Apollo Gateway: Health Check: Schema Only

gateway-health-check-schema-only.js

import { ApolloGateway } from "@apollo/gateway";
import { ApolloServer, gql } from "apollo-server-express";

const gateway = new ApolloGateway({ /* Options, maybe! */ });
const server = new ApolloServer({
  subscriptions: false,
  typeDefs: gql`
    type Query {
      book: String
    }
  `,
});

const HEALTH_CHECK_TIMEOUT_MILLIS = 2000;
server.applyMiddleware({
  app,
  path: '/',
  
  // The Apollo Server `onHealthCheck` needs to be defined to enable
  // functionality beyond merely returning "I'm okay!".  This
  // will check to see if there is a GraphQL schema ready to serve
  // requests.  It does not check for availability of downstream
  // services.
  onHealthCheck() {
    // Timeout after 2 seconds.
    let timerHandle;
    const promiseOfTimeout = new Promise((resolve, reject) => {
      timerHandle = setTimeout(
        reject,
        HEALTH_CHECK_TIMEOUT_MILLIS,
        new Error('Health check timeout')
      );
    });

    // The "{ __typename }" query only succeeds if there is a schema!
    const promiseOfResult =
      server.executeOperation({ query: "{ __typename }" })
        .then((result) => {
          // Check some criteria which validates execution at least started.
          // In this case, presence of `data` and absence of `errors`.
          // ... check more if you'd like!
          if (!result ||
              typeof result.data === 'undefined' ||
              typeof result.errors !== 'undefined'
          ) {
            console.error(result);
            throw new Error("Unexpected error during health check!");
          }
        });

    return Promise.race([promiseOfResult, promiseOfTimeout]);
  },
});