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GraphQL subscriptions

GraphQL subscriptions are a mechanism to allow clients to query for data in response to a stream of server-side events.

A GraphQL subscription looks very similar to a query, except that it uses the subscription keyword:

subscription FeedbackLikeSubscription($input: FeedbackLikeSubscribeData!) {
feedback_like_subscribe(data: $input) {
feedback {
like_count
}
}
}
  • Establishing a subscription using this GraphQL snippet will cause the application to be notified whenever an event is emitted from the feedback_like_subscribe stream.
  • feedback_like_subscribe is a subscription root field (or just subscription field), which sets up the subscription on the backend.
  • Like mutations, a subscription is handled in two separate steps. First, a server-side event occurs. Then, the query is executed.
note

Note that the event stream can be completely arbitrary, and can have no relation to the fields selected. In other words, there is no guarantee that the values selected in a subscription will have changed from notification to notification.

  • feedback_like_subscribe returns a specific GraphQL type which exposes the data we can query in response to the server-side event. In this case, we're querying for the Feedback object and its updated like_count. This allows us to show the like count in real time.

An example of a subscription payload received by the client could look like this:

{
"feedback_like_subscribe": {
"feedback": {
"id": "feedback-id",
"like_count": 321,
}
}
}

In Relay, we can declare GraphQL subscriptions using the graphql tag too:

const {graphql} = require('react-relay');

const feedbackLikeSubscription = graphql`
subscription FeedbackLikeSubscription($input: FeedbackLikeSubscribeData!) {
feedback_like_subscribe(data: $input) {
feedback {
like_count
}
}
}
`;
  • Note that subscriptions can also reference GraphQL variables in the same way queries or fragments do.

Using useSubscription to create a subscription

In order to create a subscription in Relay, we can use the useSubscription and requestSubscription APIs. Let's take a look at an example using the useSubscription API:

import type {Environment} from 'react-relay';
import type {FeedbackLikeSubscribeData} from 'FeedbackLikeSubscription.graphql';

const {graphql, useSubscription} = require('react-relay');
const {useMemo} = require('React');

function useFeedbackSubscription(
input: FeedbackLikeSubscribeData,
) {
const config = useMemo({
subscription: graphql`
subscription FeedbackLikeSubscription(
$input: FeedbackLikeSubscribeData!
) {
feedback_like_subscribe(data: $input) {
feedback {
like_count
}
}
}
`,
variables: {input},
}, [input])

return useSubscription(config);
}

Let's distill what's happening here.

  • useSubscription takes a GraphlQLSubscriptionConfig object, which includes the following fields:
    • subscription: the GraphQL literal containing the subscription, and
    • variables: the variables with which to establish the subscription.
  • In addition, useSubscription accepts a Flow type parameter. As with queries, the Flow type of the subscription is exported from the file that the Relay compiler generates.
    • If this type is provided, the GraphQLSubscriptionConfig becomes statically typed as well. It is a best practice to always provide this type.
  • Now, when the useFeedbackSubscription hook commits, Relay will establish a subscription.
    • Unlike with APIs like useLazyLoadQuery, Relay will not attempt to establish this subscription during the render phase.
  • Once it is established, whenever an event occurs, the backend will select the updated Feedback object and select the like_count fields off of it.
    • Since the Feedback type contains an id field, the Relay compiler will automatically add a selection for the id field.
  • When the subscription response is received, Relay will find a feedback object in the store with a matching id and update it with the newly received like_count value.
  • If these values have changed as a result, any components which selected these fields off of the feedback object will be re-rendered. Or, to put it colloquially, any component which depends on the updated data will re-render.
note

The name of the type of the parameter FeedbackLikeSubscribeData is derived from the name of the top-level mutation field, i.e. from feedback_like_subscribe. This type is also exported from the generated graphql.js file.

caution

The GraphQLSubscriptionConfig object passed to useSubscription should be memoized! Otherwise, useSubscription will dispose the subscription and re-establish it with every render!

Refreshing components in response to subscription events

In the previous example, we manually selected like_count. Components that select this field will be re-rendered, should we receive an updated value.

However, it is generally better to spread fragments that correspond to the components that we want to refresh in response to the mutation. This is because the data selected by components can change.

Requiring developers to know about all subscriptions that might fetch their components data (and keeping them up-to-date) is an example of the kind of global reasoning that Relay wants to avoid requiring.

For example, we might rewrite the subscription as follows:

subscription FeedbackLikeSubscription($input: FeedbackLikeSubscribeData!) {
feedback_like_subscribe(data: $input) {
feedback {
...FeedbackDisplay_feedback
...FeedbackDetail_feedback
}
}
}

Now, whenever a event in the feedback_like_subscribe event stream occurred, the data selected by the FeedbackDisplay and FeedbackDetail components will be refetched, and those components will remain in a consistent state.

note

Spreading fragments is generally preferable to refetching the data in response to subscription events, since the updated data can be fetched in a single round trip.

Executing a callback when the subscription fires, errors or is closed by the server

In addition to writing updated data to the Relay store, we may want to execute a callback whenever a subscription payload is received. We may want to execute a callback if an error is received or if an error is received or if the server ends the subscription. The GraphQLSubscriptionConfig can include the following fields to handle such cases:

  • onNext, a callback that is executed when a subscription payload is received. It is passed the subscription response (stopping at fragment spread boundaries).
  • onError, a callback that is executed when the subscription errors. It is passed the error that occured.
  • onCompleted, a callback that is executed when the server ends the subscription.

Declarative mutation directives

Declarative mutation directives and @deleteRecord work in subscriptions, too.

Manipulating connections in response to subscription events

Relay makes it easy to respond to subscription events by adding items to or removing items from connections (i.e. lists). For example, you might want to append a newly created user to a given connection. For more, see Using declarative directives.

Deleting items in response to mutations

In addition, you might want to delete an item from the store in response to a mutation. In order to do this, you would add the @deleteRecord directive to the deleted ID. For example:

subscription DeletePostSubscription($input: DeletePostSubscribeData!) {
delete_post_subscribe(data: $input) {
deleted_post {
id @deleteRecord
}
}
}

Imperatively modifying local data

At times, the updates you wish to perform are more complex than just updating the values of fields and cannot be handled by the declarative mutation directives. For such situations, the GraphQLSubscriptionConfig accepts an updater function which gives you full control over how to update the store.

This is discussed in more detail in the section on Imperatively updating store data.

Configuring the Network Layer

You will need to Configure your Network layer to handle subscriptions.

Usually GraphQL subscriptions are communicated over WebSockets, here's an example using graphql-ws:

import {
...
Network,
Observable
} from 'relay-runtime';
import { createClient } from 'graphql-ws';

const wsClient = createClient({
url:'ws://localhost:3000',
});

const subscribe = (operation, variables) => {
return Observable.create((sink) => {
return wsClient.subscribe(
{
operationName: operation.name,
query: operation.text,
variables,
},
sink,
);
});
}

const network = Network.create(fetchQuery, subscribe);

Alternatively, the legacy subscriptions-transport-ws library can be used too:

import {
...
Network,
Observable
} from 'relay-runtime';
import { SubscriptionClient } from 'subscriptions-transport-ws';

const subscriptionClient = new SubscriptionClient('ws://localhost:3000', {
reconnect: true,
});

const subscribe = (request, variables) => {
const subscribeObservable = subscriptionClient.request({
query: request.text,
operationName: request.name,
variables,
});
// Important: Convert subscriptions-transport-ws observable type to Relay's
return Observable.from(subscribeObservable);
};

const network = Network.create(fetchQuery, subscribe);

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