istanbul is already supported Multiple Process Coverage.

I want to show you how to create code coverage using istanbul.

master.js

const cluster = require('cluster');
const path = require('path');

const workerPath = path.join(__dirname, 'worker.js');

// setup cluster if running with istanbul coverage
if (process.env.running_under_istanbul) {
  // use coverage for forked process
  // disabled reporting and output for child process
  // enable pid in child process coverage filename
  cluster.setupMaster({
    exec: './node_modules/.bin/istanbul',
    args: [
      'cover',
      '--report', 'none',
      '--print', 'none',
      '--include-all-sources',
      '--include-pid',
      workerPath,
    ]
  });
} else {
  cluster.setupMaster({
    exec: workerPath,
  });
}

// fork two workers
cluster.fork();
cluster.fork();

worker.js

const koa = require('koa');

const app = koa();

app.use(function* () {
  if (this.query.uid) {
    return this.body = {
      uid: this.query.uid,
    };
  }

  this.status = 403;
  this.body = {
    error: 'please login first'
  };
});

app.listen(1984);
console.log('[%s] start listening on 1984', process.pid);

test/all.test.js

Using mocha and supertest.

const request = require('supertest');

describe('cluster coverage unittest', function () {

  before(function (done) {
    require('../master');
    const port = 1984;
    const app = {
      port: port,
      url: 'http://127.0.0.1:' + port,
      address: function () {
        return {
          port: port
        };
      },
      // mock koa api
      callback: function () {
        return app;
      },
    };
    this.app = app;
    setTimeout(done, 1500);
  });

  it('should 200 when user login', function (done) {
    request(this.app.callback())
    .get('/?uid=123')
    .expect({
      uid: '123'
    })
    .expect(200, done);
  });

  it('should 403 when anonymous user request', function (done) {
    request(this.app.callback())
    .get('/')
    .expect({
      error: 'please login first'
    })
    .expect(403, done);
  });
});

run test

Run the unittest and create coverage report

$ npm run cov

> cluster-coverage@ cov /Users/mk2/git/fengmk2.github.com/blog/2015/cluster-coverage
> npm run clean && npm run test-cov && istanbul report


> cluster-coverage@ clean /Users/mk2/git/fengmk2.github.com/blog/2015/cluster-coverage
> rm -rf coverage


> cluster-coverage@ test-cov /Users/mk2/git/fengmk2.github.com/blog/2015/cluster-coverage
> istanbul cover --include-all-sources node_modules/.bin/_mocha -- --check-leaks test/*.test.js

> istanbul report

See the cluster-coverage report