[zeromq-dev] PUSH socket blocks when no client is connected

Robert Pickering robertfpickering at fastmail.com
Mon Sep 14 16:56:18 CEST 2015

Hi all,

I'm running into a problem that PUSH sockets always block on send when
no PULL socket is connected.

First a brief note about what I'm trying to achieve: I'm trying to
distribute notifications from one server to a number of clients. The
scenario looks a lot like sub/pub, but there's an important difference
that means sub/pub is not suitable. The difference is I'd like all
clients to receive all notifications and tolerate a short disconnection
period. When a client crashes the server should queue messages for the
client until the client restarts. If the client is disconnected for too
long then messages can be thrown away.

With with SUB/PUB sockets, if a crash in the client occurs the
notification received are received by all remaining active clients but
are lost to the client that has crashed. I'm trying to implement this
scenario using PUSH/PULL sockets but running into the problem that send
blocks when no PULL client is connected.

I know the number of clients in advance, so each client has it's own
PUSH socket on the server. To send a notification I loop over all the
sockets on the server and send the notification. This works well until
one client is disconnected then it's socket starts to block, so no
further notifications can be send.

I've tried many things to get the PUSH to queue the message instead of
blocking, including setting the socket options: ZMQ_SNDBUF to 1000000,
ZMQ_SNDHWM to 10000 and ZMQ_IMMEDIATE to 1, although from my reading of
the docs none of these should be necessary.

Is there anyway to get the PUSH socket to queue the messages rather than
block when no client is connected? Or, if what I'm trying is not the
best pattern to implement the scenario described I'd be very interested
to hear ideas on other ways to implement it.

Many thanks,

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