Understanding 5 REQs to Router - scala

Question

Looking at the Scala code for this 5 REQ <--> 1 ROUTER setup:
Worker
class WorkerTask extends Runnable {
override def run: Unit = {
val rand = new Random(System.currentTimeMillis())
val context = ZMQ.context(1)
val worker = context.socket(ZMQ.REQ)
worker.connect("tcp://localhost:5555")
var total = 0
var workload = ""
do {
worker.send("Ready".getBytes, 0)
workload = new String(worker.recv(0))
Thread.sleep (rand.nextInt(1) * 1000)
total += 1
} while (workload.equalsIgnoreCase("END") == false)
printf("Completed: %d tasks\n", total)
}
}
main (Router)
def main(args: Array[String]): Unit = {
val NBR_WORKERS = 5
val context = ZMQ.context(1)
val client = context.socket(ZMQ.ROUTER)
assert(client.getType > -1)
client.bind("tcp://*:5555")
val workers = List.fill(NBR_WORKERS)(new Thread(new WorkerTask))
workers.foreach (_.start)
for (i <- 1 to (NBR_WORKERS * 10)) {
// LRU worker is next waiting in queue
val address = client.recv(0)
val empty = client.recv(0)
val ready = client.recv(0)
client.send(address, ZMQ.SNDMORE)
client.send("".getBytes, ZMQ.SNDMORE)
client.send("This is the workload".getBytes, 0)
}
for (i <- 1 to NBR_WORKERS) {
val address = client.recv(0)
val empty = client.recv(0)
val ready = client.recv(0)
client.send(address, ZMQ.SNDMORE)
client.send("".getBytes, ZMQ.SNDMORE)
client.send("END".getBytes, 0)
}
}
Running on my machine:
[info] Running net.server.RouterToReq
Completed: 21 tasks
Completed: 1 tasks
Completed: 27 tasks
Completed: 5 tasks
Completed: 1 tasks
As I partially understand the above code, 5 REQ workers accept requests from the ROUTER on port 5555.
Lastly, in the following code:
for (i <- 1 to NBR_WORKERS) {
val address = client.recv(0)
val empty = client.recv(0)
val ready = client.recv(0)
What message is the client, i.e. ROUTER, receiving?

Answer 1

The worker sends the message "Ready" repeatedly as a single frame message.
The REQ socket will add a blank delimiter frame on the front.
The ROUTER socket will prepend an identifier frame to the front of any received message to identify where it came from.
Thus the single ready message becomes a 3 frame message when you receive it on the router, this is why 3 recv calls are required.
When you send on a router socket the first frame will be removed and used to identify which client to send the message to. The REQ socket will remove all frames until it finds an empty frame and thus you only need a single recv call on the worker side.