Support GraphQL subscriptions

[kettanaito]

What

I suggest to add support for GraphQL subscriptions (example tutorial from Apollo).

Why

To support all three main GraphQL operations: query, mutation, and subscription.

How

1. Research what this new feature would imply technically.
2. Design and discuss a public API of the library for GraphQL subscriptions.
3. Implement, document, and release.

Technical notes

• GraphQL subscriptions are often implemented via WebSockets. There is a WebSockets support feature request open, which may be a pre-requisite for this task.

[hsavit1]

are there any stopgap solutions that you can recommend until this feature is built @kettanaito ?

[kettanaito]

@hsavit1 as far as I know, GraphQL subscription is an abstraction over WebSocket. Subscriptions support would require a WebSocket support as a pre-requisite (#156). I cannot recommend any solution to achieve that at the moment. WebSocket support is on the roadmap, but I wouldn't expect it any time soon, unless you wish to contribute to it.

[Nabrok]

I finally followed the advice from apollo and moved from subscription-transport-ws to graphql-ws.

Everything is working and tests even pass, but unfortunately they also spit out ...

      Unhandled GraphQL subscription error ApolloError: undefined
          at runNextTicks (node:internal/process/task_queues:61:5)
          at listOnTimeout (node:internal/timers:528:9)
          at processTimers (node:internal/timers:502:7) {
        graphQLErrors: [ Event { isTrusted: [Getter] } ],
        clientErrors: [],
        networkError: null,
        extraInfo: undefined
      }

With subscription-transport-ws I just had an unhandled GET request, which I could ignore with ...

rest.get(graphql_subscription_uri, (_req, res, ctx) => res(ctx.status(200)))

The best I could come up with so far to have graphql-ws run cleanly was to do this in the client in order to avoid sending to graphql-ws during tests ...

const link = process.env.NODE_ENV === 'test' ? httpLink : split(({ query }) => {
//const link = split(({ query }) => {
	const definition = getMainDefinition(query);
	return (
		definition.kind === 'OperationDefinition' &&
		definition.operation === 'subscription'
	);
}, wsLink, authHttpLink);

And then a handler like this ...

	graphql.operation(async (req, res, ctx) => {
		const { query } = await req.json();
		const definition = getMainDefinition(gql`${query}`);
		if (definition.kind !== 'OperationDefinition' || definition.operation !== 'subscription') {
			console.error(query);
			throw new Error("Not a subscription!");
		}
		return res(ctx.data({ all: null, possible: null, subscription: null, fields: null }));
	}),

This is kind of ugly and I don't like it. There must be a better way to just ignore subscriptions?

[Nabrok]

Okay, cleaner work around ...

In __mocks__/@apollo/client/link/subscriptions.ts

import { ApolloLink, FetchResult, Observable, Operation } from '@apollo/client/core';
import { print } from 'graphql';
import { Client } from 'graphql-ws';
  
export class GraphQLWsLink extends ApolloLink {
	constructor(private client: Client) {
		super();
	}
  
	public request(operation: Operation): Observable<FetchResult> {
		return new Observable((sink) => {
			return this.client.subscribe<FetchResult>(
				{ ...operation, query: print(operation.query) },
				{
					next: sink.next.bind(sink),
					complete: sink.complete.bind(sink),
					error: () => null
				},
			);
		});
	}
}

This is basically just the code for apollo versions < 3.5.10 but with the error function doing nothing.

Then just need the rest.get handler in my earlier post.