[zeromq-dev] Reading queued messages after disconnect

[Matt Connolly]

Dear list,

I’ve been playing around with how to ensure that a service has processed all messages that it has received (in the receiving socket queue) before shutting down. In particular, I’m using the rbczmq ruby bindings, so this question is also to see if those bindings are consistent with other languages.

Scenario:

A service has a ROUTER socket, bound to a tcp endpoint.
A client has a DEALER socket, connected to that tcp endpoint.
The client has sent one or more messages and is expecting a reply to each message.
The messages have been transmitted and are in the service’s inbound message queue waiting for processing.
The service receives a shutdown request (for example, a system being restarted for maintenance).
The service should process all received messages or store them for processing on next start-up.

—

When I do a socket.disconnect in the service, it is unable to read any message in the queue for that socket. However, if I subsequently connect that socket to a dummy endpoint, such as “ipc://foo”, then I can receive all of the messages in the queue that were sent to the real address. Once disconnected, the service is unable to send a response to the client, but at least the message can be saved, and a reply could be sent when the service comes back online.

The service should only be down for a small amount of time (just a few minutes I suppose) so for messages queued at the client side, this is not a problem because the client will wait for a reply and the io thread will continue to reconnect and send when it connects to the service.

Is the behaviour normal that socket.receive will fail with an error if the socket has no active endpoints (bind or connect)? I suppose this only makes sense for a non-blocking call, otherwise if there are no messages in queue, then the call will BlockForever™…

Also, I notice that if I disconnect/unbind the socket from the service, that ZMQ::Poller#poll and ZMQ::Socket#poll both report the socket as being readable, but reading from the socket still fails. Is this particular to rbczmq?


Cheers,
Matt

PS: happy holidays all!