Subscriptions

Get real-time updates from your GraphQL server

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.

note
Subscriptions cannot be used to listen to local client events, like subscribing to changes in the cache. Subscriptions are intended to be used to subscribe to external data changes, and have those received changes be stored in the cache. You can then leverage Apollo Client's observability model to watch for changes in the cache, using client.watchQuery, useQuery, or useFragment.

Supported subscription protocols

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

You must use the same protocol as the GraphQL endpoint you're communicating with.

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 This library is no longer actively maintained.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.

Note: Confusingly, the subscriptions-transport-ws library calls its WebSocket subprotocol graphql-ws, and the graphql-ws library calls its subprotocol graphql-transport-ws! In this article, we refer to the two libraries (subscriptions-transport-ws and graphql-ws), not the two subprotocols.

HTTP

Apollo Client provides out-of-the-box support for multipart subscriptions over HTTP when using HttpLink as your terminating link. 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

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

Relay
TypeScript
1import { createFetchMultipartSubscription } from "@apollo/client/utilities/subscriptions/relay"; 2import { Environment, Network, RecordSource, Store } from "relay-runtime"; 3 4const fetchMultipartSubs = createFetchMultipartSubscription( 5 "https://api.example.com" 6); 7 8const network = Network.create(fetchQuery, fetchMultipartSubs); 9 10export const RelayEnvironment = new Environment({ 11 network, 12 store: new Store(new RecordSource()), 13});

Defining a subscription

In your application's client, you provide a GraphQL document with a subscription operation type:

TypeScript
JavaScript
1const COMMENTS_SUBSCRIPTION: TypedDocumentNode< 2 OnCommentAddedSubscription, 3 OnCommentAddedSubscriptionVariables 4> = gql` 5 subscription OnCommentAdded($postID: ID!) { 6 commentAdded(postID: $postID) { 7 id 8 content 9 } 10 } 11`;

When Apollo Client executes the OnCommentAdded subscription, it establishes a connection to your GraphQL server and listens for response data. Unlike 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.

When your GraphQL server pushes data to the client, that object conforms to the GraphQL response format, just like it does for a query:

JSON
1{ 2 "data": { 3 "commentAdded": { 4 "id": "123", 5 "content": "What a thoughtful and well written post!" 6 } 7 } 8}

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:

Bash
1npm install graphql-ws

2. Initialize a GraphQLWsLink

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

TypeScript
index.ts
1import { GraphQLWsLink } from "@apollo/client/link/subscriptions"; 2import { createClient } from "graphql-ws"; 3 4const wsLink = new GraphQLWsLink( 5 createClient({ 6 url: "ws://localhost:4000/subscriptions", 7 }) 8);

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.

TypeScript
index.ts
1import { OperationTypeNode } from "graphql"; 2import { split, HttpLink } from "@apollo/client"; 3import { GraphQLWsLink } from "@apollo/client/link/subscriptions"; 4import { createClient } from "graphql-ws"; 5 6const httpLink = new HttpLink({ 7 uri: "http://localhost:4000/graphql", 8}); 9 10const wsLink = new GraphQLWsLink( 11 createClient({ 12 url: "ws://localhost:4000/subscriptions", 13 }) 14); 15 16// The split function takes three parameters: 17// 18// * A function that's called for each operation to execute 19// * The Link to use for an operation if the function returns a "truthy" value 20// * The Link to use for an operation if the function returns a "falsy" value 21const splitLink = split( 22 ({ operationType }) => { 23 return operationType === OperationTypeNode.SUBSCRIPTION; 24 }, 25 wsLink, 26 httpLink 27);

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:

TypeScript
JavaScript
index.ts
1import { ApolloClient, InMemoryCache } from "@apollo/client"; 2 3// ...code from the above example goes here... 4 5const client = new ApolloClient({ 6 link: splitLink, 7 cache: new InMemoryCache(), 8});

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:

TypeScript
JavaScript
1import { GraphQLWsLink } from "@apollo/client/link/subscriptions"; 2import { createClient } from "graphql-ws"; 3 4const wsLink = new GraphQLWsLink( 5 createClient({ 6 url: "ws://localhost:4000/subscriptions", 7 connectionParams: { 8 authToken: user.authToken, 9 }, 10 }) 11);

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.

note
In order to use subscriptions over multipart HTTP in a React Native application, additional configuration is required. See the React Native docs for more information.

Executing a subscription

You use the useSubscription hook to execute a subscription. 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.

TypeScript
JavaScript
1const COMMENTS_SUBSCRIPTION: TypedDocumentNode< 2 OnCommentAddedSubscription, 3 OnCommentAddedSubscriptionVariables 4> = gql` 5 subscription OnCommentAdded($postID: ID!) { 6 commentAdded(postID: $postID) { 7 id 8 content 9 } 10 } 11`; 12 13function LatestComment({ postID }: LatestCommentProps) { 14 const { data, loading } = useSubscription(COMMENTS_SUBSCRIPTION, { 15 variables: { postID }, 16 }); 17 18 return <h4>New comment: {!loading && data.commentAdded.content}</h4>; 19}

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.

note
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:

TypeScript
JavaScript
1const COMMENTS_QUERY: TypedDocumentNode< 2 CommentsForPostQuery, 3 CommentsForPostQueryVariables 4> = gql` 5 query CommentsForPost($postID: ID!) { 6 post(postID: $postID) { 7 comments { 8 id 9 content 10 } 11 } 12 } 13`; 14 15function CommentsPageWithData({ params }: CommentsPageWithDataProps) { 16 const result = useQuery(COMMENTS_QUERY, { 17 variables: { postID: params.postID }, 18 }); 19 20 return <CommentsPage {...result} />; 21}

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:

TypeScript
JavaScript
1const COMMENTS_SUBSCRIPTION: TypedDocumentNode< 2 OnCommentAddedSubscription, 3 OnCommentAddedSubscriptionVariables 4> = gql` 5 subscription OnCommentAdded($postID: ID!) { 6 commentAdded(postID: $postID) { 7 id 8 content 9 } 10 } 11`;

Next, we modify our CommentsPageWithData component to call subscribeToMore after the comments query loads.

TypeScript
JavaScript
1function CommentsPageWithData({ params }: CommentsPageWithDataProps) { 2 const { subscribeToMore, ...result } = useQuery(COMMENTS_QUERY, { 3 variables: { postID: params.postID }, 4 }); 5 6 useEffect(() => { 7 // This assumes you want to wait to start the subscription 8 // after the query has loaded. 9 if (result.data) { 10 const unsubscribe = subscribeToMore({ 11 document: COMMENTS_SUBSCRIPTION, 12 variables: { postID: params.postID }, 13 updateQuery: (prev, { subscriptionData }) => { 14 if (!subscriptionData.data) return prev; 15 const newFeedItem = subscriptionData.data.commentAdded; 16 17 return Object.assign({}, prev, { 18 post: { 19 comments: [newFeedItem, ...prev.post.comments], 20 }, 21 }); 22 }, 23 }); 24 25 return () => { 26 unsubscribe(); 27 }; 28 } 29 }, [result.data, params.postID, subscribeToMore]); 30 31 return <CommentsPage {...result} />; 32}

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.

useSubscription API reference

Options

The useSubscription Hook accepts the following options:

Operation options
TVariables

An object containing all of the GraphQL variables your query requires to execute.

Each key in the object corresponds to a variable name, and that key's value corresponds to the variable value.

Other
ApolloClient

An ApolloClient instance. By default useSubscription / Subscription uses the client passed down via context, but a different client can be passed in.

DefaultContext

Shared context between your component and your network interface (Apollo Link).

ErrorPolicy

Specifies the ErrorPolicy to be used for this operation

Record<string, any>

Shared context between your component and your network interface (Apollo Link).

FetchPolicy

How you want your component to interact with the Apollo cache. For details, see Setting a fetch policy.

boolean

If true, the hook will not cause the component to rerender. This is useful when you want to control the rendering of your component yourself with logic in the onData and onError callbacks.

Changing this to true when the hook already has data will reset the data to undefined.

() => void

Allows the registration of a callback function that will be triggered each time the useSubscription Hook / Subscription component completes the subscription.

(options: OnDataOptions<TData>) => any

Allows the registration of a callback function that will be triggered each time the useSubscription Hook / Subscription component receives data. The callback options object param consists of the current Apollo Client instance in client, and the received subscription data in data.

(error: ErrorLike) => void

Allows the registration of a callback function that will be triggered each time the useSubscription Hook / Subscription component receives an error.

boolean | ((options: Options<TData, TVariables>) => boolean)

Determines if your subscription should be unsubscribed and subscribed again when an input to the hook (such as subscription or variables) changes.

boolean

Determines if the current subscription should be skipped. Useful if, for example, variables depend on previous queries and are not ready yet.

Result

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

MaybeMasked<TData>

An object containing the result of your GraphQL subscription. Defaults to an empty object.

ErrorLike

A runtime error with graphQLErrors and networkError properties

boolean

A boolean that indicates whether any initial data has been returned

() => void

A function that when called will disconnect and reconnect the connection to the subscription. If the subscription is deduplicated, this will restart the connection for all deduplicated subscriptions.

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
    1npm install subscriptions-transport-ws

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

    JavaScript
    1import { SubscriptionClient } from "subscriptions-transport-ws";

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

    JavaScript
    1import { WebSocketLink } from "@apollo/client/link/ws";

  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:

TypeScript
1import { WebSocketLink } from "@apollo/client/link/ws"; 2import { SubscriptionClient } from "subscriptions-transport-ws"; 3 4const wsLink = new WebSocketLink( 5 new SubscriptionClient("ws://localhost:4000/subscriptions", { 6 connectionParams: { 7 authToken: user.authToken, 8 }, 9 }) 10);

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