graphql-mqtt-subscriptions

This package implements the AsyncIterator Interface and PubSubEngine Interface from the graphql-subscriptions package. It allows you to connect your subscriptions manager to an MQTT enabled Pub Sub broker to support horizontally scalable subscriptions setup. This package is an adapted version of my graphql-redis-subscriptions package.

Installation

npm install graphql-mqtt-subscriptions

Using the AsyncIterator Interface

Define your GraphQL schema with a Subscription type.

schema {
  query: Query
  mutation: Mutation
  subscription: Subscription
}

type Subscription {
    somethingChanged: Result
}

type Result {
    id: String
}

Now, create a MQTTPubSub instance.

import { MQTTPubSub } from 'graphql-mqtt-subscriptions';
const pubsub = new MQTTPubSub(); // connecting to mqtt://localhost by default

Now, implement the Subscriptions type resolver, using pubsub.asyncIterator to map the event you need.

const SOMETHING_CHANGED_TOPIC = 'something_changed';

export const resolvers = {
  Subscription: {
    somethingChanged: {
      subscribe: () => pubsub.asyncIterator(SOMETHING_CHANGED_TOPIC)
    }
  }
}

Subscriptions resolvers are not a function, but an object with subscribe method, that returns AsyncIterable.

The AsyncIterator method will tell the MQTT client to listen for messages from the MQTT broker on the topic provided, and wraps that listener in an AsyncIterator object.

When messages are received from the topic, those messages can be returned back to connected clients.

pubsub.publish can be used to send messages to a given topic.

pubsub.publish(SOMETHING_CHANGED_TOPIC, { somethingChanged: { id: "123" }});

Passing Your Own MQTT Client

The basic usage allows you to connect to an MQTT broker running on on localhost and works well for development. For production usage, it is recommended you pass your own MQTT client.

import { connect } from 'mqtt';
import { MQTTPubSub } from 'graphql-mqtt-subscriptions';

const client = connect('mqtt://test.mosquitto.org', {
  reconnectPeriod: 1000,
});

const pubsub = new MQTTPubSub({
  client
});

You can learn more on the mqtt options object here.

Dynamically Create a Topic Based on Subscription Args Passed on the Query:

export const resolvers = {
  Subscription: {
    somethingChanged: {
      subscribe: (_, args) => pubsub.asyncIterator(`${SOMETHING_CHANGED_TOPIC}.${args.relevantId}`),
    },
  },
}

Using Arguments and Payload to Filter Events

import { withFilter } from 'graphql-subscriptions';

export const resolvers = {
  Subscription: {
    somethingChanged: {
      subscribe: withFilter(
        (_, args) => pubsub.asyncIterator(`${SOMETHING_CHANGED_TOPIC}.${args.relevantId}`),
        (payload, variables) => payload.somethingChanged.id === variables.relevantId,
      ),
    },
  },
}

Changing QoS for publications or subscriptions

As specified here, the MQTT.js publish and subscribe functions takes an options object. This object can be defined per trigger with publishOptions and subscribeOptions resolvers.

const triggerToQoSMap = {
  'comments.added': 1,
  'comments.updated': 2,
};

const pubsub = new MQTTPubSub({
  publishOptions: trigger => Promise.resolve({ qos: triggerToQoSMap[trigger] }),
  
  subscribeOptions: (trigger, channelOptions) => Promise.resolve({ 
    qos: Math.max(triggerToQoSMap[trigger], channelOptions.maxQoS), 
  }),
});

Get Notified of the Actual QoS Assigned for a Subscription

MQTT allows the broker to assign different QoS levels than the one requested by the client. In order to know what QoS was given to your subscription, you can pass in a callback called onMQTTSubscribe

const onMQTTSubscribe = (subId, granted) => {
  console.log(`Subscription with id ${subId} was given QoS of ${granted.qos}`);
}

const pubsub = new MQTTPubSub({onMQTTSubscribe});

Change Encoding Used to Encode and Decode Messages

Supported encodings available here

const pubsub = new MQTTPubSub({
  parseMessageWithEncoding: 'utf16le',
});

Basic Usage with SubscriptionManager (Deprecated)

import { MQTTPubSub } from 'graphql-mqtt-subscriptions';
const pubsub = new MQTTPubSub(); // connecting to mqtt://localhost on default
const subscriptionManager = new SubscriptionManager({
  schema,
  pubsub,
  setupFunctions: {},
});

Using Trigger Transform (Deprecated)

Similar to the graphql-redis-subscriptions package, this package supports a trigger transform function. This trigger transform allows you to use the channelOptions object provided to the SubscriptionManager instance, and return a trigger string which is more detailed then the regular trigger.

Here is an example of a generic trigger transform.

const triggerTransform = (trigger, { path }) => [trigger, ...path].join('.');

Note that a path field to be passed to the channelOptions but you can do whatever you want.

Next, pass the triggerTransform to the MQTTPubSub constructor.

const pubsub = new MQTTPubSub({
  triggerTransform,
});

Lastly, a setupFunction is provided for the commentsAdded subscription field. It specifies one trigger called comments.added and it is called with the channelOptions object that holds repoName path fragment.

const subscriptionManager = new SubscriptionManager({
  schema,
  setupFunctions: {
    commentsAdded: (options, { repoName }) => ({
      'comments/added': {
        channelOptions: { path: [repoName] },
      },
    }),
  },
  pubsub,
});

Note that the triggerTransform dependency on the path field is satisfied here.

When subscribe is called like this:

const query = `
  subscription X($repoName: String!) {
    commentsAdded(repoName: $repoName)
  }
`;
const variables = {repoName: 'graphql-mqtt-subscriptions'};
subscriptionManager.subscribe({ query, operationName: 'X', variables, callback });

The subscription string that MQTT will receive will be comments.added.graphql-mqtt-subscriptions. This subscription string is much more specific and means the the filtering required for this type of subscription is not needed anymore. This is one step towards lifting the load off of the graphql api server regarding subscriptions.