How to do RPC over socket.io? - callback

Question

Suppose we have a simple echo server (tuned to be longer on the first request):
var waiting = 8000;
io.on('connection', function(socket){
socket.on('doEcho', function (data) {
setTimeout( function () {
socket.emit('echoDone', data);
}, waiting);
waiting = 1;
});
});
Then say a index.html client script does:
askEcho ("11111", function (resp) {
console.log("answer for 11111 " + resp);
});
askEcho ("22222", function (resp) {
console.log("answer for 22222 " + resp);
});
where askEcho is the following buggy function, pretending to be an RPC stub:
function askEcho (text, fun) {
socket.emit('doEcho', text);
socket.on('echoDone', function (data) {
fun ( data );
});
}
And obviously what I get is the following mess
answer for 11111 22222
answer for 22222 22222
answer for 11111 11111
answer for 22222 11111
because I installed two listeners for the same event. This can be worked around easily,
but I don't see that clear how to get the responses properly ordered at the client,
without help (programming more) at the server side.
This all seems a bit too much burden.
Can't be the askEcho function coded right and easily?

Answer 1

With socket.io you can send and receive acknowledgements via callbacks.
See this: http://socket.io/docs/#sending-and-getting-data-(acknowledgements)

Answer 2

Actually, there is a simple trick: use a counter to differentiate between the answers, and remove the callback when done (even better use once instead of on). This only poses minor changes at server and client sides.
Let's show it:
The echo server is (now without any delaying timeout):
io.on('connection', function(socket){
socket.on('doEcho', function (callCounter, data) {
socket.emit('echoDone'+callCounter, data);
});
});
In the client side (index.html) the user code is still the same, as desired:
askEcho ("11111", function (resp) {
console.log("answer for 11111 " + resp);
});
askEcho ("22222", function (resp) {
console.log("answer for 22222 " + resp);
});
and this is the stub askEcho that works properly in teamwork with the server side:
var callCounter = 0;
function askEcho (text, fun) {
var localCallCounter = ++callCounter;
socket.emit('doEcho', localCallCounter, text);
socket.once('echoDone'+localCallCounter, function (data) {
// not required: socket.removeListener ('echoDone'+localCallCounter);
fun ( data );
});
}