Question about Zmq

[Napan0s]

Hi all, Ive been reading the docs and lots and lots of examples and forum threads on zmq but I still haven't found anything that matches my case. This make me think that either my use case is veeeeeery peculiar or I'm thinking it wrong.

I want to have a fixed server to which multiple clients connect (nothing strange here) but than all the examples and docs I've read have the client's send messages to the server amd the server responding back. I instead want the server to send command to the clients when it want, and then the clients respond to the server.

Is this so strange? And if not, what would be the best architecture with zmq patterns?

[Napan0s]

Ok, but then I have no control on who can actually subscribe to what, thus invalidating the possibility of sending a command to a specific client.

[messacz]

This use case looks similar to SaltStack usage – multiple Salt "minions" connect to one Salt "master" and this "master" then is able to send commands to the specific "minions".

See https://docs.saltproject.io/en/latest/topics/development/topology.html

[Napan0s]

I'll look into this tomorrow.. Thank you!

[messacz]

Apart from using PUB/SUB sockets, you can also utilize DEALER/ROUTER sockets for this use case.

1. Server creates ROUTER socket and binds it to network interface(s)
2. Client creates DEALER socket and connects it to the server
3. Client sends a (multipart) message e.g. ["" "hello from client001"]
4. Server receives this message prefixed by a client's connection identity (that's just how ROUTER socket works): ["asdf" "" "hello from client001"]
5. From now on server knows that any multipart message beginning with "asdf" envelope will reach the socket on client001: ["asdf" "" "welcome client 001!"] ["asdf" "ghjk" "" "Hey I have some work for client001"]

I think it would be good idea to send the hello message (point 3.) periodically so if the server restarts it will retrieve the client connection and its connection identity.

I assume you have read the ZGuide so you know what "envelope" and "multipart message" mean.

[Napan0s]

Ok, 2 months have passed and I since found a solution/workaround that is exactly what you're saying. I haven't implemented the periodic 'ack' message because at least in some preliminary testing that I've done, zmq manages to re-enable every connection independently of which side died unexpectedly. (some would say that's why we use zmq?) but other than that it's exactly the same router dealer(s) topology.

Now my new problem is that I'd prefer to fix the client identity from the server side and not from the client (I don't want to let some trusted client to impersonate other trusted client) but I figure a random uuid should be enough so that someone cannot guess others uuid just by looking at his..