[systemd-devel] Zero downtime restart of Web application (Nginx, Puma, JRuby)

[David Timothy Strauss]

On Thu, Jul 14, 2016, 19:07 Kai Hendry <hendry at webconverger.com> wrote:

> I would love to see that 10 lines of shell you claimed, but I think you
> might be underestimating the fine work that went into Dokku!
>

It's not so much underestimating the work in Dokku as much as leveraging
what systemd and a tool like haproxy provide for services.

Here's what a script would do with no socket activation, assuming you're
sending traffic to the services with a tool like haproxy and have an
interface like a control socket [1]:

   1. Tell haproxy to stop sending traffic to service instance A.
   2. systemctl restart instance-a.service
   3. Tell haproxy to start sending traffic to service instance A and stop
   to B.
   4. systemctl restart instance-b.service
   5. Tell haproxy to start sending traffic to service instance B.

Alternatively, you could track state of a flip-flop and stabilize on only
one service instance at a time after the restart.

[1] https://cbonte.github.io/haproxy-dconv/configuration-1.5.html#9.2
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