[zeromq-dev] Cleaning up file descriptors for dead router peers

[Pieter Hintjens]

Do you think there's any way to reproduce this in the lab, e.g.
killing a peer before it can shut down TCP properly?

On Tue, Jun 23, 2015 at 10:08 PM, Marcin Romaszewicz <marcin at brkt.com> wrote:
> Hi All,
>
> I've got an issue with ZMQ_ROUTER sockets which I'm having a hard time
> working around, and I'd love some advice, but I suspect the answer is that
> what I want to do isn't possible.
>
> Say I have a router socket listening on a port, and I have peers connecting
> and disconnecting randomly over TCP. These peers have random identities for
> all intents and purposes.
>
> Most of the time, a peer will disconnect "cleanly", meaning the TCP
> connection is terminated via FIN or RST packets, ZMQ cleans up the file
> descriptor.
>
> However, some of the time, my peer will die silently, effectively due to
> network outage or power outage or something.
>
> In these cases, the router socket keeps the file descriptor around forever.
> I know that the peer is dead because all my peers heartbeat to each other,
> and the heartbeats have gone away. I thought that trying to send some data
> to a dead peer would tear down that connection, since the underlying TCP
> socket would eventually start erroring, but it doesn't, zmq must be dropping
> my packet before sending it to the underlying socket.
>
> The socket monitor tells me that someone has connected to the router socket
> on on its bound port with a specific file descriptor, but I've got so many
> of these coming in that I can't associate a specific file descriptor with a
> specific peer.
>
> TCP keep-alives don't work all that well in raising errors in a dead
> connection.
>
> What I know on the app side due to my heartbeats is that peer XYZ is dead.
> I'd like to tell the router socket to close the underlying file descriptor.
> What I know via the monitor is that I have a bunch of file descriptors open,
> but I can't map them to peers. If I could, I'd just call os.close() on that
> file descriptor and hopefully ZMQ would handle this gracefully.
>
> Eventually, in a few hours of uptime, my process hits the os file descriptor
> limit, and stops receiving new connections on the zeromq level. I can have
> the process quit when it detects this, but that forces all the functioning
> peers to reconnect and re-do some work, so I'd like to avoid it.
>
> I scanned the previous discussions about it, and there has been mention of
> exposing this somehow, but I don't see anything along these lines in the
> latest API. (looking at 4.1.2 release).
>
> Any suggestions on how I could work around this?
>
> I'm thinking of extending the socket monitor to have a new event type, like
> ZMQ_PEER_CONNECT/DISCONNECT which passes back the peer ID and file
> descriptor, but I've not gone through the zmq code enough yet to know how
> much work this would be.
>
> Thanks in advance,
> -- Marcin
>
>
>
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