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

[Pieter Hintjens]

For what it's worth, we just merged a pull request that adds
connection heartbeating. It could be fun to see if this solves your
problem. (In theory it should...)

https://github.com/zeromq/libzmq/pull/1448


On Wed, Jun 24, 2015 at 6:48 PM, Marcin Romaszewicz <marcin at brkt.com> wrote:
> Yes, you can easily reproduce this by pulling a network cable or shutting
> the host down before it can do any sort of TCP connection cleanup. I'm
> seeing it in AWS when instances get terminated, because they're given so
> little time to respond to TERM that connections aren't cleaned up.
>
> The iptables approach which Francis mentioned should work as well.
>
> I'll see if I can come up with a simple example of reproducing this. It
> might be even possible to repro this on a single machine simply by
> suspending a peer.
>
> -- Marcin
>
> On Wed, Jun 24, 2015 at 2:47 AM, Pieter Hintjens <ph at imatix.com> wrote:
>>
>> 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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>> >
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