Non-Terminating ZeroMQ Load Balancer - scala

Question

Given the following slightly modified Load Balancer ZeroMQ code:
package net.broker
import org.zeromq.ZMQ
object LruQueue2 {
class ClientTask(name: String) extends Runnable {
override def run(): Unit = {
val context = ZMQ.context(1)
val client = context.socket(ZMQ.REQ)
client.setIdentity(name.getBytes)
client.connect("tcp://localhost:5555")
// send request, get reply
client.send("HELLO".getBytes, 0)
val reply = client.recv(0)
println(s"${new String(client.getIdentity)} received: ${new String(reply)}")
}
}
class WorkerTask(name: String) extends Runnable {
override def run(): Unit = {
val context = ZMQ.context(1)
val worker = context.socket(ZMQ.REQ)
worker.connect("tcp://localhost:5556")
worker.setIdentity(name.getBytes)
worker.send("READY".getBytes, 0)
while(true) {
val clientAddr = worker.recv(0)
val empty = worker.recv(0)
val clientMsg = worker.recv(0)
worker.send(clientAddr, ZMQ.SNDMORE)
worker.send("".getBytes, ZMQ.SNDMORE)
worker.send("WORLD".getBytes, 0)
println(s"${new String(worker.getIdentity)}: 3-frames to client: ${new String(clientAddr)}")
}
}
}
def main(args: Array[String]): Unit = {
val NOFLAGS = 0
// worker using REQ socket to do LRU routing
val NBR_CLIENTS = 1
val NBR_WORKERS = 1
val context = ZMQ.context(1)
val frontend = context.socket(ZMQ.ROUTER)
val backend = context.socket(ZMQ.ROUTER)
frontend.bind("tcp://*:5555")
backend.bind("tcp://*:5556")
val clients = (1 to NBR_CLIENTS).toList.map{ i => new Thread(new ClientTask(s"CLIENT$i"))}
val workers = (1 to NBR_CLIENTS).toList.map{ i => new Thread(new WorkerTask(s"WORKER$i"))}
clients.foreach(_.start)
workers.foreach(_.start)
val workerQueue = scala.collection.mutable.Queue[Array[Byte]]()
val poller = context.poller(2)
poller.register(backend, ZMQ.Poller.POLLIN)
poller.register(frontend, ZMQ.Poller.POLLIN)
var clientNbr = NBR_CLIENTS
while(true) {
println("begin to poll")
poller.poll()
println("done polling")
println("clientNbr:" + clientNbr)
println("workerQueue.length: " + workerQueue.length)
if(clientNbr == 0) {
sys.exit(0)
}
else if(poller.pollin(0) && clientNbr > 0) {
val workerAddr = backend.recv(NOFLAGS)
val empty = backend.recv(NOFLAGS)
val clientAddrOrReadyMsg = backend.recv(NOFLAGS)
workerQueue.enqueue(workerAddr)
if(new String(clientAddrOrReadyMsg) == "READY") {
// nothing to do - worker is letting us know that he's ready to work
}
else {
// retrieve remaining 2 frames of client message
// [Empty][Client Message of "HELLO"]
val empty = backend.recv(0)
val workerResponse = backend.recv(0)
frontend.send(clientAddrOrReadyMsg, ZMQ.SNDMORE)
frontend.send("".getBytes, ZMQ.SNDMORE)
frontend.send(workerResponse, NOFLAGS)
clientNbr -= 1
}
}
else if (poller.pollin(1) && workerQueue.nonEmpty) {
val clientAddr = frontend.recv(0)
val empty = frontend.recv(0)
val clientMsg = frontend.recv(0)
backend.send(workerQueue.dequeue(), ZMQ.SNDMORE)
backend.send("".getBytes, ZMQ.SNDMORE)
backend.send(clientAddr, ZMQ.SNDMORE)
backend.send("".getBytes, ZMQ.SNDMORE)
backend.send(clientMsg, NOFLAGS)
}
else {}
}
}
}
When I run the above code, I see the following output:
[info] Running net.broker.LruQueue2
[info] begin to poll
[info] done polling
[info] clientNbr:1
[info] workerQueue.length: 0
[info] begin to poll
[info] done polling
[info] clientNbr:1
[info] workerQueue.length: 1
[info] begin to poll
[info] WORKER1: 3-frames to client: CLIENT1
[info] done polling
[info] clientNbr:1
[info] workerQueue.length: 0
[info] begin to poll
[info] CLIENT1 received: WORLD
It appears that it's stuck on poller.poll(), per the output print statements.
How can I debug this behavior?