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List:       systemd-devel
Subject:    Re: [systemd-devel] Alternatives to RequiresOverridable= ?
From:       Andrei Borzenkov <arvidjaar () gmail ! com>
Date:       2021-04-11 4:59:57
Message-ID: 48a3ed41-ec19-6624-449c-065a1a0a7ecc () gmail ! com
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On 10.04.2021 21:54, Cameron Sparr wrote:
> > Requires/Wants/RequiresOverridable= without After= is useless.
> 
> Thanks for the reply. I'm curious about this statement, do you mean it is useless \
> in general without After= or just in the context of our use-case? 

General. Read man systemd.unit, read this list archives - this question
pops up every month.

Requires= is useless to define startup dependencies unless you can
ensure that start job for required unit completes (successfully or not)
before start job for requiring unit is selected for processing.

> I should probably clarify the use-case too. cloud-final.service runs after \
> cloud-init.service finishes. So if ecs.service starts at the same time as \
> cloud-final.service this is acceptable. Is this the behavior that Requires= alone \
> gives us? 

I do not understand this question.

And if I understand correctly it can be overridden if the user
explicitly starts ecs.service using 'systemctl start' ?
> 

Requires= cannot be overridden.

> On 4/9/21, 11:45 PM, "systemd-devel on behalf of Andrei Borzenkov" \
> <systemd-devel-bounces@lists.freedesktop.org on behalf of arvidjaar@gmail.com> \
> wrote: 
> On 10.04.2021 03:07, Cameron Sparr wrote:
> > Hello, I work for Amazon ECS and I've been working on a change to one of our \
> > systemd services. From what I could tell in documentation I found online, it \
> > seemed that RequiresOverridable= was the perfect fit for our use-case. 
> > 
> > When I built a package using this field, however, I got a message saying that \
> > this option is obsolete, which led me to this mailing list message: \
> > https://lists.freedesktop.org/archives/systemd-devel/2015-November/034880.html 
> > 
> > So my question is, what would be the alternative to using RequiresOverridable? \
> > What got our attention to use this flag was that user input would be able to \
> > override the requirement, which is exactly what we want. Does Requires= also \
> > provide that capability? From our testing it _seems_ like it does but I don't see \
> > it called out in the documentation anywhere. 
> > 
> > If it helps, I can describe our use-case below:
> > 
> > 
> > 1.       We have a service that executes user-defined bash scripts on system \
> > startup called (simplifying) cloud-final.service. 
> > 2.       We have a service called ecs.service that runs the ecs daemon service. \
> > This service's configuration file is usually made by the user scripts run in \
> > cloud-final.service 
> > 3.       So we wanted to make sure ecs.service starts after cloud-final.service. \
> > To accomplish this we put After=cloud-final.service in ecs.service. 
> > 4.       But now we would also like users to be able to override ecs.service \
> > waiting for cloud-final.service to finish. Because cloud-final allows users to \
> > execute arbitrary bash scripts they should be able to run "systemctl start ecs" \
> > and the ecs service will start. 
> 
> After= dependencies are relevant only for jobs that are currently
> present in job queue. If ecs.server does not pull in cloud-final.service
> with Wants= or Requires=, you can start it explicitly without any delay.
> 
> Of course if when you request starting ecs.service the
> cloud-final.service is still being activated (its start job is active),
> then ecs.service will wait for activation to complete. There is no way
> around it I am aware of.
> 
> > 5.       So the solution we were going to do was split ecs into two services:
> > 
> > a.       ecs-ready.service which has After=cloud-final.service
> > 
> > b.       ecs.service which has RequiresOverridable=ecs-ready.service
> > 
> 
> Requires/Wants/RequiresOverridable= without After= is useless. And it
> sounds like you tested Requires= without After= when you say "it seems
> to work". RequiresOverridable= with After= would still attempt to start
> required unit and wait for it. It would have ignored failure to start
> required unit, but that is not what you want.
> 
> > 6.       The idea above being that normally ecs.service would start with \
> > ecs-ready (and thus after cloud-final), but if the user explicitly requested it \
> > could be started without having to wait for after cloud-final. 
> > 
> > 
> > _______________________________________________
> > systemd-devel mailing list
> > systemd-devel@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/systemd-devel
> > 
> 
> _______________________________________________
> systemd-devel mailing list
> systemd-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/systemd-devel
> 
> 

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