How to send response in koa

I understand that when I set this.body and all middleware finished executing the response is sent.

What I want is to send the response at some point and then execute some remaining actions.

Simply setting this.body and then doing yield next doesn’t seem to do it - it doesn’t seem to automatically send the response after the current middleware. I might be wrong, please correct me if this is the case.

This is the simplified code of what I’ve tried:

var router = require('koa-router');
router.get('/test', store, process);

function *store(next) {
  yield storeToDB(this.request.body);
  this.body = 'OK';
  // it seems like I need to do something here to send response, like res.send('OK') in express
  try { yield next; }
  catch(e) { console.log('processing error:', e); }
}

function *process() {
  yield processData(this.request.body);
}

The response is sent after process, not after store as I need.

I understand that some separate process can read from DB on some notifications or schedule, but I wanted to keep it simple yet…

[felixfbecker]

But instead of @madbence’s example I would suggest to do

const someSeriousBackgroundOperation = co.wrap(function* () {
  yield verySlowDBQuery();
  yield someOutOfProcessImageProcessingWithCUDA();
  ...
})

app.use(function* (next) {
  yield* next();
  someSeriousBackgroundOperation();
});

So once async functions ship in V8 you only have to replace co.wrap with async and yield with await.

[yanickrochon]

If speed is an issue, I’d setup my koa app in one process and any other long data manipulation in another process. In pseudo code, this could be something like

var app = createKoaApp();
var pub = createExternalProcessPublisher();

app.use(function *(next) {
  this.body = yield getRequestedInformation(this.req);
  pub.send(collectedRequestInformation(this.req));
});

And, in some external process

var rcv = createReceiver();

rcv.on('message', function (data) {
  /* handle data to log or whatever else with it */
});

This way, the HTTP server is dedicated to return whatever the user wants and nothing else. All other processing and error handling is done externally, independent on the user request.