graphql-mongodb-subscriptions

This repository was bootstrapped by cloning graphql-redis-subscriptions and also referenced the graphql-postgres-subscriptions.

This package implements the PubSubEngine Interface from the graphql-subscriptions package and also the new AsyncIterator interface. It allows you to connect your subscriptions manager to a MongoDB Pub Sub mechanism to support multiple subscription manager instances.

Installation

At first, install the graphql-mongodb-subscriptions package:

npm install graphql-mongodb-subscriptions

As the graphql-subscriptions package is declared as a peer dependency, you might receive warning about an unmet peer dependency if it's not installed already by one of your other packages. In that case you also need to install it too:

npm install graphql-subscriptions

Using as AsyncIterator

Define your GraphQL schema with a Subscription type:

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

type Subscription {
    somethingChanged: Result
}

type Result {
    id: String
}

Now, let's create a simple MongodbPubSub instance:

import { MongodbPubSub } from 'graphql-mongodb-subscriptions';
const pubsub = new MongodbPubSub();

Now, implement your Subscriptions type resolver, using the 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.

Calling the method asyncIterator of the MongodbPubSub instance will send MongoDB a SUBSCRIBE message to the topic provided and will return an AsyncIterator binded to the MongodbPubSub instance and listens to any event published on that topic. Now, the GraphQL engine knows that somethingChanged is a subscription, and every time we will use pubsub.publish over this topic, the MongodbPubSub will PUBLISH the event over MongoDB to all subscribed instances and those in their turn will emit the event to GraphQL using the next callback given by the GraphQL engine.

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

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 both 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,
      ),
    },
  },
}

Configuring MongodbPubSub

MongodbPubSub constructor can be passed a configuration object to enable some advanced features.

MongoPubSubChannelOptions are used to change the default options for the capped collection that will be created. View MongoDB capped collection docs

export type CommonMessageHandler = (message: any) => any;

export interface MongoPubSubChannelOptions {
  size: number;
  max: number;
}

export interface PubSubMongoDbOptions {
  connectionDb: Db;
  channelName?: string;
  channelOptions?: MongoPubSubChannelOptions;
  connectionListener?: (event: string, data: any) => void;
  commonMessageHandler?: CommonMessageHandler;
}

commonMessageHandler

The common message handler gets called with the received document from MongoDB. You can transform the message before it is passed to the individual filter/resolver methods of the subscribers. This way it is for example possible to inject one instance of a DataLoader which can be used in all filter/resolver methods.

const getDataLoader = () => new DataLoader(...)
const commonMessageHandler = ({attributes: {id}, data}) => ({id, dataLoader: getDataLoader()})
const pubsub = new PostgresPubSub({ client, commonMessageHandler });

export const resolvers = {
  Subscription: {
    somethingChanged: {
      resolve: ({ id, dataLoader }) => dataLoader.load(id)
    }
  }
};

Test

npm run test