Mount Hasura on Apollo federated gateway

shim.js

const { ApolloServer } = require("apollo-server");
const gql = require("graphql-tag");
const fetch = require("node-fetch");
const {
  introspectionQuery,
  buildClientSchema,
  printSchema
} = require("graphql");

const typeDefs = gql`

  schema {
    query: query_root
  }

  type _Service {
    sdl: String
  }
  
  type query_root {
    _service: _Service!
  }

`;

const hasuraURL = "https://somehasuraurl.com/v1/graphql";

async function getHasuraSchema() {
  return await fetch(hasuraURL, {
    method: "POST",
    headers: {
      Accept: "application/json",
      "Content-Type": "application/json"
    },
    body: JSON.stringify({
      query: introspectionQuery
    })
  })
    .then(res => res.json())
    .then(introspectionJSON => {
      delete introspectionJSON.data.__schema.subscriptionType; // apollo-gateway does not support subscriptions as of now and having subscription type throws a wierd error!
      return printSchema(buildClientSchema(introspectionJSON.data));
    });
}

const resolvers = {
  query_root: {
    _service: async () => {
      var hasuraSchema = await getHasuraSchema();
      return { sdl: hasuraSchema };
    }
  }
};

const schema = new ApolloServer({ typeDefs, resolvers });

schema.listen({ port: process.env.PORT }).then(({ url }) => {
  console.log(`schema ready at ${url}`);
});