How to Log Receive Message in ZMQ Proxy? - c++

Question

In ZMQ Proxy, we have 2 types of sockets, DEALER and ROUTER. Also, I've tried to use the capture socket, but it didn't work based on what exactly I looked for.
I'm looking for a way to log what message my proxy server receives.

Answer 1

Q : a way to log what message my proxy server receives.
The simplest way is to make use of an API v4+ directly supported logging via a ManInTheMiddle-"capture" socket:
// [ROUTER]--------------------------------------+++++++
// |||||||
// [DEALER]---------------*vvvvvvvv *vvvvvvv
int zmq_proxy (const void *frontend, const void *backend, const void *capture);
// [?]---------------------------------------------------------------*^^^^^^^
Where the capture ought be either of { ZMQ_PUB | ZMQ_DEALER | ZMQ_PUSH | ZMQ_PAIR }
If the capture socket is not NULL, the proxy shall send all messages, received on both frontend and backend, to the capture socket.
If this ZeroMQ API-granted is not meeting your expectation, feel free to express your expectations in as sufficiently detailed manner as needed ( and implement either an "external" capture-socket payload { message-content | socket_monitor() }-based filtering or one may design a brand new, user-defined logging-proxy, where your expressed features will get implemented with a use of your custom use-case specific requirements, implemented in your application-specific code, resorting to re-use but the clean and plain ZeroMQ API for all the DEALER-inbound/outbound-ROUTER message-passing and log-filtering/processing logic. )
There is no other way I can imagine to take place and solve the task.

Answer 2

It also works with a pair of PAIR sockets. As soon as one end of a pair of sockets is connected to the capture socket, messages are sent to the capture sockets AND to the other end of the proxy.
http://zguide.zeromq.org/page:all#ZeroMQ-s-Built-In-Proxy-Function
and
http://api.zeromq.org/3-2:zmq-proxy
and
http://zguide.zeromq.org/page:all#Pub-Sub-Tracing-Espresso-Pattern
helped me.
This code in python demonstrates it:
import zmq, threading, time
def peer_run(ctx):
""" this is the run method of the PAIR thread that logs the messages
going through the broker """
sock = ctx.socket(zmq.PAIR)
sock.connect("inproc://peer") # connect to the caller
sock.send(b"") # signal the caller that we are ready
while True:
try:
topic = sock.recv_string()
obj = sock.recv_pyobj()
except Exception:
topic = None
obj = sock.recv()
print(f"\n !!! peer_run captured message with topic {topic}, obj {obj}. !!!\n")
def proxyrun():
""" zmq broker run method in separate thread because zmq.proxy blocks """
xpub = ctx.socket(zmq.XPUB)
xpub.bind(xpub_url)
xsub = ctx.socket(zmq.XSUB)
xsub.bind(xsub_url)
zmq.proxy(xpub, xsub, cap)
def pubrun():
""" publisher run method in a separate thread, publishes 5 messages with topic 'Hello'"""
socket = ctx.socket(zmq.PUB)
socket.connect(xsub_url)
for i in range(5):
socket.send_string(f"Hello {i}", zmq.SNDMORE)
socket.send_pyobj({'a' : 123})
time.sleep(0.01)
ctx = zmq.Context()
xpub_url = "ipc://xpub"
xsub_url = "ipc://xsub"
#xpub_url = "tcp://127.0.0.1:5567"
#xsub_url = "tcp://127.0.0.1:5568"
# set up the capture socket pair
cap = ctx.socket(zmq.PAIR)
cap.bind("inproc://peer")
cap_th = threading.Thread(target=peer_run, args=(ctx,), daemon=True)
cap_th.start()
cap.recv() # wait for signal from peer thread
print("cap received message from peer, proceeding.")
# start the proxy
th_proxy=threading.Thread(target=proxyrun, daemon=True)
th_proxy.start()
# create req/rep socket just to prove that pub/sub can run alongside it
zmq_rep_sock = ctx.socket(zmq.REP)
zmq_rep_sock.bind("ipc://ghi")
# create sub socket and connect it to proxy's pub socket
zmq_sub_sock = ctx.socket(zmq.SUB)
zmq_sub_sock.connect(xpub_url)
zmq_sub_sock.setsockopt(zmq.SUBSCRIBE, b"Hello")
# create the poller
poller = zmq.Poller()
poller.register(zmq_rep_sock, zmq.POLLIN)
poller.register(zmq_sub_sock, zmq.POLLIN)
# create publisher thread and start it
th_pub = threading.Thread(target=pubrun, daemon=True)
th_pub.start()
# receive publisher's messages ordinarily
while True:
events = dict(poller.poll())
print(f"received events: {events}")
if zmq_rep_sock in events:
message = zmq_rep_sock.recv_pyobj()
print(f"received zmq_rep_sock {message}")
elif zmq_sub_sock in events:
topic = zmq_sub_sock.recv_string()
message = zmq_sub_sock.recv_pyobj()
print(f"received zmq_sub_sock {topic} , {message}")
output
cap received message from peer, proceeding.
!!! peer_run captured message with topic None, obj b'\x80\x03}q\x00X\x01\x00\x00\x00aq\x01K{s.'. !!!
received events: {<zmq.sugar.socket.Socket object at 0x76310f70>: 1}
received zmq_sub_sock Hello 1 , {'a': 123}
!!! peer_run captured message with topic Hello 2, obj {'a': 123}. !!!
received events: {<zmq.sugar.socket.Socket object at 0x76310f70>: 1}
received zmq_sub_sock Hello 2 , {'a': 123}
!!! peer_run captured message with topic Hello 3, obj {'a': 123}. !!!
received events: {<zmq.sugar.socket.Socket object at 0x76310f70>: 1}
received zmq_sub_sock Hello 3 , {'a': 123}
!!! peer_run captured message with topic Hello 4, obj {'a': 123}. !!!
received events: {<zmq.sugar.socket.Socket object at 0x76310f70>: 1}
received zmq_sub_sock Hello 4 , {'a': 123}
Be aware of the slow joiner problem, hence the sleep command in the publisher/