[DRE-maint] unicorn: native systemd service

[Eric Wong]

Christos Trochalakis <yatiohi at ideopolis.gr> wrote:
> On Thu, Jun 25, 2015 at 11:26:26PM +0000, Eric Wong wrote:
> >With socket activation, you should just be able to kill unicorn using
> >SIGQUIT (just master, or even all workers) and restart without ever
> >dropping a connection.  I do NOT suggest using SIGTERM for unicorn,
> >since that'll cause the master to kill all workers ASAP.
> 
> Yes, you are right socket activation is also an option! I have made some
> experiments with a simple rack app to test it.
> 
> systemd uses the LISTEN_FDS env variable that is an integer indicating the
> number of inherited file descriptors. Those FDs have consecutive numbers
> starting from `SD_LISTEN_FDS_START` which is `3` (man sd_listen_fds).
> 
> So for example if LISTEN_FDS="2", UNICORN_FD should be "3,4". I used a
> simple wrapper script for that. Here is the full configuration:

OK, I'll probably add LISTEN_FDS and LISTEN_PID support to unicorn
directly so the wrapper is unnecessary.

<snip>

> KillMode=mixed

I don't think KillMode=mixed is necessary, here.  systemd can send
SIGQUIT to workers.

<snip>

> TCP socket options are not applied by unicorn on inherited sockets (TCPSocket
> === sock is false). systemd socket files have support for most options now but
> we might want unicorn to `setsockopt` them as well. For example,
> 'DeferAcceptSec', 'KeepAliveIntervalSec', 'NoDelay' are supported since v216, so
> they are not available in jessie (v215).

They are now :)

http://bogomips.org/unicorn-public/m/1435373879-4299-1-git-send-email-e@80x24.org.txt

We don't have KeepAliveIntervalSec
(aka TCP_KEEPINTVL) since it's not-portable and probably over
Are you sure about that?

> socket activation is a really interesting setup, but personally I would not run
> it with a large application. Clients would have to wait for the new master to
> be up and running before a reply is returned, and that could take tenths of
> seconds. The USR2 rexec solves that problem since both old and new workers are
> accepting on the socket and we can kill the old ones when we are ready. In that
> case the PIDFile trick is handy to support zero downtime restarts with no
> latency.

Is it possible to make systemd fire up two unicorn masters?
That would be a nice feature to have with socket activation.