Subscriptions in Apollo Server
Persistent GraphQL read operations
Apollo Server 3 removes built-in support for subscriptions. You can reenable support as described below.
Subscriptions are not currently supported in Apollo Federation.
Subscriptions are long-lasting GraphQL read operations that can update their result whenever a particular server-side event occurs. Most commonly, updated results are pushed from the server to subscribing clients. For example, a chat application's server might use a subscription to push newly received messages to all clients in a particular chat room.
Because subscription updates are usually pushed by the server (instead of polled by the client), they usually use the WebSocket protocol instead of HTTP.
Important: Compared to queries and mutations, subscriptions are significantly more complex to implement. Before you begin, confirm that your use case requires subscriptions.
Schema definition
Your schema's Subscription
type defines top-level fields that clients can subscribe to:
type Subscription {
postCreated: Post
}
The postCreated
field will update its value whenever a new Post
is created on the backend, thus pushing the Post
to subscribing clients.
Clients can subscribe to the postCreated
field with a GraphQL string like this:
subscription PostFeed {
postCreated {
author
comment
}
}
Each subscription operation can subscribe to only one field of the
Subscription
type.
Enabling subscriptions
Beginning in Apollo Server 3, subscriptions are not supported by the "batteries-included"
apollo-server
package. To enable subscriptions, you must first swap to theapollo-server-express
package (or any other Apollo Server integration package that supports subscriptions).The following steps assume you've already swapped to
apollo-server-express
.
To run both an Express app and a separate subscription server, we'll create an http.Server
instance that effectively wraps the two and becomes our new listen
er. We'll also create a SubscriptionServer
, which requires a few tweaks to our existing code.
Install
subscriptions-transport-ws
and@graphql-tools/schema
:npm install subscriptions-transport-ws @graphql-tools/schema
Add the following imports to the file where you initialize your
ApolloServer
instance (we'll use these in later steps):index.jsimport { createServer } from 'http'; import { execute, subscribe } from 'graphql'; import { SubscriptionServer } from 'subscriptions-transport-ws'; import { makeExecutableSchema } from '@graphql-tools/schema';
Next, in order to set up both the HTTP and WebSocket servers, we need to create an
http.Server
. Do this by passing your Expressapp
to thecreateServer
function, which we imported from thehttp
module:index.js// This `app` is the returned value from `express()`. const httpServer = createServer(app);
Create an instance of
GraphQLSchema
(if you don't already).If you already pass the
schema
option to theApolloServer
constructor (instead oftypeDefs
andresolvers
), you can skip this step.The
SubscriptionServer
(which we'll instantiate next) doesn't taketypeDefs
andresolvers
options. Instead, it takes an executableGraphQLSchema
. We can pass thisschema
object to both theSubscriptionServer
andApolloServer
. This way, we make sure that the same schema is being used in both places.index.jsconst schema = makeExecutableSchema({ typeDefs, resolvers }); // ... const server = new ApolloServer({ schema, });
Create the
SubscriptionServer
.index.jsconst subscriptionServer = SubscriptionServer.create({ // This is the `schema` we just created. schema, // These are imported from `graphql`. execute, subscribe, }, { // This is the `httpServer` we created in a previous step. server: httpServer, // This `server` is the instance returned from `new ApolloServer`. path: server.graphqlPath, }); // Shut down in the case of interrupt and termination signals // We expect to handle this more cleanly in the future. See (#5074)[https://github.com/apollographql/apollo-server/issues/5074] for reference. ['SIGINT', 'SIGTERM'].forEach(signal => { process.on(signal, () => subscriptionServer.close()); });
Finally, we need to adjust our existing
listen
call.Most Express applications call
app.listen(...)
. Change this tohttpServer.listen(...)
with the same arguments. This way, the server starts listening on the HTTP and WebSocket transports simultaneously.
A completed example of migrating subscriptions is shown below:
Resolving a subscription
Resolvers for Subscription
fields differ from resolvers for fields of other types. Specifically, Subscription
field resolvers are objects that define a subscribe
function:
const resolvers = {
Subscription: {
postCreated: {
// More on pubsub below
subscribe: () => pubsub.asyncIterator(['POST_CREATED']),
},
},
// ...other resolvers...
};
The subscribe
function must return an object of type AsyncIterator
, a standard interface for iterating over asynchronous results. In the example above, an AsyncIterator
is generated by pubsub.asyncIterator
(more on this below).
The PubSub
class
The
PubSub
class is not recommended for production environments, because it's an in-memory event system that only supports a single server instance. After you get subscriptions working in development, we strongly recommend switching it out for a different subclass of the abstractPubSubEngine
class. Recommended subclasses are listed in ProductionPubSub
libraries.
Apollo Server uses a publish-subscribe (pub/sub) model to track events that update active subscriptions. The graphql-subscriptions
library provides the PubSub
class as a basic in-memory event bus to help you get started:
To use the graphql-subscriptions
package, first install it like so:
npm install graphql-subscriptions
A PubSub
instance enables your server code to both publish
events to a particular label and listen for events associated with a particular label. We can create a PubSub
instance like so:
import { PubSub } from 'graphql-subscriptions';
const pubsub = new PubSub();
Publishing an event
You publish an event with the publish
method of a PubSub
instance:
pubsub.publish('POST_CREATED', {
postCreated: {
author: 'Ali Baba',
comment: 'Open sesame'
}
});
- The first parameter is the name of the event label you're publishing to, as a string.
- You don't need to register a label name before publishing to it.
- The second parameter is the payload associated with the event.
- The payload should include whatever data is necessary for your resolvers to populate the associated
Subscription
field and its subfields.
- The payload should include whatever data is necessary for your resolvers to populate the associated
When working with GraphQL subscriptions, you publish
an event whenever a subscription's return value should be updated. One common cause of such an update is a mutation, but any back-end logic might result in changes that should be publish
ed.
As an example, let's say our GraphQL API supports a createPost
mutation:
type Mutation {
createPost(author: String, comment: String): Post
}
A basic resolver for createPost
might look like this:
const resolvers = {
Mutation: {
createPost(parent, args, context) { // Datastore logic lives in postController return postController.createPost(args); }, },
// ...other resolvers...
};
Before we persist the new post's details in our datastore, we can publish
an event that also includes those details:
const resolvers = {
Mutation: {
createPost(parent, args, context) {
pubsub.publish('POST_CREATED', { postCreated: args }); return postController.createPost(args);
},
},
// ...other resolvers...
};
Next, we can listen for this event in our Subscription
field's resolver.
Listening for events
An AsyncIterator
object listens for events that are associated with a particular label (or set of labels) and adds them to a queue for processing. You create an AsyncIterator
by calling the asyncIterator
method of PubSub
:
pubsub.asyncIterator(['POST_CREATED']);
You pass this method an array containing the names of all event labels that the AsyncIterator
should listen for.
Every Subscription
field resolver's subscribe
function must return an AsyncIterator
object. This brings us back to the code sample at the top of Resolving a subscription:
const resolvers = {
Subscription: {
postCreated: {
subscribe: () => pubsub.asyncIterator(['POST_CREATED']),
},
},
// ...other resolvers...
};
With this subscribe
function set, Apollo Server uses the payloads of POST_CREATED
events to push updated values for the postCreated
field.
Filtering events
Sometimes, a client should only receive updated subscription data if that data meets certain criteria. To support this, you can call the withFilter
helper function in your Subscription
field's resolver.
Example
Let's say our server provides a commentAdded
subscription, which should notify clients whenever a comment is added to a specified code repository. A client can execute a subscription that looks like this:
subscription($repoName: String!){
commentAdded(repoFullName: $repoName) {
id
content
}
}
This presents a potential issue: our server probably publishes a COMMENT_ADDED
event whenever a comment is added to any repository. This means that the commentAdded
resolver executes for every new comment, regardless of which repository it's added to. As a result, subscribing clients might receive data they don't want (or shouldn't even have access to).
To fix this, we can use the withFilter
helper function to control updates on a per-client basis.
Here's an example resolver for commentAdded
that uses the withFilter
function:
import { withFilter } from 'graphql-subscriptions';
const resolvers = {
Subscription: {
commentAdded: { subscribe: withFilter( () => pubsub.asyncIterator('COMMENT_ADDED'), (payload, variables) => { // Only push an update if the comment is on // the correct repository for this operation return (payload.commentAdded.repository_name === variables.repoFullName); }, ),
}
},
// ...other resolvers...
};
The withFilter
function takes two parameters:
- The first parameter is exactly the function you would use for
subscribe
if you weren't applying a filter. - The second parameter is a filter function that returns
true
if a subscription update should be sent to a particular client, andfalse
otherwise (Promise<boolean>
is also allowed). This function takes two parameters of its own:payload
is the payload of the event that was published.variables
is an object containing all arguments the client provided when initiating their subscription.
Use withFilter
to make sure clients get exactly the subscription updates they want (and are allowed to receive).
Basic runnable example
An example server is available on GitHub and CodeSandbox:
The server exposes one subscription (numberIncremented
) that returns an integer that's incremented on the server every second. Here's an example subscription that you can run against your server:
subscription IncrementingNumber {
numberIncremented
}
After you start up the server in CodeSandbox, follow the instructions in the browser to test out running the numberIncremented
subscription in Apollo Sandbox. You'll probably need to configure the subscriptions endpoint (wss://YOUR_SANDBOX_ID.sse.codesandbox.io/graphql
) in Sandbox's Settings tab. You'll see the subscription's value update every second.
Operation context
When initializing context for a query or mutation, you usually extract HTTP headers and other request metadata from the req
object provided to the context
function.
For subscriptions, you extract this metadata from the connection
object instead. This object adheres to the ExecutionParams
interface.
Because all operation types use the same context
initialization function, you should check which of req
or connection
is present for each incoming request:
const server = new ApolloServer({
context: ({ req, connection }) => {
if (connection) { // Operation is a Subscription
// Obtain connectionParams-provided token from connection.context
const token = connection.context.authorization || "";
return { token };
} else { // Operation is a Query/Mutation
// Obtain header-provided token from req.headers
const token = req.headers.authorization || "";
return { token };
}
},
});
This is especially important because metadata like auth tokens are sent differently depending on the transport.
onConnect
and onDisconnect
You can define functions that the subscription server executes whenever a subscription request connects (onConnect
) or disconnects (onDisconnect
).
Defining an onConnect
function provides the following benefits:
- You can reject a particular incoming connection by throwing an exception or returning
false
inonConnect
. This is especially useful for authentication. - If
onConnect
returns an object, that object's fields are added to the WebSocket connection'scontext
object. This is not the operationcontext
that's passed between resolvers. However, you can transfer these values from theconnection
'scontext
when you initialize operation context.
You provide these function definitions to the constructor object of a SubscriptionServer
, like so:
SubscriptionServer.create({
schema,
execute,
subscribe,
onConnect(connectionParams, webSocket, context) { console.log('Connected!')
},
onDisconnect(webSocket, context) {
console.log('Disconnected!')
},});
These functions are passed the following parameters:
Name / Type | Description |
---|---|
| Passed to An object containing parameters included in the request, such as an authentication token. For details, see Authenticate over WebSocket in the Apollo Client documentation. |
| The connecting or disconnecting |
| Context object for the WebSocket connection. This is not the |
Example: Authentication with onConnect
On the client, SubscriptionsClient
supports adding token information to connectionParams
(example) that will be sent with the first WebSocket message. On the server, all GraphQL subscriptions are delayed until the connection has been fully authenticated and the onConnect
callback returns a truthy value.
The connectionParams
argument in the onConnect
callback contains the information passed by the client and can be used to validate user credentials.
The GraphQL context can also be extended with the authenticated user data to enable fine grain authorization.
async function findUser(authToken) {
// find a user by auth token
};
SubscriptionServer.create({
schema,
execute,
subscribe,
async onConnect(connectionParams, webSocket) {
if (connectionParams.authToken) {
const currentUser = await findUser(connectionParams.authToken);
return { currentUser };
}
throw new Error('Missing auth token!');
},
{ server, path }
});
The example above looks up a user based on the auth token that is sent with the first initialization message on the transport, then returns the user object as a Promise
. The user object found will be available as context.currentUser
in your GraphQL resolvers.
In case of an authentication error, the Promise
will be rejected, which prevents the client's connection.
Production PubSub
libraries
As mentioned above, the PubSub
class is not recommended for production environments, because its event-publishing system is in-memory. This means that events published by one instance of your GraphQL server are not received by subscriptions that are handled by other instances.
Instead, you should use a subclass of the PubSubEngine
abstract class that you can back with an external datastore such as Redis or Kafka.
The following are community-created PubSub
libraries for popular event-publishing systems:
If none of these libraries fits your use case, you can also create your own PubSubEngine
subclass. If you create a new open-source library, click Edit on GitHub to let us know about it!
The graphql-ws
transport library
This page explains how to connect the subscriptions-transport-ws
library to your Apollo Server. However, as of 2021, this library is not actively maintained. A newer alternative to subscriptions-transport-ws
called graphql-ws
is currently more actively maintained, and its README explains how to use it with Apollo Server. The two libraries implement different protocols for GraphQL subscriptions over websockets, so you will need to adjust your client to support graphql-ws
. At the time of release of Apollo Server 3, the protocol used in the graphql-ws
library is not yet supported by GraphQL Playground or Apollo Explorer, which is why we do not yet document it as our main recommendation for a subscriptions library. If the ability to interact with subscriptions in GraphQL Playground or Apollo Explorer is not important to you, you may prefer using graphql-ws
over subscriptions-transport-ws
.