Using an external PubSub engine

[emmanuelvacher]

Hello,

I would like to know if it is possible to use graphql-mqtt-subscriptions along with this package. More precisely, I'd like someone to explain how to wrap a pubsub with an AsyncIterator since the apollo client needs one for the subscription to be effective.

My final goal is to use a MQTT broker to handle subscriptions instead of the in-memory pubsub engine provided by your package.

Thank you for your help.

[dotansimha]

@emmanuelvacher

Basically, you can implement any custom PubSub, the only thing that is important is to return AsyncIterator as your subscription stream.

You can implement your own logic and transform your MQTT subscription into an AsyncIterator and use it as your subscriptions stream.

AsyncIterator API should implement the AsyncIterator Symbol, next, return and throw methods.
This is an example for a simple AsyncIterator creator, that maps Redis subscriptions into an AsyncIterator (haven't tested it, but it should work 😆 ):

import { $$asyncIterator } from 'iterall';

const redisClient = createClient({ host, port, password });

const redisEventToAsyncIterator = (redisClient: RedisClient, eventName: string) => {
  let promiseResolve;

  redisClient.on('message', (channel: string, message: string) => {
    const parsedMessage = JSON.parse(message);

    if (promiseResolve && eventName === channel) {
      promiseResolve(parsedMessage);
    }
  });

  return {
    next() {
      return new Promise(resolve => {
        promiseResolve = resolve;
      }).then(value => ({
        done: false,
        value,
      }));
    },
    return() {
      return Promise.resolve({ done: true, value: undefined });
    },
    throw(e: Error) {
      return Promise.reject(e);
    },
    [$$asyncIterator]() {
      return this;
    },
  };
};

Then, you can use it as your subscriptions stream, in your resolver:

const REDIS_EVENT_NAME = 'something_changed';

const resolvers = {
   Subscription: {
        somethingChanged: {
             subscribe: () => redisEventToAsyncIterator(client, REDIS_EVENT_NAME),
        }
   }
}

And then each time your Redis instance publishes a message - it will map this message to your GraphQL subscription.

You can see more implementations of AsyncIterator here:

• AsyncIterator from AsyncIterator with filter logic
• AsyncIterator from EventEmitter
• AsyncIterator from Promise
• Empty AsyncIterator
• Error to AsyncIterator

The graphql-mqtt-subscriptions and graphql-redis-subscriptions uses the old API of subscriptions, and it needs an update (@davidyaha), but you can simply implement it now with a custom AsyncIterator.

[emmanuelvacher]

@dotansimha Thank you very much, I'm going to try that.

[dotansimha]

@emmanuelvacher did you managed to implement your own AsyncIterator? can we close this issue?

[emmanuelvacher]

@dotansimha yes I managed to implement my own AsyncIterator thanks to your advice 👍 you can close this issue.