Question

I’m having a strange problem with ZeroMQ, in which some messages get stuck, and just get unstuck when new messages arrive. It’s like the new messages push the stuck messages on the door (terrible comparison, i know).

My code is quite simple:

rep.php

$context = new ZMQContext;
$receiver = new ZMQSocket($context, ZMQ::SOCKET_PULL);
$receiver->connect("tcp://localhost:8022");
$receiver2 = new ZMQSocket($context, ZMQ::SOCKET_PULL);
$receiver2->connect("tcp://localhost:8024");

for (;;) {
    echo $receiver->recv() . PHP_EOL;
    echo $receiver2->recv() . PHP_EOL;
}

cnt.php and cnt2.php (same code, different ports)

$context = new ZMQContext;
$work = new ZMQSocket($context, ZMQ::SOCKET_PUSH);
$work->bind('tcp://*:8022');
$work->send('Hello World');

cnt.php sends to 8022 and cnt2.php to 8024. They get executed from time to time and send messages to rep.php. However, some messages get stuck. If i sent 4 messages from cnt.php, nothing is received, but when i send 1 from cnt2.php, i get 5 messages at once. Any ideas?

Answer 1

I am no PHP expert here but guessing the syntax and functionality. Please correct me if I am wrong

echo $receiver->recv() . ' - ' . $receiver2->recv();

recv() should be a blocking call.

1. $receiver->recv() blocks until some message is received.
2. But echo does not echo the message immediately
3. You are again blocked on $receiver2->recv()
4. Only when you send message from the other file does echo work because it is waiting on $receiver2->recv()

Since you want to process the socket recv() independently, you should use polling or event based asynchronous I/O.

[Attempted solution]

$poll = new ZMQPoll();
$poll->add($receiver, ZMQ::POLL_IN);
$poll->add($receiver2, ZMQ::POLL_IN);
$readable = $writeable = array();
while(true) {
     $events = $poll->poll($readable, $writeable);
     foreach($readable as $socket) {
               $message = $socket->recv();
               echo $message, PHP_EOL;
     }
}

Adapted from : http://zguide.zeromq.org/php:rrbroker