But how do I implement backpressure in zeromq? I've used it in production and it was great when it worked, but it was scary because its queue of messages could get arbitrarily large. I think I even saw an OoM when a service went down and something just queued indefinitely.

Edit: just checked the FAQ and apparently it will now block or drop messages. I sort of wish it would explicitly error instead of block, so as not to require the caller to queue in a thread or something.