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.

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

type Subscription {
somethingChanged: Result
}

type Result {
id: String
}
`

Now, create a MQTTPubSub instance.

`javascript
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.

`javascript
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.

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

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

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

Using Arguments and Payload to Filter Events

`javascript
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,
),
},
},
}
`

Passing your own client object

The basic usage is great for development and you will be able to connect to any mqtt enabled server running on your system seamlessly.
For production usage, it is recommended you pass your own MQTT client.

`javascript
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.

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.

`javascript
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

`javascript
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

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

Basic Usage with SubscriptionManager (Deprecated)

`javascript
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.

`javascript
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.

`javascript
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.
`javascript
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:

`javascript
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.