graphql-subscriptions

2018-10-13 admin

graphql-subscriptions是什么

什么是graphql-subscriptions,GraphQL subscriptions for node.js

graphql-subscriptions使用教程帮助文档

npm version GitHub license

graphql-subscriptions

GraphQL subscriptions is a simple npm package that lets you wire up GraphQL with a pubsub system (like Redis) to implement subscriptions in GraphQL.

You can use it with any GraphQL client and server (not only Apollo).

Installation

npm install graphql-subscriptions graphql or yarn add graphql-subscriptions graphql

This package should be used with a network transport, for example subscriptions-transport-ws.

TypeScript

If you are developing a project that uses this module with TypeScript:

  • ensure that your tsconfig.json lib definition includes "esnext.asynciterable"
  • npm install @types/graphql or yarn add @types/graphql

Getting started with your first subscription

To begin with GraphQL subscriptions, start by defining a GraphQL Subscription type in your schema:

type Subscription {
    somethingChanged: Result
}

type Result {
    id: String
}

Next, add the Subscription type to your schema definition:

schema {
  query: Query
  mutation: Mutation
  subscription: Subscription
}

Now, let’s create a simple PubSub instance - it is a simple pubsub implementation, based on EventEmitter. Alternative EventEmitter implementations can be passed by an options object to the PubSub constructor.

import { PubSub } from 'graphql-subscriptions';

export const pubsub = new PubSub();

Now, implement your Subscriptions type resolver, using the pubsub.asyncIterator to map the event you need:

const SOMETHING_CHANGED_TOPIC = 'something_changed';

export const resolvers = {
  Subscription: {
    somethingChanged: {
      subscribe: () => pubsub.asyncIterator(SOMETHING_CHANGED_TOPIC),
    },
  },
}

Subscriptions resolvers are not a function, but an object with subscribe method, that returns AsyncIterable.

Now, the GraphQL engine knows that somethingChanged is a subscription, and every time we use pubsub.publish over this topic - it will publish it using the transport we use:

pubsub.publish(SOMETHING_CHANGED_TOPIC, { somethingChanged: { id: "123" }});

Note that the default PubSub implementation is intended for demo purposes. It only works if you have a single instance of your server and doesn’t scale beyond a couple of connections. For production usage you’ll want to use one of the PubSub implementations backed by an external store. (e.g. Redis)

Filters

When publishing data to subscribers, we need to make sure that each subscribers get only the data it need.

To do so, we can use withFilter helper from this package, which wraps AsyncIterator with a filter function, and let you control each publication for each user.

withFilter API:

  • asyncIteratorFn: (rootValue, args, context, info) => AsyncIterator<any> : A function that returns AsyncIterator you got from your pubsub.asyncIterator.
  • filterFn: (payload, variables, context, info) => boolean | Promise<boolean> - A filter function, executed with the payload (the published value), variables, context and operation info, must return boolean or Promise<boolean> indicating if the payload should pass to the subscriber.

For example, if somethingChanged would also accept a variable with the ID that is relevant, we can use the following code to filter according to it:

import { withFilter } from 'graphql-subscriptions';

const SOMETHING_CHANGED_TOPIC = 'something_changed';

export const resolvers = {
  Subscription: {
    somethingChanged: {
      subscribe: withFilter(() => pubsub.asyncIterator(SOMETHING_CHANGED_TOPIC), (payload, variables) => {
        return payload.somethingChanged.id === variables.relevantId;
      }),
    },
  },
}

Note that when using withFilter, you don’t need to wrap your return value with a function.

Channels Mapping

You can map multiple channels into the same subscription, for example when there are multiple events that trigger the same subscription in the GraphQL engine.

const SOMETHING_UPDATED = 'something_updated';
const SOMETHING_CREATED = 'something_created';
const SOMETHING_REMOVED = 'something_removed';

export const resolvers = {
  Subscription: {
    somethingChanged: {
      subscribe: () => pubsub.asyncIterator([ SOMETHING_UPDATED, SOMETHING_CREATED, SOMETHING_REMOVED ]),
    },
  },
}

Payload Manipulation

You can also manipulate the published payload, by adding resolve methods to your subscription:

const SOMETHING_UPDATED = 'something_updated';

export const resolvers = {
  Subscription: {
    somethingChanged: {
      resolve: (payload, args, context, info) => {
        // Manipulate and return the new value
        return payload.somethingChanged;
      },
      subscribe: () => pubsub.asyncIterator(SOMETHING_UPDATED),
    },
  },
}

Usage with callback listeners

Your database might have callback-based listeners for changes, for example something like this:

const listenToNewMessages = (callback) => {
  return db.table('messages').listen(newMessage => callback(newMessage));
}

// Kick off the listener
listenToNewMessages(message => {
  console.log(message);
})

The callback function would be called every time a new message is saved in the database. Unfortunately, that doesn’t play very well with async iterators out of the box because callbacks are push-based, where async iterators are pull-based.

We recommend using the callback-to-async-iterator module to convert your callback-based listener into an async iterator:

import asyncify from 'callback-to-async-iterator';

export const resolvers = {
  Subscription: {
    somethingChanged: {
      subscribe: () => asyncify(listenToChanges),
    },
  },
}

Custom AsyncIterator Wrappers

The value you should return from your subscribe resolver must be an AsyncIterator.

You can use this value and wrap it with another AsyncIterator to implement custom logic over your subscriptions.

For example, the following implementation manipulate the payload by adding some static fields:

import { $$asyncIterator } from 'iterall';

export const withStaticFields = (asyncIterator: AsyncIterator<any>, staticFields: Object): Function => {
  return (rootValue: any, args: any, context: any, info: any): AsyncIterator<any> => {

    return {
      next() {
        return asyncIterator.next().then(({ value, done }) => {
          return {
            value: {
              ...value,
              ...staticFields,
            },
            done,
          };
        });
      },
      return() {
        return Promise.resolve({ value: undefined, done: true });
      },
      throw(error) {
        return Promise.reject(error);
      },
      [$$asyncIterator]() {
        return this;
      },
    };
  };
};

You can also take a look at withFilter for inspiration.

For more information about AsyncIterator:

PubSub Implementations

It can be easily replaced with some other implementations of PubSubEngine interface. There are a couple of them out there:

You can also implement a PubSub of your own, by using the exported interface PubSubEngine from this package.

SubscriptionManager @deprecated

SubscriptionManager is the previous alternative for using graphql-js subscriptions directly, and it’s now deprecated.

If you are looking for its API docs, refer to a previous commit of the repository

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