Send message to multiple servers pyzmq

Question

If I have one client connect to multiple servers, and try to send a message,

socket = context.socket(zmq.REQ)
socket.connect ("tcp://127.0.0.1:5565")
socket.connect ("tcp://127.0.0.1:5566")
socket.connect ("tcp://127.0.0.1:5567")
socket.send("Hello all")

only one server will actually get the message. The documentation says that pyzmq preforms some simple load balancing across all available servers.

Is there a way send a message to all servers, rather than just one?

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Background:

I am trying to control a network of raspberry pis with my computer. I need to send a message to all of them at once, but I can't use PUB/SUB model, because then they all need to respond to that message.

I have one requester (master computer) that sends a request to all of the repliers (raspberry pis), and they all reply individually. For example I could send one message asking to get the reading from a tempurature sensor, and I want all of the raspberry pis to get read a tempurature sensor and send it back.

Answer 1

Yes.

Use an appropriate Formal Communication Pattern.

ZMQ.REQ formalism indeed expects, that the component is asking some other process, via sending a REQUEST, to do some job in response to the message. Thus the multiple exgress targets the .connect() has built a transport relation with, are served in a round-robin mode, selecting one after another, in a fair-queue-policy mode. So the component works but for a different purpose, than you are asking it to do.

Solution

Try some more complex Formal Communication Pattern that "spreads" the message to all relevant peers ( PUB/SUB alike ) but more complex, smarter, fail-safe derived schemes, that would serve your Raspberry PI solution needs.

The greatest strength of the ZeroMQ is in that it off-loads the low-level details from you and leaves you an immense power in designing all the needed distributed scaleable Formal Communication Patterns, that you need. Forget about just the few primitives ( building blocks ) directly listed in the ZeroMQ binding. Think about your abstract message/event processing scheme and then assemble ZeroMQ elements to meet that scheme.

ZeroMQ [socket] is not a hose from A to B. It is rather an access port for dialogues with smart Formal Communication Pattern Nodes. You may benefit, that [socket] may work over many transport classes at the same time ... so your Formal Communication Patterns may span over L3-networks [TCP:] + go into [IPC:] and [INPROC:] process-to-process channels inside the [localhost].

All working in parallel ( well, sure - almost in parallel once inspected in lower detail )

All working in smooth co-integrated environment.

Where to source from?

A best next step you may do for this is IMHO to get a bit more global view, which may sound complicated for the first few things one tries to code with ZeroMQ, but if you at least jump to the page 265 of the Code Connected, Volume 1 [asPdf->], if it were not the case of reading step-by-step there.

The fastest-ever learning-curve would be to have first an un-exposed view on the Fig.60 Republishing Updates and Fig.62 HA Clone Server pair for a possible High-availability approach and then go back to the roots, elements and details.