Wait for socketio event inside express route

Question

I'm using express,socketio and socketio-client in my application. (I not very comfortable with nodejs stack...)

to summarize my application flow : Client => node/express API + Socketoi server <=> nodejs (Socketio-client)

1. browser send request to a nodejs/express (route /api)
2. Do some request headers overwrites with middlewares
3. In the route '/', server sends an emit to a nodejs (Socketio-client)
4. after executing some logic, socketio-client emit an event with the logics result
5. I need this result to be send in the response to the client

My code below:

router.get('/*', function (req, res) {
  //emit data for socketio-client to apply some logic
  app.io.sockets.emit('req', {
    reqheader : req.headers,
    requrl : req.protocol + "://" + req.headers.host + req.url,
    reqmethod : req.method
  });
  console.log("after emit");
  //I use callback to make response wait for socketio server to catch event from client
  waitforevent(req, res, function (__res) {
    console.log("callback" );
    res.end(__res.body);
    res.sendStatus(__res.statusCode);
    //res.end();
  });
  function waitforevent(req, res, callback) {
    console.log("waiting for event" );
    app.io.__socket.on('respp', function (data) {
        //console.log("no response yet \n" + JSON.parse(data) );
        __res = JSON.parse(data);
        console.log("event catched...");
        callback(__res);
    });
  }
});

My problem : This works only the first time I send a Get http://localhost:3000/api frome the browser. __res.body is printed in the browser.

req 1
after emit
waiting for event
event catched...
callback
Error: Can't set headers after they are sent.   
**GET /api 200 73.841 ms - -**

req 2
after emit
waiting for event

Next request will just wait for server to respond, which is, I suspect, not happening because the app.io.__socket.on('respp', function (data){...} is never catched by the server.

After sending more request (while the others are waiting), I noticed this warning in server logs:

(node) warning: possible EventEmitter memory leak detected. 11 respp listeners added. Use emitter.setMaxListeners() to increase limit.

Is there other ways to catch events in a route before sending response to clients?

Answer 1

You could remove the event listener when the socket closes to avoid the event listener leak:

router.get('/*', function (req, res) {
  app.io.sockets.emit('req', {
    reqheader : req.headers,
    requrl : req.protocol + "://" + req.headers.host + req.url,
    reqmethod : req.method
  });

  req.socket.on('close', function() {
    app.io.__socket.removeListener('respp', resppHandler);
  });
  app.io.__socket.on('respp', resppHandler);

  function resppHandler(data) {
    data = JSON.parse(data);
    res.statusCode = data.statusCode;
    res.end(data.body);
  }
});

I'm not sure if app.io.__socket should really be app.io.sockets or not, but I copied it as-is from your code, assuming you know what you're doing.

Additionally, you may wish to add some sort of timeout so as not to keep the request waiting indefinitely.

Answer 2

I solved with once:

var app = require('express')();
var server = require('http').Server(app);
var io = require('socket.io')(server);

var socket;
io.on('connection', function (sock) {
    console.log('Connected');
    socket = sock;
});

server.listen(3000);

app.get('/*', function (req, res) {
    socket.once('event', function (data) {
        if (data.error) {
            console.log('is an error');
            res.status(400).json(data);
        } else {
            console.log('is ok');
            res.status(200).json(data);
        }
    });
    io.emit('ask-for-event', { data: data });
});