On Thu, 16 Aug 2012 15:33:29 +0300 Ciprian Dorin Craciun <ciprian.crac...@gmail.com> wrote:
> On Thu, Jul 26, 2012 at 1:09 PM, Peter Lemenkov <lemen...@gmail.com> wrote: > > Hello All. > > I'm trying to write a systemd service for epmd (Erlang Port Mapper > > Daemon, if someone is curious). its only purpose is to open a TCP port > > 4369 at 0.0.0.0 and act as a simple messaging (very simple actually) > > bus between erlang nodes. I'd like to run it w/o anything and under a > > most restricted system account. So far I'm using this service: > > > (Notice: Slightly off topic for systemd, but very on topic for EPMD...) > > Please be aware that starting `epmd` under a service monitoring > system like systemd (or others) is not 100% possible under the current > circumstances. For a detailed explanation please see the following > thread on the Erlang mailing list: > http://erlang.org/pipermail/erlang-questions/2011-October/061802.html > > The summary is that if someone starts an Erlang application before > your `epmd` instance starts you'll end up with your `epmd` dieing and > being restarted by systemd in a continuous loop, because each Erlang > application tries to start its own `epmd` instance. (Thus there is a > race condition here.) In the thread above I've found a workaround for > this issue but it's not quite "documented"... > > On the other hand if you would modify `epmd` to use socket > activation at least the issue will be less likely to happen. (It can > still happen if someone starts the Erlang application even before > systemd.) > Other workarounds might be one of the recent patches proposed here on the list - either the one that controls "failure" exit code (provided epmd returns "special" code on such conflicts) or the one that stops restarts after some time, though I guess latter might be undesirable, since if epmd gets started along with some service, it might also be killed along with it at some point. Another semi-fix might be just rate-limiting the restarts, which should already be possible. Whole problem seem to be at least a bit superficial though - if every erlang daemon has Require=epmd.service, the only way to create it is by starting erlang stuff and epmd from some user session, which is also a way to break pretty much every other daemon which can only have one instance (e.g. starting mysqld by hand will break mysqld.service). -- Mike Kazantsev // fraggod.net
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