slog.DebugContext(ctx, "debug 03")

	deepProcess(ctx)
}