Implementing GraphQL subscriptions

After explaining GraphQL subscriptions, let's look at how we can use Apollo Server to implement them in our application. Because subscriptions utilize WebSocket rather than HTTP, we must use a second GraphQL endpoint created for subscriptions that uses the WebSocket protocol rather than HTTP:

1. Begin by installing the packages listed as follows:

   npm install subscriptions-transport-ws graphql-subscriptions 

2. Add the following imports to the server/src/index.ts file:

   import { createServer } from 'http'; 
   import { execute, subscribe } from 'graphql'; 
   import { SubscriptionServer } from 'subscriptions-
     transport-ws';

3. Following that, we must launch two servers, HTTP and WebSocket. As a result, we must create http.Server by providing our Express app to the createServer() function in the body of the startApolloServer() function, as seen here:

   async function startApolloServer() {
     const PORT...