[systemd-devel] Behavior of BindsTo when stopping a service

[Amit Saha]

On Sun, Jan 28, 2018 at 5:04 PM Andrei Borzenkov <arvidjaar at gmail.com>
wrote:

> 28.01.2018 02:11, Amit Saha пишет:
> > On Sun, 28 Jan 2018 at 6:22 am, Andrei Borzenkov <arvidjaar at gmail.com>
> > wrote:
> >
> >> 26.01.2018 05:11, Amit Saha пишет:
> >>> Hello,
> >>>
> >>> I have a systemd service - drainconnections which I want to to make
> sure
> >>> finishes stopping before systemd starts stopping another service. What
> I
> >>> also want is to if I stop supervisord service, drainconnections should
> be
> >>> stopped first.
> >>>
> >>> It seems like BindsTo is what I need for establishing the coupling
> >> between
> >>> the two units.
> >>
> >> If your description of services relationship is accurate and complete,
> >> BindsTo is too strong here; you really need just PartsOf.
> >
> >
> > Thanks. PartOf looks like something more suitable. Would having PartOf in
> > supervisord obey the before after relationship setup by drainconnections
> > service?
> >
>
> PartsOf (like BindsTo) is orthogonal to After/Before. PartsOf/BindsTo
> define what to do, After/Before define when to do.
>
> > I will give it a shot.
>

Thanks. Tried it and that is how it works.



> >
> >>
> >>
> >>> The following unit file describes drainconnections:
> >>>
> >>>
> >>> [Unit]
> >>> Description=Drain Connections
> >>> After=supervisord.service
> >>> BindsTo=supervisord.service
> >>>
> ...
> >>>
> >>> In addition, I also needed to add BindsTo=drainconnections to the
> >>> supervisord service.
> >>>
> >>
> >> This will create dependency loop with your unit definition shown above.
> >
> >
> > Wouldn’t the Before/After take care of breaking the dependency loop? That
> > seems to be what I observed.
> >
>
> Probably I had to elaborate. If you need full BindsTo semantic (which is
> superset of Requires) it only works as documented if you also have
> corresponding After/Before which creates loop. Otherwise Requires part
> is mostly useless. If you are only interested in propagating stop
> request and "stopping on surprise removal" part of BindsTo then this is
> probably OK, but this will make your units definitions rather confusing.
>

Thanks, I think your suggestion of using PartOf is more suitable for my use
case here - which is basically making sure that I can do some work before
systemd shuts down the system (killing supervisord) and use After/Before to
ensure the when.

This is my current unit file:

[Unit]
Description=Drain Connections
After=supervisord.service
PartOf=supervisord.service

[Service]
Type=oneshot
RemainAfterExit=True
ExecStart=/bin/true
ExecStop=/usr/bin/touch /var/shuttingdown1
ExecStop=/usr/bin/sleep 60
ExecStop=/usr/bin/touch /var/shuttingdown2

[Install]
WantedBy=multi-user.target


It looks like I added the BindsTo=drainconnectins.service to supervisord as
well for a use case where during shutdown, if supervisord was stopped
first, it would first stop drainconnections.service. But I don't need that
- since systemd figures out it needs to first stop drainconnections.service
before it stops supervisord.

I think the only change I would make to the supervisord configuration is so
that it always starts drainconnections.service when it's started.
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