Subscriptions

In addition to queries and mutations, GraphQL supports a third operation type: subscriptions.

Like queries, subscriptions enable you to fetch data. Unlike queries, subscriptions maintain an active connection to your GraphQL server (most commonly via WebSocket). This enables your server to push updates to the subscription's result over time.

Subscriptions are useful for notifying your client in real time about changes to back-end data, such as the creation of a new object or updates to an important field.

When to use subscriptions

In the majority of cases, your client should not use subscriptions to stay up to date with your backend. Instead, you should poll intermittently with queries, or re-execute queries on demand when a user performs a relevant action.

You should use subscriptions for the following:

• Small, incremental changes to large objects. Repeatedly polling for a large object is expensive, especially when most of the object's fields rarely change. Instead, you can fetch the object's initial state with a query, and your server can proactively push updates to individual fields as they occur.

• Low-latency, real-time updates. For example, a chat application's client wants to receive new messages as soon as they're available.

Defining a subscription

You define a subscription on both the server side and the client side, just like you do for queries and mutations.

Server side

You define available subscriptions in your GraphQL schema as fields of the Subscription type. The following commentAdded subscription notifies a subscribing client whenever a new comment is added to a particular blog post (specified by postID):

type Subscription {
  commentAdded(postID: ID!): Comment
}

For more information on implementing support for subscriptions on the server side, see the Apollo Server documentation for subscriptions.

Client side

In your application's client, you define the shape of each subscription you want Apollo Client to execute, like so:

const COMMENTS_SUBSCRIPTION = gql`
  subscription OnCommentAdded($repoFullName: String!) {
    commentAdded(repoFullName: $repoFullName) {
      id
      content
    }
  }
`;

When Apollo Client executes the onCommentAdded subscription, it establishes a connection to your GraphQL server and listens for response data. Unlike with a query, there is no expectation that the server will immediately process and return a response. Instead, your server only pushes data to your client when a particular event occurs on your backend.

Whenever your GraphQL server does push data to a subscribing client, that data conforms to the structure of the executed subscription, just like it does for a query:

{
  "data": {
    "commentAdded": {
      "id": "123",
      "content": "What a thoughtful and well written post!"
    }
  }
}

Setting up the transport

Because subscriptions maintain a persistent connection, they can't use the default HTTP transport that Apollo Client uses for queries and mutations. Instead, Apollo Client subscriptions most commonly communicate over WebSocket, via the community-maintained subscriptions-transport-ws library.

1. Install required libraries

Apollo Link is a collection of libraries that help you customize Apollo Client's network communication. One of these libraries is @apollo/client/link/ws, which enables communication over WebSocket.

Install both @apollo/client/link/ws and subscriptions-transport-ws in your project, like so:

npm install @apollo/client subscriptions-transport-ws

2. Initialize a WebSocketLink

Import and initialize a WebSocketLink object in the same project file where you initialize ApolloClient:

import { WebSocketLink } from '@apollo/client/link/ws';

const wsLink = new WebSocketLink({
  uri: `ws://localhost:5000/`,
  options: {
    reconnect: true
  }
});

Replace the value of the uri option with the URL of your GraphQL server. This is often a localhost URL during development.

3. Use different transports for different operations

Apollo Client should use your WebSocketLink for subscriptions, but it shouldn't use it for queries or mutations. For those operations, Apollo Client should use HTTP as usual. To support this, the @apollo/client library provides a split function that lets you use one of two different Links, according to the result of a boolean check.

The following example expands on the previous one by initializing both a WebSocketLink and an HttpLink. It then uses the split function to combine those two Links into a single Link that uses one or the other according to the type of operation being executed.

import { split, HttpLink } from '@apollo/client';
import { getMainDefinition } from '@apollo/client/utilities';
import { WebSocketLink } from '@apollo/client/link/ws';

const httpLink = new HttpLink({
  uri: 'http://localhost:3000/'
});

const wsLink = new WebSocketLink({
  uri: `ws://localhost:5000/`,
  options: {
    reconnect: true
  }
});

// The split function takes three parameters:
//
// * A function that's called for each operation to execute
// * The Link to use for an operation if the function returns a "truthy" value
// * The Link to use for an operation if the function returns a "falsy" value
const splitLink = split(
  ({ query }) => {
    const definition = getMainDefinition(query);
    return (
      definition.kind === 'OperationDefinition' &&
      definition.operation === 'subscription'
    );
  },
  wsLink,
  httpLink,
);

Using this logic, queries and mutations will use HTTP as normal, and subscriptions will use WebSocket.

4. Authenticate over WebSocket (optional)

It is often necessary to authenticate a client before allowing it to receive subscription results. To do this, you can provide a connectionParams option to the WebSocketLink constructor, like so:

import { WebSocketLink } from '@apollo/client/link/ws';

const wsLink = new WebSocketLink({
  uri: `ws://localhost:5000/`,
  options: {
    reconnect: true,
    connectionParams: {      authToken: user.authToken,    },});

Your WebSocketLink passes the connectionParams object to your server whenever it connects. If your server has a SubscriptionsServer object that's listening for WebSocket connections, it receives the connectionParams object and can use it to perform authentication, along with any other connection-related tasks.

Executing a subscription

You use Apollo Client's useSubscription Hook to execute a subscription from React. Like useQuery, useSubscription returns an object from Apollo Client that contains loading, error, and data properties you can use to render your UI.

The following example component uses the subscription we defined earlier to render the most recent comment that's been added to a specified blog post. Whenever the GraphQL server pushes a new comment to the client, the component re-renders with the new comment.

const COMMENTS_SUBSCRIPTION = gql`
  subscription OnCommentAdded($postID: ID!) {
    commentAdded(postID: $postID) {
      id
      content
    }
  }
`;

function LatestComment({ postID }) {
  const { data: { commentAdded }, loading } = useSubscription(
    COMMENTS_SUBSCRIPTION,
    { variables: { postID } }
  );
  return <h4>New comment: {!loading && commentAdded.content}</h4>;
}

Subscribing to updates for a query

Whenever a query returns a result in Apollo Client, that result includes a subscribeToMore function. You can use this function to execute a followup subscription that pushes updates to the query's original result.

The subscribeToMore function is similar in structure to the fetchMore function that's commonly used for handling pagination. The primary difference is that fetchMore executes a followup query, whereas subscribeToMore executes a subscription.

As an example, let's start with a standard query that fetches all of the existing comments for a given blog post:

const COMMENTS_QUERY = gql`
  query CommentsForPost($postID: ID!) {
    post(postID: $postID) {
      comments {
        id
        content
      }
    }
  }
`;

function CommentsPageWithData({ params }) {
  const result = useQuery(
    COMMENTS_QUERY,
    { variables: { postID: params.postID } }
  );
  return <CommentsPage {...result} />;
}

Let's say we want our GraphQL server to push an update to our client as soon as a new comment is added to the post. First we need to define the subscription that Apollo Client will execute when the COMMENTS_QUERY returns:

const COMMENTS_SUBSCRIPTION = gql`
  subscription OnCommentAdded($postID: ID!) {
    commentAdded(postID: $postID) {
      id
      content
    }
  }
`;

Next, we modify our CommentsPageWithData function to add a subscribeToNewComments property to the CommentsPage component it returns. This property is a function that will be responsible for calling subscribeToMore after the component mounts.

function CommentsPageWithData({ params }) {
  const { subscribeToMore, ...result } = useQuery(
    COMMENT_QUERY,
    { variables: { postID: params.postID } }
  );

  return (
    <CommentsPage
      {...result}
      subscribeToNewComments={() =>        subscribeToMore({          document: COMMENTS_SUBSCRIPTION,          variables: { postID: params.postID },          updateQuery: (prev, { subscriptionData }) => {            if (!subscriptionData.data) return prev;            const newFeedItem = subscriptionData.data.commentAdded;            return Object.assign({}, prev, {              post: {                comments: [newFeedItem, ...prev.post.comments]              }            });          }        })      }    />
  );
}

In the example above, we pass three options to subscribeToMore:

• document indicates the subscription to execute.
• variables indicates the variables to include when executing the subscription.
• updateQuery is a function that tells Apollo Client how to combine the query's currently cached result (prev) with the subscriptionData that's pushed by our GraphQL server. The return value of this function completely replaces the current cached result for the query.

Finally, in our definition of CommentsPage, we tell the component to subscribeToNewComments when it mounts:

export class CommentsPage extends Component {
  componentDidMount() {
    this.props.subscribeToNewComments();
  }
}

useSubscription API reference

Note: If you're using React Apollo's Subscription render prop component, the option/result details listed below are still valid (options are component props and results are passed into the render prop function). The only difference is that a subscription prop (which holds a GraphQL subscription document parsed into an AST by gql) is also required.

Options

The useSubscription Hook accepts the following options:

Result

After being called, the useSubscription Hook returns a result object with the following properties: