Async Send and Listen with amqp.node - amqp

Question

I am trying to understand why when using amqp.node (https://github.com/squaremo/amqp.node), it seems like I queue all my messages before I ever dequeue or handle these messages. I would expect the sending and receiving to be interleaved.
send-receive-send-receive
But I get
send-send-receive-receive
While inspecting my processes I see that I will first go through all of the sends before i get to any of the receives. I have written a simple demonstration of the problem I'm having
var amqp = require('amqplib');
var prettyHrtime = require('pretty-hrtime');
var when = require('when');
var firstMesssage = false;
var time, diff, totaltime;
var received = 0;
amqp.connect('amqp://localhost', { noDelay: false } ).then(function(conn) {
process.once('SIGINT', function() { conn.close(); });
return conn.createChannel().then(function(ch) {
var ok = ch.assertQueue('hello', {durable: false});
ok = ok.then(function(_qok) {
return ch.consume('hello', function(msg) {
if(!firstMesssage){
time = process.hrtime();
firstMesssage = true;
}
received++;
console.log(new Date().getTime());
console.log(" [x] Received '%s' %s", msg.content.toString(), received);
if(received >= 10000){
diff = process.hrtime(time);
var words = prettyHrtime(diff);
console.log(words);
}
}, {noAck: true});
});
return ok.then(function(_consumeOk) {
console.log(' [*] Waiting for messages. To exit press CTRL+C');
});
});
}).then(null, console.warn);
var numberToSend = 10000;
var numberDone = 0;
amqp.connect('amqp://localhost', { noDelay: false } ).then(function(conn) {
return when(conn.createChannel().then(function(ch) {
var q = 'hello';
var msg = 'Hello World!';
var ok = ch.assertQueue(q, {durable: false});
function sendMsg(){
ch.sendToQueue(q, new Buffer(msg));
console.log(" [x] Sent '%s'", msg, new Date().getTime());
numberDone++;
}
return ok.then(function(_qok) {
while(numberDone < numberToSend){
sendMsg();
}
console.log('*** done');
return ch.close();
});
})).ensure(function() { conn.close(); });
}).then(null, console.warn);

Answer 1

Node is single threaded so it can't process any of the incoming messages until your while loop that does all the sending has finished.
Try wrapping your sendMsg in a setImmediate call and see how that changes things (it'll have to be converted to a promise and chained so that the channel and connect don't close before everything sends):
function sendMsg(resolve){
setImmediate(function() {
ch.sendToQueue(q, new Buffer(msg));
console.log(" [x] Sent '%s'", msg, new Date().getTime());
resolve();
});
}
return ok.then(function(_qok) {
var sendOk = when.promise(sendMsg);
while(numberDone < numberToSend){
sendOk = sendOk.then(function() {
return when.promise(sendMsg);
});
numberDone++;
}
return sendOk.finally(function() {
console.log('*** done');
ch.close();
});
});
You can see the difference in even just 5 messages:
[x] Sent 'Hello World!' 1428467734962
[*] Waiting for messages. To exit press CTRL+C
[x] Sent 'Hello World!' 1428467734963
1428467734965
[x] Received 'Hello World!' 1
[x] Sent 'Hello World!' 1428467734965
[x] Sent 'Hello World!' 1428467734965
[x] Sent 'Hello World!' 1428467734966
[x] Sent 'Hello World!' 1428467734966
*** done
1428467735004
[x] Received 'Hello World!' 2
1428467735005
[x] Received 'Hello World!' 3
1428467735005
[x] Received 'Hello World!' 4
1428467735005
[x] Received 'Hello World!' 5