ZeroMQ REQ/REP server error handling

Question

I am trying to use the ZeroMQ rep/req and cannot figure out how to handle server side errors. Look at the code from here:

socket.bind("tcp://*:%s" % port)
while True:
    #  Wait for next request from client
    message = socket.recv()
    print "Received request: ", message
    time.sleep (1)  
    socket.send("World from %s" % port)

My problem is what happens if the client calls socket.send() and then hangs or crashes. Wouldn't the server just get stuck on socket.send() or socket.recv() forever?

Note that it is not a problem with TCP sockets. With TCP sockets I can simply break the connection. With ZMQ, the connections are implicitly managed for me and I don't know if it is possible to break a 'session' or 'connection' and start over.

score 6 · Answer 1 · answered Feb 07 '14 at 22:33

6

You can terminate ZMQ sockets much the same way you terminate TCP sockets.

socket.close()

If you need to wait on a message but only up for a finite amount of time you can pass a timeout flag to socket.recv(timeout=1024) and then handle the timeout error case the same way you would when a TCP socket timeouts or disconnects. If you need to manage several sockets all of which may be in an error state then the Poller class will let you accomplish this.

answered Feb 07 '14 at 22:33

Stephen Diehl

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On the server side, if I do socket.close(), do I close the session corresponding to THE instance of the client or do I close every session, or do I close the listening socket? – Tom Bennett Feb 07 '14 at 22:36
Depends on the socket class, there's different behaviors for REQ/REP vs PUB/SUB etc. – Stephen Diehl Feb 07 '14 at 22:38
I am only interested in the REP socket. – Tom Bennett Feb 07 '14 at 23:35
I was wondering about this, too. I modified the "lazy pirate server" example mentioned below, and found that the server seems not to have an issue if the client dies. So, thankfully, there seems to be no issue, unless I'm missing something. – Stuart Berg Mar 03 '19 at 20:39

score 2 · Answer 2 · answered Feb 08 '14 at 16:14

2

The ZMQ Z-guide offers lots of good hints on how to structure your services to handle different scenarios.

I think chapter 4 can be of interest to you, especially the Lazy Pirate Pattern.

Check out the examples of Lazy Pirate Server and Lazy Pirate Client.

answered Feb 08 '14 at 16:14

StianE

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I read those. They offer client side reliability. My problem is simpler. I just want to make sure the REP server does not get stuck. – Tom Bennett Feb 08 '14 at 17:30

score 0 · Answer 3 · answered May 29 '18 at 13:50

In general,

Make sure you setsockopt() on the socket such that send and recv will not block forever. (Temporary blocking – be it client or server – is OK, but infinite blocking is bad because your application cannot do anything else)
In the event that any of the I/O got an error,
- If you are the client, close() the current socket and re-create a new one to establish a new connection
- If you are the server, there's nothing else to do, you simply are waiting for a new connection. You will want to explore the Poller class.

ZeroMQ REQ/REP server error handling

3 Answers3