pyzmq-docker | Micro-services as modules with Docker and pyzmq | Microservice library

This is mostly a docker networking question, and isn't specific to pyzmq or zeromq. You would have the same issues with anything trying to connect from a container to the host.

To be clear, in this example, you have a server running on the host (zmq_sub.py, which calls bind), and you want to connect to it from a client running inside a docker container (zmq_pub.py).

Since the docker container is the one connecting, you don't need to do any docker port exposing or forwarding. EXPOSE and forwarding ports are only for making it possible to connect to a container (i.e. bind is called in the container), not to make outbound connections from a container, which is what's going on here.

The main thing here is that when it comes to networking with docker, you should think of each container as a separate machine on a local network. In order to be connectable from other containers or the host, a service should bind onto all interfaces (or at least an accessible interface). Binding on localhost in a container means that only other processes in that container should be able to talk to it. Similarly, binding localhost on the host means that docker containers shouldn't be able to connect.

So the first change is that your bind url should be: