On Wed, Jan 09, 2019 at 06:00:31PM +0100, Simon Matter via CentOS wrote:
> Maybe things _could_ be done the right way with systemd, but it doesn't
> happen because it quickly starts to be very complex and it's a lot of work
> to do it for a complete distribution. It just doesn't happen - or at least
> did not happen in all the years since its introduction.
There are a couple ways that systemd can handle service startup in a
way that dependent services can gracefully start up after it. One way
is to have systemd open the socket, then hand it to the service when
it is ready. This requires quite a bit of hacking and I don't think
is as reliable, it's more of the inetd way of doing things.
Another is to have the code send a message when it is ready. This
isn't really that complicated, you can look at the change in
postgresql's git here:
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=7d17e683fcc28a1b371c7dd02935728cd2cbf9bf
Basically, when the database is ready, it calls the C function:
sd_notify(0, "READY=1");
and when it's shutting down, it runs:
sd_notify(0, "STOPPING=1");
To be honest, that's not too complicated. It does require minor
changes to the code to support systemd, but you can replace idle loops
in shell scripts with a smarter database (which knows when it is
ready) telling PID 1 that it is ready.
--
Jonathan Billings <billings@xxxxxxxxxx>
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