LRU queue broker using high-level API in PHP

<?php
/*
* Least-recently used (LRU) queue device
* Demonstrates use of the zmsg class
* @author Ian Barber <ian(dot)barber(at)gmail(dot)com>
*/

include "zmsg.php";

define("NBR_CLIENTS", 10);
define("NBR_WORKERS", 3);

// Basic request-reply client using REQ socket
function client_thread() {
$context = new ZMQContext();
$client = new ZMQSocket($context, ZMQ::SOCKET_REQ);
$client->connect("ipc://frontend.ipc");

// Send request, get reply
$client->send("HELLO");
$reply = $client->recv();
printf("Client: %s%s", $reply, PHP_EOL);
}

// Worker using REQ socket to do LRU routing
function worker_thread () {
$context = new ZMQContext();
$worker = $context->getSocket(ZMQ::SOCKET_REQ);
$worker->connect("ipc://backend.ipc");

// Tell broker we're ready for work
$worker->send("READY");

while(true) {
$zmsg = new Zmsg($worker);
$zmsg->recv();

//// Additional logic to clean up workers. //
if($zmsg->address() == "END") {
exit();
}

printf ("Worker: %s\n", $zmsg->body());

$zmsg->body_set("OK");
$zmsg->send();
}
}

function main() {
for($client_nbr = 0; $client_nbr < NBR_CLIENTS; $client_nbr++) {
$pid = pcntl_fork();
if($pid == 0) {
client_thread();
return;
}
}

for($worker_nbr = 0; $worker_nbr < NBR_WORKERS; $worker_nbr++) {
$pid = pcntl_fork();
if($pid == 0) {
worker_thread();
return;
}
}

$context = new ZMQContext();
$frontend = new ZMQSocket($context, ZMQ::SOCKET_ROUTER);
$backend = new ZMQSocket($context, ZMQ::SOCKET_ROUTER);
$frontend->bind("ipc://frontend.ipc");
$backend->bind("ipc://backend.ipc");

// Logic of LRU loop
// - Poll backend always, frontend only if 1+ worker ready
// - If worker replies, queue worker as ready and forward reply
// to client if necessary
// - If client requests, pop next worker and send request to it

// Queue of available workers
$available_workers = 0;
$worker_queue = array();
$writeable = $readable = array();

while($client_nbr > 0) {
$poll = new ZMQPoll();

// Poll front-end only if we have available workers
if($available_workers > 0) {
$poll->add($frontend, ZMQ::POLL_IN);
}

// Always poll for worker activity on backend
$poll->add($backend, ZMQ::POLL_IN);
$events = $poll->poll($readable, $writeable);

if($events > 0) {
foreach($readable as $socket) {
// Handle worker activity on backend
if($socket === $backend) {
// Queue worker address for LRU routing
$zmsg = new Zmsg($socket);
$zmsg->recv();
assert($available_workers < NBR_WORKERS);
$available_workers++;
array_push($worker_queue, $zmsg->unwrap());

if($zmsg->body() != "READY") {
$zmsg->set_socket($frontend)->send();

// exit after all messages relayed
$client_nbr--;
}
} else if($socket === $frontend) {
$zmsg = new Zmsg($socket);
$zmsg->recv();
$zmsg->wrap(array_shift($worker_queue), "");
$zmsg->set_socket($backend)->send();
$available_workers--;
}
}
}
}

// Clean up our worker processes
foreach($worker_queue as $worker) {
$zmsg = new Zmsg($backend);
$zmsg->body_set('END')->wrap($worker, "")->send();
}

sleep(1);
}

main();