How do I clear the buffer upon start/exit in ZMQ socket? (to prevent server from connecting with dead clients)

python, pyzmq, sockets, zeromq

Solution

Answering my own question-

The problem is that the first client sends a message which the server accepts when it starts running, regardless of the status of the client.

To prevent this, 2 things have to be done. The most important thing is to use socket.close() to close the client connection. Secondly, the LINGER parameter can be set to a low value or zero. This clears the buffer after the timeout value from the time the socket is closed.

class client:
    def clientInit(self):
...
            self.socket.setsockopt(zmq.LINGER, 100)   
...

def clientQuit(self):
        try:
            self.socket.send('bye')
            self.socket.recv() 
        except:
            print "Error disconnecting."    
        self.socket.close()

Problem

I am using a REQ/REP type socket for ZMQ communication in python. There are multiple clients that attempt to connect to one server. Timeouts have been added in the client script to prevent indefinite wait. The problem is that when the server is not running, and a client attempts to establish connection, it's message gets added to the queue buffer, which should not even exist at this moment ideally. When the script starts running and a new client connects, the previous client's data is taken in first by the server. This should not happen. When the server starts, it assumes a client is connected to it since it had tried to connect previously, and could not exit cleanly (since the server was down). In the code below, when the client tries the first time, it gets `ERR 03: Server down` which is correct, followed by `Error disconnecting`. When server is up, I get `ERR 02: Server Busy` for the first client which connects. This should not occur. The client should be able to seamlessly connect with the server now that it's up and running. Server Code: ``` import zmq def server_fn(): context = zmq.Context() socket = context.socket(zmq.REP) socket.bind("tcp://192.168.1.14:5555") one=1 while one == 1: message = socket.recv() #start process if valid new connection if message == 'hello': socket.send(message) #ACK #keep session alive until application ends it. while one == 1: message = socket.recv() print("Received request: ", message) #exit connection if message == 'bye': socket.send(message) break #don't allow any client to connect if already busy if message == 'hello': socket.send ('ERR 00') continue #do all data communication here else: socket.send('ERR 01: Connection Error') return server_fn() ``` Client Code: ``` import zmq class client: def clientInit(self): hello='hello' #zmq connection self.context = zmq.Context() print("Connecting to hello world server...") self.socket = self.context.socket(zmq.REQ) self.socket.connect("tcp://192.168.1.14:5555") #RCVTIMEO to prevent forever block self.socket.setsockopt(zmq.RCVTIMEO, 5000) #SNDTIME0 is needed since script may not up up yet self.socket.setsockopt(zmq.SNDTIMEO, 5000) try: self.socket.send(hello) except: print "Sending hello failed." try: echo = self.socket.recv() if hello == echo: #connection established. commStatus = 'SUCCESS' elif echo == 'ERR 00': #connection busy commStatus = "ERR 00. Server busy." else: #connection failed commStatus="ERR 02" except: commStatus = "ERR 03. Server down." return commStatus def clientQuit(self): try: self.socket.send('bye') self.socket.recv() except: print "Error disconnecting." cObj = client() commStatus=cObj.clientInit() print commStatus cObj.clientQuit() ``` PS - I have a feeling the solution may lie in the correct usage of socket.bind and socket.connect.

Original source