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.
;const pubsub = ; // 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'; const resolvers = Subscription: somethingChanged: pubsub
Subscriptions resolvers are not a function, but an object with
subscribe
method, that returnsAsyncIterable
.
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;
Dynamically Create a Topic Based on Subscription Args Passed on the Query:
const resolvers = Subscription: somethingChanged: pubsub
Using Arguments and Payload to Filter Events
; const resolvers = Subscription: somethingChanged: subscribe:
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.
;; const client = ; const pubsub = 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.
const triggerToQoSMap = 'comments.added': 1 'comments.updated': 2; const pubsub = Promise Promise;
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 = { console;} const pubsub = onMQTTSubscribe;
Change Encoding Used to Encode and Decode Messages
Supported encodings available here
const pubsub = parseMessageWithEncoding: 'utf16le';
Basic Usage with SubscriptionManager (Deprecated)
;const pubsub = ; // connecting to mqtt://localhost on defaultconst 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;
Note that a
path
field to be passed to thechannelOptions
but you can do whatever you want.
Next, pass the triggerTransform
to the MQTTPubSub
constructor.
const pubsub = 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 = schema setupFunctions: 'comments/added': channelOptions: path: repoName pubsub;
Note that the
triggerTransform
dependency on thepath
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;
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.