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 are long-lasting operations that can change their result over time. They can maintain an active connection to your GraphQL server (most commonly via WebSocket), enabling the server to push updates to the subscription's result.

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 (such as clicking a button).

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.

Supported subscription protocols

The GraphQL spec does not define a specific protocol for sending subscription requests. Apollo Client supports the following protocols for subscriptions:

• WebSocket, using one of the following subprotocols:
  • graphql-ws
  • subscriptions-transport-ws (⚠️ unmaintained)
• HTTP, using chunked multipart responses (Apollo Client 3.7.11 and later)

WebSocket subprotocols

The first popular JavaScript library to implement subscriptions over WebSocket is called subscriptions-transport-ws. This library is no longer actively maintained. Its successor is a library called graphql-ws. These two libraries do not use the same WebSocket subprotocol, so you need to use the same subprotocol that your GraphQL endpoint uses.

The WebSocket setup section below uses graphql-ws. If your endpoint uses subscriptions-transport-ws, see this section for differences in configuration.

HTTP

To use Apollo Client with a GraphQL endpoint that supports multipart subscriptions over HTTP, make sure you're using version 3.7.11 or later.

Aside from updating your client version, no additional configuration is required! Apollo Client automatically sends the required headers with the request if the terminating HTTPLink is passed a subscription operation.

Usage with Relay or urql

To consume a multipart subscription over HTTP in an app using Relay or urql, Apollo Client provides network layer adapters that handle the parsing of the multipart response format.

Relay

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import { createFetchMultipartSubscription } from "@apollo/client/utilities/subscriptions/relay";
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import { Environment, Network, RecordSource, Store } from "relay-runtime";
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const fetchMultipartSubs = createFetchMultipartSubscription(
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  "https://api.example.com"
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);
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const network = Network.create(fetchQuery, fetchMultipartSubs);
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export const RelayEnvironment = new Environment({
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  network,
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  store: new Store(new RecordSource()),
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});

urql

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import { createFetchMultipartSubscription } from "@apollo/client/utilities/subscriptions/urql";
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import { Client, fetchExchange, subscriptionExchange } from "@urql/core";
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const url = "https://api.example.com";
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const multipartSubscriptionForwarder = createFetchMultipartSubscription(
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  url
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);
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const client = new Client({
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  url,
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  exchanges: [
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    fetchExchange,
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    subscriptionExchange({
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      forwardSubscription: multipartSubscriptionForwarder,
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    }),
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  ],
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});

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):

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

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:

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

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:

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

WebSocket setup

1. Install required libraries

Apollo Link is a library that helps you customize Apollo Client's network communication. You can use it to define a link chain that modifies your operations and routes them to the appropriate destination.

To execute subscriptions over WebSocket, you can add a GraphQLWsLink to your link chain. This link requires the graphql-ws library. Install it like so:

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npm install graphql-ws

2. Initialize a GraphQLWsLink

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

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import { GraphQLWsLink } from '@apollo/client/link/subscriptions';
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import { createClient } from 'graphql-ws';
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const wsLink = new GraphQLWsLink(createClient({
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  url: 'ws://localhost:4000/subscriptions',
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}));

Replace the value of the url option with your GraphQL server's subscription-specific WebSocket endpoint. If you're using Apollo Server, see Setting a subscription endpoint.

3. Split communication by operation (recommended)

Although Apollo Client can use your GraphQLWsLink to execute all operation types, in most cases it should continue using HTTP for queries and mutations. This is because queries and mutations don't require a stateful or long-lasting connection, making HTTP more efficient and scalable if a WebSocket connection isn't already present.

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 GraphQLWsLink 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.

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import { split, HttpLink } from '@apollo/client';
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import { getMainDefinition } from '@apollo/client/utilities';
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import { GraphQLWsLink } from '@apollo/client/link/subscriptions';
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import { createClient } from 'graphql-ws';
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const httpLink = new HttpLink({
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  uri: 'http://localhost:4000/graphql'
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});
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const wsLink = new GraphQLWsLink(createClient({
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  url: 'ws://localhost:4000/subscriptions',
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}));
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// The split function takes three parameters:
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//
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// * A function that's called for each operation to execute
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// * The Link to use for an operation if the function returns a "truthy" value
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// * The Link to use for an operation if the function returns a "falsy" value
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const splitLink = split(
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  ({ query }) => {
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    const definition = getMainDefinition(query);
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    return (
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      definition.kind === 'OperationDefinition' &&
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      definition.operation === 'subscription'
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    );
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  },
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  wsLink,
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  httpLink,
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);

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

4. Provide the link chain to Apollo Client

After you define your link chain, you provide it to Apollo Client via the link constructor option:

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import { ApolloClient, InMemoryCache } from '@apollo/client';
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// ...code from the above example goes here...
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const client = new ApolloClient({
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  link: splitLink,
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  cache: new InMemoryCache()
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});

5. 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 GraphQLWsLink constructor, like so:

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import { GraphQLWsLink } from '@apollo/client/link/subscriptions';
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import { createClient } from 'graphql-ws';
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const wsLink = new GraphQLWsLink(createClient({
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  url: 'ws://localhost:4000/subscriptions',
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  connectionParams: {
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    authToken: user.authToken,
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  },
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}));

Your GraphQLWsLink passes the connectionParams object to your server whenever it connects. Your server receives the connectionParams object and can use it to perform authentication, along with any other connection-related tasks.

Subscriptions via multipart HTTP

No additional libraries or configuration are required. Apollo Client adds the required headers to your request when the default terminating HTTPLink receives a subscription operation at the uri specified when initializing the link or Apollo Client instance.

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.

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

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.

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

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const COMMENTS_QUERY = gql`
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  query CommentsForPost($postID: ID!) {
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    post(postID: $postID) {
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      comments {
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        id
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        content
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      }
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    }
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  }
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`;
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function CommentsPageWithData({ params }) {
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  const result = useQuery(
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    COMMENTS_QUERY,
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    { variables: { postID: params.postID } }
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  );
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  return <CommentsPage {...result} />;
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}

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:

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

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.

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

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:

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export function CommentsPage({subscribeToNewComments}) {
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  useEffect(() => subscribeToNewComments(), []);
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  return <>...</>
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}

useSubscription API reference

Options

The useSubscription Hook accepts the following options:

Result

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

The older subscriptions-transport-ws library

If your server uses subscriptions-transport-ws instead of the newer graphql-ws library, you need to make a few changes to how you set up your link:

1. Instead of npm install graphql-ws:

   Bash

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   npm install subscriptions-transport-ws

   Copy

2. Instead of import { createClient } from 'graphql-ws':

   JavaScript

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   import { SubscriptionClient } from 'subscriptions-transport-ws'

   Copy

3. Instead of import { GraphQLWsLink } from '@apollo/client/link/subscriptions':

   JavaScript

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

   Copy

4. The options you pass to new SubscriptionClient differ slightly from those passed to createClient:

   • The first argument passed to the SubscriptionClient constructor is the URL for your subscription server.
   • The connectionParams option is nested under an options object called options instead of being at the top level. (You can also pass the new SubscriptionClient constructor arguments directly to new WebSocketLink.)
   • See the subscriptions-transport-ws README for complete SubscriptionClient API docs.

After you create your wsLink, everything else in this article still applies: useSubscription, subscribeToMore, and split links work exactly the same way for both implementations.

The following is an example of a typical WebSocketLink initialization:

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import { WebSocketLink } from "@apollo/client/link/ws";
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import { SubscriptionClient } from "subscriptions-transport-ws";
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const wsLink = new WebSocketLink(
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  new SubscriptionClient("ws://localhost:4000/subscriptions", {
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    connectionParams: {
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      authToken: user.authToken
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    }
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  })
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);

More details on WebSocketLink's API can be found in its API docs.