[zeromq-dev] multi-connect REQ socket send routing

Carl Jenkins cjenkins5614 at gmail.com
Sat Jun 6 20:57:53 CEST 2020

Hi. I posted this last weekend, and further posted this on the pyzmq GitHub
Issues, as well as the IRC channel. But I was not able to get any response.

I was able to reproduce this with Cppzmq as well, so it’s not a Python

What I really would like to know is, how do people do multiple server
req-rep without a broker? Zeromq seems to offer this with one single socket
connecting to multiple servers, judging by the standard doc, but in reality
it hangs forever if any one of all servers disappears. It must be a common
use case at scale, and how do people resolve it? Do they create a socket
for each server, and cycle through them manually, or I’m missing something
obvious here.


On Sat, May 30, 2020 at 11:52 AM Carl Jenkins <cjenkins5614 at gmail.com>

> Hi. Just to follow up on the question. I think detecting dead server
> during REQ is covered in Chapter 4 Reliable Req-Rep Patterns, and the lazy
> pirate pattern deals with this case. In my situation, I ended up having
> another SUB socket receive heartbeats from servers and reconnect to the
> healthy ones when recv() times out.
> This is still a bit involved (one more socket on client side, and some
> communication between the health checker and main thread) and led me into
> testing some simple cases. What I found was that if the binding servers are
> REP, and client is REQ, when one server dies, client won’t automatically
> choose a different connected server to send the request. However, if the
> binding server is REQ, and clients are REP, when one client dies, the
> server is able to fall back to other healthy clients.
> The code is at
> https://pastebin.com/RSJfdef1.
> Best,
> Carl
> On Mon, May 25, 2020 at 3:23 PM Carl Jenkins <cjenkins5614 at gmail.com>
> wrote:
>> Hello,
>> I started using zeromq for a project and have bee reading the guide
>> (almost done with chapter 3. There is one occasion where I want to have a
>> REQ client talk to multiple servers. I don’t plan to use the routing/load
>> balancing pattern, with the intent to avoid single point of failure. If a
>> server dies, I’d like the client to switch to a different server to do
>> send-recv.
>> I have realized recv would block if that very host it’s talking to dies,
>> and my solution is to set RCVTIMEO to a finite number (and LINGER to a
>> finite number as well when socket terminates). However, for the send part,
>> I can see send always seems to go through (I think due to buffering), and
>> the subsequent recv would time out. This means send is still doing simple
>> round-robin, otherwise recv would have been smooth, given other servers are
>> healthy. I’m OK with buffering and send not reporting one server dying, but
>> how can I ask zeromq to neglect the dead server from the round robin list?
>> This seems to be the documented behavior from
>> https://zeromq.org/socket-api/, but my tests above didn’t reproduce it:
>> If no services are available, then any send operation on the socket will
>> block until at least one service becomes available. The REQ socket will not
>> discard any messages.
>> Thanks,
>> Carl
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