Load balancing broker in PHP

<?php
/*
* Least-recently used (LRU) queue device
* Clients and workers are shown here as IPC as PHP
* does not have threads.
* @author Ian Barber <ian(dot)barber(at)gmail(dot)com>
*/

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) {
// Read and save all frames until we get an empty frame
// In this example there is only 1 but it could be more
$address = $worker->recv();

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

// Get request, send reply
$request = $worker->recv();
printf ("Worker: %s%s", $request, PHP_EOL);

$worker->send($address, ZMQ::MODE_SNDMORE);
$worker->send("", ZMQ::MODE_SNDMORE);
$worker->send("OK");
}
}

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
$worker_addr = $socket->recv();
assert($available_workers < NBR_WORKERS);
$available_workers++;
array_push($worker_queue, $worker_addr);

// Second frame is empty
$empty = $socket->recv();
assert(empty($empty));

// Third frame is READY or else a client reply address
$client_addr = $socket->recv();

if ($client_addr != "READY") {
$empty = $socket->recv();
assert(empty($empty));
$reply = $socket->recv();
$frontend->send($client_addr, ZMQ::MODE_SNDMORE);
$frontend->send("", ZMQ::MODE_SNDMORE);
$frontend->send($reply);

// exit after all messages relayed
$client_nbr--;
}
} elseif ($socket === $frontend) {
// Now get next client request, route to LRU worker
// Client request is [address][empty][request]
$client_addr = $socket->recv();
$empty = $socket->recv();
assert(empty($empty));
$request = $socket->recv();

$backend->send(array_shift($worker_queue), ZMQ::MODE_SNDMORE);
$backend->send("", ZMQ::MODE_SNDMORE);
$backend->send($client_addr, ZMQ::MODE_SNDMORE);
$backend->send("", ZMQ::MODE_SNDMORE);
$backend->send($request);

$available_workers--;
}
}
}
}

// Clean up our worker processes
foreach ($worker_queue as $worker) {
$backend->send($worker, ZMQ::MODE_SNDMORE);
$backend->send("", ZMQ::MODE_SNDMORE);
$backend->send('END');
}

sleep(1);
}

main();