[ClusterLabs Developers] RA as a systemd wrapper -- the right way?

Jan Pokorný jpokorny at redhat.com
Mon Nov 6 20:01:35 EST 2017

On 07/11/17 01:02 +0300, Andrei Borzenkov wrote:
> 06.11.2017 22:38, Valentin Vidic пишет:
>> On Fri, Oct 13, 2017 at 02:07:33PM +0100, Adam Spiers wrote:
>>> I think it depends on exactly what you mean by "synchronous" here. You can
>>> start up a daemon, or a process which is responsible for forking into a
>>> daemon, but how can you know for sure that a service is really up and
>>> running?  Even if the daemon ran for a few seconds, it might die soon after.
>>> At what point do you draw the line and say "OK start-up is now over, any
>>> failures after this are failures of a running service"?  In that light,
>>> "systemctl start" could return at a number of points in the startup process,
>>> but there's probably always an element of asynchronicity in there.
>>> Interested to hear other opinions on this.
>> systemd.service(5) describes a started (running) service depending
>> on the service type:
>> simple  - systemd will immediately proceed starting follow-up units (after exec)
>> forking - systemd will proceed with starting follow-up units as soon as
>>           the parent process exits
>> oneshot - process has to exit before systemd starts follow-up units
>> dbus    - systemd will proceed with starting follow-up units after the
>>           D-Bus bus name has been acquired
>> notify  - systemd will proceed with starting follow-up units after this
>>           notification message has been sent
>> Obviously notify is best here
> forking, dbus and notify all allow daemon to signal to systemd that
> deamon is ready to service request. Unfortunately ...
>> but not all daemons implement sending
>> sd_notify(READY=1) when they are ready to serve clients.
> ... as well as not all daemons properly daemonize itself or register on
> D-Bus only after they are ready.

Sharing the sentiment about the situation, arising probably primarily
from daemon authors never been pushed to indicate full ability to
provide service precisely because 1/ it's not the primary objective of
init systems -- the only thing they would need to comply with
regarding getting these daemons started (as opposed to real
service-oriented supervisors, which is also the realm of HA, right?),
and 2/ even if it had been desirable to indicate that, no formalized
interface (and in turn, system convolutions) that would become
widespread was devised for that purpose.  On the other hand, sd_notify
seems to reconcile that in my eyes (+1 to Valetin's qualifying it the best the
above options) as it doesn't impose any other effect (casting extra
interpretation on, say, a fork event makes it possibly not intended or
at least not-well-timed side-effect of the main, intended effect).

To elaborate more, historically, it's customary to perform double fork
in the daemons to make them as isolated from controlling terminals and
what not as possible.  But it may not be desirable to perform anything
security sensitive prior to at least the first fork, hence with
"forking", you've already lost the preciseness of "ready" indication,
unless there is some further synchronization between the parent and
its child processes (I am yet to see that in practice).  So I'd say,
unless the daemon is specifically fine-tuned, both forking and dbus
types of services are bound to carry some amount of asynchronicity as
mentioned.  To the distaste of said service supervisors that strive to
maximize service usefulness over the considerable timeframe, which is
way more than ticking the "should be running OK because it got started
by me without any early failure" checkbox.

The main issue (though sometimes workable) of sd_notify approach is
that in your composite application you may not have a direct "consider
me ready" hook throughout the underlying stack, and tying it with
processing of the first request is out of question because it's timing
is not guaranteed (if it's ever to arrive).

Sorry, didn't add much to the discussion, getting rid of
asynchronities is tough in the world that wasn't widely intrested
in poll/check-less "true ready" state.

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