Error handling

Error handling

The event nature of WebSocket protocol defines how app errors should be handled. Once a connection has been established between the client and the server, communitation between these two points should be handled only through events - that also concerns errors.

Avoid throwing exceptions explicitly when possible. If the exception is thrown, try to catch it and send Event instead.

export const saveUser$: WsEffect = event$ =>
  event$.pipe(
    matchEvent('SAVE_USER'),
    map(event => event.payload),
    mergeMap(Dao.saveUser),
    catchError(error => of({
      type: 'SAVE_USER_ERROR',
      payload: error.message,
    })),
  );

If you are building a reusable middleware or you really need to throw an exception, @marblejs/core exports a dedicated event error constructor which can be intercepted via listener error handler. The EventError constructor takes as a first parameter an event object and as a second parameter a string message. Optionally you can pass the third argument, which can contain any other error related value.

new EventError(event, 'some error message', additionalData);

Custom error handling effect

By default Marble.js comes with simple and lightweight error handling effect, but you can define a custom one if you want.

error.ws-effect.ts

export const customError$: WsErrorEffect<ThrownError> = (event$, client, meta) =>
  req$.pipe(
    map(event => ({
      type: 'ERROR',
      payload: meta.error.message
    }),
  );

As any other Effect, error handler maps the stream of errored events to objects of type Event (type, payload). The WsErrorEffect can retrieve from the third argument an intercepted error object which can be used for error handling-related logic.

To connect the custom error effect, all you have to do is to attach it to error$ property in webSocketListener config object.

webSocket.listener.ts

import { webSocketListener } from '@marblejs/websockets';
import { customError$ } from './error.ws-effect';

const webSocketServer = webSocketListener({
  middlewares,
  effects,
  // Custom error effect 👇
  error$: customError$,
});