index.js

const express = require('express');
const { graphqlHTTP } = require('express-graphql');
const { stitchSchemas } = require('@graphql-tools/stitch');
const { stitchingDirectives } = require('@graphql-tools/stitching-directives');
const { stitchingDirectivesTransformer } = stitchingDirectives();
const { buildSchema, print } = require('graphql');
const { fetch } = require('cross-fetch');

function makeRemoteExecutor(url) {
  return async ({ document, variables }) => {
    const query = typeof document === 'string' ? document : print(document);
    const fetchResult = await fetch(url, {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({ query, variables }),
    });
    return fetchResult.json();
  };
}

async function fetchRemoteSchema(executor) {
  const { data } = await executor({ document: '{ _sdl }' });
  return buildSchema(data._sdl);
}

async function makeGatewaySchema() {
  const accountsExec = makeRemoteExecutor('http://localhost:4001/graphql');
  const productsExec = makeRemoteExecutor('http://localhost:4002/graphql');
  const reviewsExec = makeRemoteExecutor('http://localhost:4003/graphql');

  return stitchSchemas({
    subschemaConfigTransforms: [stitchingDirectivesTransformer],
    subschemas: [
      {
        schema: await fetchRemoteSchema(accountsExec),
        executor: accountsExec,
      },
      {
        schema: await fetchRemoteSchema(productsExec),
        executor: productsExec,
      },
      {
        schema: await fetchRemoteSchema(reviewsExec),
        executor: reviewsExec,
      }
    ]
  });
}

makeGatewaySchema().then(schema => {
  const app = express();
  app.use('/graphql', graphqlHTTP({ schema, graphiql: true }));
  app.listen(4000, () => console.log('gateway running at http://localhost:4000/graphql'));
});