ZeroMQ PUB socket buffers all my out going data when it is connecting

Question

I noticed that a zeromq PUB socket will buffers all outgoing data if it is connecting, for example

import zmq
import time
context = zmq.Context()

# create a PUB         


        

Answer 1

I feel this behavior is the semantic of zmq_connect(). That is: when zmq_connect() returns success, then the connection is conceptually established, and thus your connecting-PUB starts queuing message instead of dropping.

Following excerpt from "ZMQ Guide" is a hint for this:

In theory with ØMQ sockets, it does not matter which end connects, and which end binds. However with PUB-SUB sockets, if you bind the SUB socket and connect the PUB socket, the SUB socket may receive old messages, i.e. messages sent before the SUB started up. This is an artifact of the way bind/connect works. It's best to bind the PUB and connect the SUB, if you can.

Following section in zmq_connect() has some hints, shown below:

Key differences to conventional sockets

Generally speaking, conventional sockets present a synchronous interface to either connection-oriented reliable byte streams (SOCK_STREAM), or connection-less unreliable datagrams (SOCK_DGRAM). In comparison, ØMQ sockets present an abstraction of an asynchronous message queue, with the exact queueing semantics depending on the socket type in use. Where conventional sockets transfer streams of bytes or discrete datagrams, ØMQ sockets transfer discrete messages.

ØMQ sockets being asynchronous means that the timings of the physical connection setup and tear down, reconnect and effective delivery are transparent to the user and organized by ØMQ itself. Further, messages may be queued in the event that a peer is unavailable to receive them.