Process multiple POST requests with emitters/listeners

Question

Code:

app.post('/new', urlencodedParser, function (req, res) {
  if (!req.body) return res.sendStatus(400);

  emitter.on('response', function (output) {
    res.status(200).json({ip: `${output}`, id: `${random_id}`});
  });
});

Problem:

The problem I'm having right now (I'm using express) is I can't make multiple requests to the app. Express returns the expected value the first time, and every time after that:

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

The rest of the application is a collection of functions triggered by event emitters and listeners. Think of it as a string of emitters triggering listeners and executing 4 or 5 functions in succession, and, at the end, returning a value.

This works flawlessly for the first time, but never again. Note that requests are expected to come in from different machines, not a single machine.

I know res.end() will end the response but no combination of Express functions makes it work the second time. Is there a way to use emitters/listeners and Express responses?

Thanks in advance!

EDIT: more code

emitter.on('newRequest', persistentDisk);
emitter.on('newDisk', persistentDisk);
emitter.on('newDeployment', newDeployment);
emitter.on('newService', createService);
emitter.on('checkDeployment', checkDeployment);
emitter.on('returnIp', returnIp);

emitter.on('deleteRequest', deleteApp);

app.post('/new', urlencodedParser, function (req, res) {
  if (!req.body) return res.sendStatus(400);

  var random_id = randomize('a', 8);

  var user_id = req.body.user_id;
  var stripe_id = req.body.stripe_id;
  var app_name = req.body.app_name;
  var gpu_count = req.body.gpu_count;
  var cpu_count = req.body.cpu_count;
  var memory_count = req.body.memory_count;

  emitter.emit('newRequest', random_id, user_id, stripe_id, app_name, gpu_count, cpu_count, memory_count);
  emitter.on('response', function (output) {
    res.send({ip: `${output}`, id: `${random_id}`});
  });
    async function persistentDisk(random_id, user_id, stripe_id, app_name, gpu_count, cpu_count, memory_count) {
      try {
            emitter.emit('newDeployment', random_id, user_id, stripe_id, app_name, gpu_count, cpu_count, memory_count);
            emitter.emit('newService', random_id, user_id, stripe_id, app_name, gpu_count, cpu_count, memory_count);
      } catch (err) {
        console.log(err);
        errors.report(err);
      }
    }

async function newDeployment(random_id, user_id, stripe_id, app_name, gpu_count, cpu_count, memory_count) {
  try {
    var deployConfig = JSON.parse(`...`);

      emitter.emit('checkDeployment', random_id, user_id, stripe_id, app_name, gpu_count, cpu_count, memory_count);

  } catch (err) {
    console.log(err);
    errors.report(err);
  }
}

async function createService(random_id, user_id, stripe_id, app_name, gpu_count, cpu_count, memory_count) {
  try {
    var exposeConfig = JSON.parse(`...`);

      emitter.emit('returnIp', random_id, user_id, stripe_id, app_name, gpu_count, cpu_count, memory_count)
  } catch (err) {
    console.log(err);
    errors.report(err);
  }
}

async function checkDeployment(random_id, user_id, stripe_id, app_name, gpu_count, cpu_count, memory_count) {
  try {
  } catch (err) {
    console.log(err);
    errors.report(err);
  }
}

async function returnIp(random_id, user_id, stripe_id, app_name, gpu_count, cpu_count, memory_count) {
  try {
    var service = JSON.parse(`{"ip":"127.0.0.1"}`)
    jq.run('.ip', service, { input: 'json' }).then((output) => {
      emitter.emit('response', output)
      if (output.toString() === "null") {
        setInterval(() => {
          console.log('value is null');
          emitter.emit('returnIp', random_id, user_id, stripe_id, app_name, gpu_count, cpu_count, memory_count)
        }, 15000);
        }
      });
  } catch (err) {
    console.log(err);
    errors.report(err);
  }
}

This produces a response (127.0.0.1, importantly) the first time.

Answer 1

Changing to:

app.post('/new', urlencodedParser, function (req, res) {
  if (!req.body) return res.sendStatus(400);

  emitter.once('response', function (output) {
    res.status(200).json({ip: `${output}`, id: `${random_id}`});
  });
});

(from emitter.on to emitter.once) resolves the issue.