# Add a logger

Our application currently prints the "Starting HTTP server" message to standard out, and errors to standard error. Both, standard out and error are also a form of global state. We should print to a logger object.

We'll use Zap (opens new window) in this section of the tutorial but you should be able to use any logging system.

  1. Provide a Zap logger to the application. In this tutorial, we'll use zap.NewExample (opens new window), but for real applications, you should use zap.NewProduction or build a more customized logger.

  2. Add a field to hold the logger on EchoHandler, and in NewEchoHandler add a new logger argument to set this field.

    type EchoHandler struct {
      log *zap.Logger
    func NewEchoHandler(log *zap.Logger) *EchoHandler {
      return &EchoHandler{log: log}
  3. In the EchoHandler.ServeHTTP method, use the logger instead of printing to standard error.

    func (h *EchoHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
      if _, err := io.Copy(w, r.Body); err != nil {
        h.log.Warn("Failed to handle request", zap.Error(err))
  4. Similarly, update NewHTTPServer to expect a logger and log the "Starting HTTP server" message to that.

    func NewHTTPServer(lc fx.Lifecycle, mux *http.ServeMux, log *zap.Logger) *http.Server {
      srv := &http.Server{Addr: ":8080", Handler: mux}
        OnStart: func(ctx context.Context) error {
          ln, err := net.Listen("tcp", srv.Addr)
          if err != nil {
            return err
          log.Info("Starting HTTP server", zap.String("addr", srv.Addr))
          go srv.Serve(ln)
  5. (Optional) You can use the same Zap logger for Fx's own logs as well.

    func main() {
        fx.WithLogger(func(log *zap.Logger) fxevent.Logger {
          return &fxevent.ZapLogger{Logger: log}

    This will replace the [Fx] messages with messages printed to the logger.

  6. Run the application.

    {"level":"info","msg":"initialized custom fxevent.Logger","function":"main.main.func1()"}
    {"level":"info","msg":"OnStart hook executing","callee":"main.NewHTTPServer.func1()","caller":"main.NewHTTPServer"}
    {"level":"info","msg":"Starting HTTP server","addr":":8080"}
    {"level":"info","msg":"OnStart hook executed","callee":"main.NewHTTPServer.func1()","caller":"main.NewHTTPServer","runtime":"6.292µs"}
  7. Post a request to it.

    $ curl -X POST -d 'hello' http://localhost:8080/echo

What did we just do?

We added another component to the application with fx.Provide, and injected that into other components that need to print messages. To do that, we only had to add a new parameter to the constructors.

In the optional step, we told Fx that we'd like to provide a custom logger for Fx's own operations. We used the existing fxevent.ZapLogger to build this custom logger from our injected logger, so that all logs follow the same format.