Full Schema Stitching with Apollo Server

转自： https://tomasalabes.me/blog/nodejs/graphql/apollo/2018/09/18/schema-stitiching-apollo.html

Full Schema Stitching with Apollo Server

In this post we will see how we can stitch the queries/mutations and subscriptions from an Apollo Server and its Links packages. Plus a few tips to make it more interesting ;)

Our Goal

Our goal is creating a single apollo-link that will let Apollo stitch all requests going to the server and send them to the right downstream server.

 

apollo-stitching

 

Once we have the function that creates the link, the overall code should look something like this:

import { introspectSchema, makeRemoteExecutableSchema, mergeSchemas } from 'graphql-tools';

export const createSchema = async () => {
    const { link } = createLink(); // -> this is the method that we will create

    const introspectedSchema = await introspectSchema(link);
    const schema = makeRemoteExecutableSchema({
        link,
        schema: introspectedSchema
    });

    return mergeSchemas({
        schemas: [
            schema
            // more schemas
        ]
    });
};

Ok, let’s stitch!

Mutations / Queries using “apollo-link-http”

This one is pretty straight forward, apollo-link-http redirects http/s queries to the provided URL. I use node-fetchwhich is the recommended fetch package for Node.

import { HttpLink } from 'apollo-link-http';
import fetch from 'node-fetch';

const createHttpLink = (gqlServerUrl) => {
    const uri = `http://${gqlServerUrl}/graphql`;
    return new HttpLink({
        fetch,
        uri
    });
};

Subscriptions using “apollo-link-ws”

Now subscriptions. We need to use apollo-link-ws It’s similar to the http-link except we need to first create a SubscriptionClient where we pass the url, options and the websockets node implementation. In the previous link we passed the fetch implementation, now is the websocket’s one.

import WebSocket from 'ws';
import { WebSocketLink } from 'apollo-link-ws';
import { SubscriptionClient } from 'subscriptions-transport-ws';
import { WebSocketLink } from 'apollo-link-ws';

const createWsLink = (gqlServerUrl) => {
    const wsUri = `ws://${gqlServerUrl}/subscriptions`;
    const wsClient = new SubscriptionClient(
        wsUri,
        {
            reconnect: true // if connection is lost, retry
        },
        WebSocket
    );

    return new WebSocketLink(wsClient);
};

All options for the SubscriptionClient are here.

Creating the Link + a Retry Mechanism

This is where we add the “magic”. We will use the above 2 links, with a split and retry strategies.

We will:

1. Create the HTTP and WS links
2. Using split, depending on the requested operation we will evaluate what link is needed
3. Using RetryLink, we will check if there was an error connecting to the server, and retry the connection if necessary

import { RetryLink } from 'apollo-link-retry';
import { getMainDefinition } from 'apollo-utilities';

const createLink = () => {
    const projectGqlServer = `your-gql-server:3000`;
    const httpLink = createHttpLink(projectGqlServer);
    const wsLink = createWsLink(projectGqlServer);

    const link = new RetryLink({
        // these are the defaults, change them as you will
        delay: {
            initial: 300, // The number of milliseconds to wait before attempting the first retry.
            max: Infinity, // The maximum number of milliseconds that the link should wait for any retry
            jitter: true // Whether delays between attempts should be randomized.
        },
        attempts: {
            max: 5, // The max number of times to try a single operation before giving up.
            retryIf: (error, _operation) => !!error // A predicate function that can determine whether a particular response should be retried.
        }
    }).split( // using "Directional Composition" of links
        ({ query }) => {
            const { kind, operation } = getMainDefinition(query);
            return kind === 'OperationDefinition' && operation === 'subscription';
        },
        wsLink,
        httpLink
    );
    return { link };
};

Conclusion

With this you should be able to route all types of operations received by this server to the remote server.

There’re other strategies using other link packages but I haven’t tried them, as some I think don’t apply to the server but to the client. But I think this gives you the base for the stitching, from here you can take it to where you need it.

As always, hope it helps! And share if it does!

Cheers!