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Subscriptions

GraphQL can be used to perform reads with queries and writes with mutations. However, oftentimes clients want to get updates pushed to them from the server when data they care about changes. To support that, GraphQL has a third operation: subscription. TypeGraphQL of course has great support for subscription, using the graphql-subscriptions package created by Apollo GraphQL.

Creating Subscriptions

Subscription resolvers are similar to queries and mutation resolvers but slightly more complicated.

First we create a normal class method as always, but this time annotated with the @Subscription() decorator.

class SampleResolver {
  // ...
  @Subscription()
  newNotification(): Notification {
    // ...
  }
}

Then we have to provide the topics we wish to subscribe to. This can be a single topic string, an array of topics or a function to dynamically create a topic based on subscription arguments passed to the query. We can also use TypeScript enums for enhanced type safety.

class SampleResolver {
  // ...
  @Subscription({
    topics: "NOTIFICATIONS", // single topic
    topics: ["NOTIFICATIONS", "ERRORS"] // or topics array
    topics: ({ args, payload, context }) => args.topic // or dynamic topic function
  })
  newNotification(): Notification {
    // ...
  }
}

We can also provide the filter option to decide which topic events should trigger our subscription. This function should return a boolean or Promise<boolean> type.

class SampleResolver {
  // ...
  @Subscription({
    topics: "NOTIFICATIONS",
    filter: ({ payload, args }) => args.priorities.includes(payload.priority),
  })
  newNotification(): Notification {
    // ...
  }
}

Now we can implement the subscription resolver. It will receive the payload from a triggered topic of the pubsub system using the @Root() decorator. There, we can transform it to the returned shape.

class SampleResolver {
  // ...
  @Subscription({
    topics: "NOTIFICATIONS",
    filter: ({ payload, args }) => args.priorities.includes(payload.priority),
  })
  newNotification(
    @Root() notificationPayload: NotificationPayload,
    @Args() args: NewNotificationsArgs,
  ): Notification {
    return {
      ...notificationPayload,
      date: new Date(),
    };
  }
}

Triggering subscription topics

Ok, we've created subscriptions, but what is the pubsub system and how do we trigger topics?

They might be triggered from external sources like a database but also in mutations, e.g. when we modify some resource that clients want to receive notifications about when it changes.

So, let us assume we have this mutation for adding a new comment:

class SampleResolver {
  // ...
  @Mutation(returns => Boolean)
  async addNewComment(@Arg("comment") input: CommentInput) {
    const comment = this.commentsService.createNew(input);
    await this.commentsRepository.save(comment);
    return true;
  }
}

We use the @PubSub() decorator to inject the pubsub into our method params. There we can trigger the topics and send the payload to all topic subscribers.

class SampleResolver {
  // ...
  @Mutation(returns => Boolean)
  async addNewComment(@Arg("comment") input: CommentInput, @PubSub() pubSub: PubSubEngine) {
    const comment = this.commentsService.createNew(input);
    await this.commentsRepository.save(comment);
    // here we can trigger subscriptions topics
    const payload: NotificationPayload = { message: input.content };
    await pubSub.publish("NOTIFICATIONS", payload);
    return true;
  }
}

For easier testability (mocking/stubbing), we can also inject the publish method by itself bound to a selected topic. This is done by using the @PubSub("TOPIC_NAME") decorator and the Publisher<TPayload> type:

class SampleResolver {
  // ...
  @Mutation(returns => Boolean)
  async addNewComment(
    @Arg("comment") input: CommentInput,
    @PubSub("NOTIFICATIONS") publish: Publisher<NotificationPayload>,
  ) {
    const comment = this.commentsService.createNew(input);
    await this.commentsRepository.save(comment);
    // here we can trigger subscriptions topics
    await publish({ message: input.content });
    return true;
  }
}

And that's it! Now all subscriptions attached to the NOTIFICATIONS topic will be triggered when performing the addNewComment mutation.

Using a custom PubSub system

By default, TypeGraphQL uses a simple PubSub system from grapqhl-subscriptions which is based on EventEmitter. This solution has a big drawback in that it will work correctly only when we have a single instance (process) of our Node.js app.

For better scalability we'll want to use one of the PubSub implementations backed by an external store like Redis with the graphql-redis-subscriptions package.

All we need to do is create an instance of PubSub according to the package instructions and then provide it to the TypeGraphQL buildSchema options:

const myRedisPubSub = getConfiguredRedisPubSub();

const schema = await buildSchema({
  resolvers: [__dirname + "/**/*.resolver.ts"],
  pubSub: myRedisPubSub,
});

Creating a Subscription Server

The bootstrap guide and all the earlier examples used apollo-server to create an HTTP endpoint for our GraphQL API.

Fortunately, to make subscriptions work, we don't need to manually provide a transport layer that doesn't have constraints of HTTP and can do a push-based communication (WebSockets). The apollo-server package has built-in subscriptions support using websockets, so it works out of the box without any changes to our bootstrap config. However, if we want, we can provide the subscriptions property of the config object:

// Create GraphQL server
const server = new ApolloServer({
  schema,
  subscriptions: {
    path: "/subscriptions",
    // other options and hooks, like `onConnect`
  },
});

And it's done! We have a working GraphQL subscription server on /subscriptions, along with the normal HTTP GraphQL server.

Examples

See how subscriptions work in a simple example.

For production usage, it's better to use something more scalable like a Redis-based pubsub system - a working example is also available. However, to launch this example you need to have a running instance of Redis and you might have to modify the example code to provide your connection parameters.