Simple client/server ZMQ in Python to send multiple lines per request
python, pyzmq, zeromq
Solution
Well, you came up with my preferred solution, that is, to just send the whole thing as a single message, using separate frames if necessary. That said, the reason it will allow you to only send a single reply is because you're using a `REQ-REP` socket pair and when using such a pair you must follow a simple "request-reply-request-reply" pattern. Each communication must start with one request, and the next message must be one reply, and the next one request, etc.
To work around this you have several options:
- As you did, package up everything you want to send in a single message. In your case, so long as there are not constraints that prevent it, that would be the preferred option because all of the data is actually one logical message, coming from one logical file, and it's all going to be composed into a single logical piece of data on the receiving end, I assume.
- If you want to keep the `REQ-REP` socket pair, then you can send a request, followed by a reply, and then your next request can just be "MORE" or something like that, followed by the next piece of data, and just keep on requesting "MORE" until your client replies with something like "DONE", at which point you know you have all of the data and can quit requesting more.
- Or, if you prefer to be able to do multiple replies to a single request, or request multiple times before receiving a reply, then you'll want to use a `ROUTER-DEALER` socket pair instead of `REQ-REP`. In this case, your `DEALER` socket would take the place of your `REQ` socket, and `ROUTER` would take the place of `REP`. Check out the manual and ask a new question if you get stuck on how to implement `ROUTER-DEALER`.
Problem
This is my first exposure to ZMQ under Python and I want the server to sent multiple lines when it receives a request from the client. The code that I added to the example offered by ZMQ on the server side is: ``` with open("test.txt", 'r') as f: for line in f: socket.send_string(line.rstrip("\n")) ``` The question is how to make the server send all lines or how to make the client not to send a request before server finishes sending all the lines from `test.txt` Client ``` import zmq context = zmq.Context() print("Connecting to hello world server") socket = context.socket(zmq.REQ) socket.connect("tcp://localhost:5555") for request in range(10): print("Sending request %s" % request) socket.send(b"Hello") message = socket.recv() print("Received reply %s [ %s ]" % (request, message)) ``` Server ``` import time import zmq context = zmq.Context() socket = context.socket(zmq.REP) socket.bind("tcp://*:5555") while True: # Wait for next request from client message = socket.recv() print("Received request: %s" % message) # Do some 'work' time.sleep(1) # Send reply back to client with open("test.txt", 'r') as f: for line in f: socket.send_string(line.rstrip("\n")) ``` Client log ``` Connecting to hello wolrd server Sending request 0 Received reply 0 [ This is test line 1 ] Sending request 1 ``` This is where it stops, as the server generated the error shown bellow: Server log ``` line 324, in send_string return self.send(u.encode(encoding), flags=flags, copy=copy) File "socket.pyx", line 571, in zmq.backend.cython.socket.Socket.send (zmq/backend/cython/socket.c:5319) File "socket.pyx", line 618, in zmq.backend.cython.socket.Socket.send (zmq/backend/cython/socket.c:5086) File "socket.pyx", line 181, in zmq.backend.cython.socket._send_copy (zmq/backend/cython/socket.c:2081) File "checkrc.pxd", line 21, in zmq.backend.cython.checkrc._check_rc (zmq/backend/cython/socket.c:6032) zmq.error.ZMQError: Operation cannot be accomplished in current state Process finished with exit code 1 ``` test.txt ``` This is test line 1 This is test line 2 This is test line 3 This is test line 4 This is test line 5 ```