Learn about apollo-graphql Subscription - node.js

Question

I'm presently studying Graphql(apollo) and utilising Node.js for the backend.
I've made some progress with queries and mutations, but I'm stuck at subscription. I've seen several videos and read some blogs, but I'm having trouble grasping it since they're utilising front-end frameworks, such as react, and I'm not familiar with react or any other front-end javascript frameworks.
I'm solely interested in learning it for the backend.
Is there anyone who can assist me with this?
I've got three separate queries (or whatever they're called) that I'm working on.
User,
Post and
Comment
Now I want to generate a subscription whenever someone adds a new comment or creates a new post.
For users, comments, and posts, I already have a mutation for add, update and remove.
There is currently no authentication or authorization in place.

Answer 1

Your question is more to help me implement this rather than actually asking an issue or problem you faced after trying out some steps in series of order.
This might help you when learning and implementing GraphQL subscription;
https://www.apollographql.com/docs/apollo-server/data/subscriptions/
https://www.youtube.com/watch?v=R2IrBI4-7Kg&t=698s
If you have tried something and faced a wall, please add your code and what you have done, so I could help you on where you are wrong or what you could do hopefully. Cheers.

Answer 2

The solutions I'm providing are crafted on #apollo/server v.4, with expressMiddleware and mongodb/mongoose on the backend and subscribeToMore with updateQuery on the client-side instead of the useSubscription hook.
The graphql-transport-ws transport library is no longer supported; instead, use graphql-ws.
The implementation differentiates three main collections: User, Post, and Comment, as well as subscription for post creation, post modification, and user authentication.
Likewise, as of 12.2022, the following setup and configuration apply.
Subscription on the backend:
Install the following dependencies:
$ npm i #apollo/server #graphql-tools/schema graphql-subscriptions graphql-ws ws cors body-parser mongoose graphql express
I'm assuming you've already configured your MongoDB models; if not, you might want to look at this repo for a basic setup.
Set up schema types and resolvers, such as this one.
// typeDefs.js
const typeDefs = `#graphql
type User {
id: ID!
email: String!
posts: [Post]!
commentsMade: [Comment]!
}
type Token {
value: String!
}
input UpdatePostInput {
title: String!
}
type Post {
id: ID!
title: String!
postedBy: User
comments: [Comment]!
}
input CommentInput {
text: String!
}
type Comment {
id: ID!
text: String!
commenter: User!
commentFor: Post!
}
type Query {
users: [User]!
user(id: ID!): User!
posts: [Post!]!
post(id: ID!): Post!
comment(id: ID!): Comment!
comments: [Comment!]!
}
type Mutation {
signup(email: String!, password: String!): User
signin(email: String!, password: String!): Token
createPost(title: String): Post
createComment(postId: String!, commentInput: CommentInput!): Comment
updatePost(postId: ID!, updatePostInput: UpdatePostInput!): Post
}
type Subscription {
postAdded: Post
commentAdded: Comment
postUpdated: Post
}
`
export default typeDefs
// resolvers.js
import dotenv from 'dotenv'
import { PubSub } from 'graphql-subscriptions'
import mongoose from 'mongoose'
import { GraphQLError } from 'graphql'
import bcrypt from 'bcrypt'
import UserModel from '../models/User.js'
import PostModel from '../models/Post.js'
import CommentModel from '../models/Comment.js'
dotenv.config()
...
const pubsub = new PubSub()
const User = UserModel
const Post = PostModel
const Comment = CommentModel
const secret = process.env.TOKEN_SECRET
const resolvers = {
Query: {...},
Mutation: {
...
createPost: async (_, args, contextValue) => {
const authUser = contextValue.authUser
if (!authUser) {
throw new GraphQLError('User is not authenticated', {
extensions: {
code: 'UNAUTHENTICATED',
http: { status: 401 },
},
})
}
const post = new Post({
...args,
postedBy: mongoose.Types.ObjectId(authUser.id),
})
try {
const savedPost = await post.save()
authUser.posts = authUser.posts.concat(post._id)
await authUser.save()
const addedPost = {
id: savedPost.id,
title: savedPost.title,
postedBy: savedPost.postedBy,
comments: savedPost.comments,
}
// subscription postAdded with object iterator POST_ADDED
pubsub.publish('POST_ADDED', { postAdded: addedPost })
return post
} catch (error) {
throw new GraphQLError(`Error: ${error.message}`, {
extensions: {
code: 'BAD_USER_INPUT',
http: { status: 400 },
argumentName: args,
},
})
}
},
updatePost: async (_, args, contextValue) => {
const authUser = contextValue.authUser
if (!authUser) {
throw new GraphQLError('User is not authenticated', {
extensions: {
code: 'UNAUTHENTICATED',
http: { status: 401 },
},
})
}
try {
const post = await Post.findByIdAndUpdate(
args.postId,
args.updatePostInput,
{ new: true }
)
.populate('comments')
.populate('postedBy')
const updatedPost = {
id: post.id,
title: post.title,
postedBy: post.postedBy,
comments: post.comments,
}
// subscription postUpdated with object iterator POST_UPDATED
pubsub.publish('POST_UPDATED', { postUpdated: updatedPost })
return post
} catch (error) {
throw new GraphQLError(`Error: ${error.message}`, {
extensions: {
code: 'BAD_REQUEST',
http: { status: 400 },
argumentName: args,
},
})
}
},
},
// resolvers for post addition and post modification using subscribe function
Subscription: {
postAdded: {
subscribe: () => pubsub.asyncIterator('POST_ADDED'),
},
commentAdded: {
subscribe: () => pubsub.asyncIterator('COMMENT_ADDED'),
},
postUpdated: {
subscribe: () => pubsub.asyncIterator('POST_UPDATED'),
},
},
//Hard-coding the default resolvers is appropriate in some cases,
// but I think it is required in fields with references to other
//database models to avoid returning null field values.
Post: {
id: async (parent, args, contextValue, info) => {
return parent.id
},
title: async (parent) => {
return parent.title
},
postedBy: async (parent) => {
const user = await User.findById(parent.postedBy)
.populate('posts', { id: 1, title: 1, comments: 1, postedBy: 1 })
.populate('commentsMade')
//console.log('id', user.id)
//console.log('email', user.email)
return user
},
comments: async (parent) => {
return parent.comments
},
},
Comment: {
id: async (parent, args, contextValue, info) => {
return parent.id
},
text: async (parent, args, contextValue, info) => {
return parent.text
},
commenter: async (parent, args, contextValue, info) => {
const user = await User.findById(parent.commenter)
.populate('posts')
.populate('commentsMade')
return user
},
commentFor: async (parent, args, contextValue, info) => {
const post = await Post.findById(parent.commentFor)
.populate('comments')
.populate('postedBy')
return post
},
},
...
}
export default resolvers
The code in the main entry server file (e.g. index.js) may look like this, e.g.
import dotenv from 'dotenv'
import { ApolloServer } from '#apollo/server'
import { expressMiddleware } from '#apollo/server/express4'
import { ApolloServerPluginDrainHttpServer } from '#apollo/server/plugin/drainHttpServer'
import { makeExecutableSchema } from '#graphql-tools/schema'
import { WebSocketServer } from 'ws'
import { useServer } from 'graphql-ws/lib/use/ws'
import express from 'express'
import http from 'http'
import cors from 'cors'
import bodyParser from 'body-parser'
import jwt from 'jsonwebtoken'
import UserModel from './models/User.js'
import typeDefs from './tpeDefs.js'
import resolvers from './resolvers.js'
import mongoose from 'mongoose'
dotenv.config()
mongoose.set('strictQuery', false)
let db_uri
if (process.env.NODE_ENV === 'development') {
db_uri = process.env.MONGO_DEV
}
mongoose.connect(db_uri).then(
() => {
console.log('Database connected')
},
(err) => {
console.log(err)
}
)
const startGraphQLServer = async () => {
const app = express()
const httpServer = http.createServer(app)
const schema = makeExecutableSchema({ typeDefs, resolvers })
const wsServer = new WebSocketServer({
server: httpServer,
path: '/',
})
const serverCleanup = useServer({ schema }, wsServer)
const server = new ApolloServer({
schema,
context: async ({ req }) => {
const authHeader = req.headers['authorization']
const token = authHeader && authHeader.split(' ')[1]
if (token) {
const decoded = jwt.verify(token, process.env.TOKEN_SECRET)
const authUser = await UserModel.findById(decoded.id)
.populate('commentsMade')
.populate('posts')
return { authUser }
}
},
plugins: [
ApolloServerPluginDrainHttpServer({ httpServer }),
{
async serverWillStart() {
return {
async drainServer() {
await serverCleanup.dispose()
},
}
},
},
],
})
await server.start()
app.use(
'/',
cors(),
bodyParser.json(),
expressMiddleware(server, {
context: async ({ req }) => {
const authHeader = req.headers['authorization']
const token = authHeader && authHeader.split(' ')[1]
if (token) {
const decoded = jwt.verify(token, process.env.TOKEN_SECRET)
const authUser = await UserModel.findById(decoded.id)
.populate('commentsMade')
.populate('posts')
return { authUser }
}
},
})
)
const PORT = 4000
httpServer.listen(PORT, () =>
console.log(`Server is now running on http://localhost:${PORT}`)
)
}
startGraphQLServer()
END. It's time to run some tests and checks in the Apollo Explorer sandbox. Be conscientious about defining the required default resolvers to avert the Apollo server from sending null values on your behalf.
To view the code and implementation, go to this repository.
Happy coding!