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.

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 or useQuery .

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)

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 This library is no longer actively maintained.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

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

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});

urql

1import { createFetchMultipartSubscription } from "@apollo/client/utilities/subscriptions/urql";
2import { Client, fetchExchange, subscriptionExchange } from "@urql/core";
3
4const url = "https://api.example.com";
5
6const multipartSubscriptionForwarder = createFetchMultipartSubscription(
7  url
8);
9
10const client = new Client({
11  url,
12  exchanges: [
13    fetchExchange,
14    subscriptionExchange({
15      forwardSubscription: multipartSubscriptionForwarder,
16    }),
17  ],
18});

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):

1type Subscription {
2  commentAdded(postID: ID!): Comment
3}

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:

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`;