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* Systemd unit parser and s6 generator
@ 2023-02-27 16:25 Oliver Schad
  2023-02-28  2:02 ` Laurent Bercot
  2023-04-09  7:43 ` Jonathan de Boyne Pollard
  0 siblings, 2 replies; 4+ messages in thread
From: Oliver Schad @ 2023-02-27 16:25 UTC (permalink / raw)
  To: supervision

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Hi everybody

I don't know, if somebody has tried that ever. However we want to
continue to use Ubuntu on s6 and packages today our s6 "units" manually.

We've build a generator for that, that creates a base of service. But
the call of this generator is once by a human being which manually
decides which options make sense. Later he/she modifies maybe the
generated result.

We've discussed internally if we change that process and try to write a
systemd unit parser, because all units are there in Ubuntu.

If we could catch 90% of all cases, we need, we would be happy.
If it would take 2 weeks of work, that would be fine.

Did somebody of you try to implement something? What are your thoughts?

Best Regards
Oli

-- 
Automatic-Server AG •••••
Oliver Schad
Geschäftsführer
Hardstr. 46
9434 Au | Schweiz

www.automatic-server.com | oliver.schad@automatic-server.com
Tel: +41 71 511 31 11 | Mobile: +41 76 330 03 47

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^ permalink raw reply	[flat|nested] 4+ messages in thread

* Re: Systemd unit parser and s6 generator
  2023-02-27 16:25 Systemd unit parser and s6 generator Oliver Schad
@ 2023-02-28  2:02 ` Laurent Bercot
  2023-04-24  8:06   ` Laurent Bercot
  2023-04-09  7:43 ` Jonathan de Boyne Pollard
  1 sibling, 1 reply; 4+ messages in thread
From: Laurent Bercot @ 2023-02-28  2:02 UTC (permalink / raw)
  To: Oliver Schad, supervision

>We've discussed internally if we change that process and try to write a
>systemd unit parser, because all units are there in Ubuntu.
>
>If we could catch 90% of all cases, we need, we would be happy.
>If it would take 2 weeks of work, that would be fine.
>
>Did somebody of you try to implement something? What are your thoughts?

  Hi Oli,

  This is a subject that comes up regularly in the #s6 IRC channel. And
I always answer something like this:

  The difficulty of converting systemd services to s6 services is not a
syntax problem. The unit file syntax is mostly fine.

  The difficulty is the difference of world modelization between systemd
and s6. systemd's view is very holistic, every component can depend on
another one or rely on a systemd-only feature, and the unit file syntax
reflects that; while s6's view is more local, every service is its own
independent thing.
  But more importantly, the way systemd maps the system into concepts is
pretty different from the way s6 maps the system into concepts, and 
these
two views are not 1-to-1 compatible. Translating a setup between systemd
and s6 requires intelligence; it is not possible to write an automated
tool that does it accurately and idiomatically without going *deep* into
things.

  In practice, yes, it is possible to write a converter that will take
systemd.service files and turn them into s6 service directories, and
that will work "most" of the time, depending on how your service files
look. Most directives in systemd.service are directly translatable to
s6 features. However, as soon as you start using esoteric systemd
directives, the complexity explodes, and it's all a balancing act on
what you want to support vs. how difficult it is. And only
systemd.service files are automatically convertible; other unit files
such as systemd.socket, systemd.path and systemd.target are all 
dependent
on the systemd-centric system view and it is impossible to treat them
without analyzing the entire system.

  I still owe you a couple hours of work, so what I can do, if it is of
interest to you, is list all the directives in a service file and
rate their conversion difficulty, so you can then evaluate your own
service files and assess the feasability of an automated conversion 
tool.
90% coverage is doable if your services are super simple and don't rely
on systemd-specific features, but it's very easy to get lost in the 
weeds.

--
  Laurent


^ permalink raw reply	[flat|nested] 4+ messages in thread

* Re: Systemd unit parser and s6 generator
  2023-02-27 16:25 Systemd unit parser and s6 generator Oliver Schad
  2023-02-28  2:02 ` Laurent Bercot
@ 2023-04-09  7:43 ` Jonathan de Boyne Pollard
  1 sibling, 0 replies; 4+ messages in thread
From: Jonathan de Boyne Pollard @ 2023-04-09  7:43 UTC (permalink / raw)
  To: supervision

> [...]  We've discussed internally if we change that process and try to 
> write a systemd unit parser, because all units are there in Ubuntu. If 
> we could catch 90% of all cases, we need, we would be happy. If it 
> would take 2 weeks of work, that would be fine. Did somebody of you 
> try to implement something? What are your thoughts?
>
So it seems that I am, years later, still the only person in the world 
to have done this (except not targetting skarnet toolsets). (-:

As the voice of experience:

Yes, you'll not get total coverage.  You'll get "good enough for most 
cases", though I couldn't put an exact percentage on that.

No, with the s6 tooling as it stands, you'll not be able to do 
everything.  One of the things that one has to add is support for 
systemd's idiosyncratic non-UCSPI TCP/UDP service handling. Contrast the 
--systemd-compatibility flag to the tcp-socket-listen program in the 
nosh toolset to what s6-tcpserver4-socketbinder in s6-networking does.

* https://jdebp.uk/Softwares/nosh/guide/commands/tcp-socket-listen.xml

* https://skarnet.org/software/s6-networking/s6-tcpserver4-socketbinder.html

That flag is used by convert-systemd-units to handle the conversion of 
socket units.

* https://jdebp.uk/Softwares/nosh/guide/commands/convert-systemd-units.xml

Most of the issues for s6 will be things like that, minor tweaks for 
(sometimes reasonable, sometimes downright absurd) systemd quirks and 
the odd missing tool.  I wrote a set-control-group-knob command, for 
example, used by convert-systemd-units to translate the various unit 
file settings that end up twiddling knobs on control groups.  I haven't 
seen a skarnet tool for doing that, yet; so that will be one necessary 
addition, for example.

* https://jdebp.uk/Softwares/nosh/guide/commands/set-control-group-knob.xml

s6-networking also does not appear to have UDP listening tools, which 
socket units for UDP sockets need.  Timer units need tools like 
time-pause-until and time-env-add, similarly.  Notice the 
--systemd-compatibility flag for that latter, too, and the notions like 
systemd month lengths.

* https://jdebp.uk/Softwares/nosh/guide/commands/time-pause-until.xml

* https://jdebp.uk/Softwares/nosh/guide/commands/time-env-add.xml

There is, furthermore, a lot of stuff that you'll find that s6/nosh/&c. 
service run scripts do a lot better than systemd can.  As you'll see 
from the convert-systemd-units(1) manual page, I added quite a number of 
things to the systemd unit file syntax for things that one can do in the 
aforementioned, such as UCSPI access control mechanisms (UCSPI being 
entirely alien to systemd) for starters.


^ permalink raw reply	[flat|nested] 4+ messages in thread

* Re: Systemd unit parser and s6 generator
  2023-02-28  2:02 ` Laurent Bercot
@ 2023-04-24  8:06   ` Laurent Bercot
  0 siblings, 0 replies; 4+ messages in thread
From: Laurent Bercot @ 2023-04-24  8:06 UTC (permalink / raw)
  To: Oliver Schad, supervision

>what I can do, if it is of
>interest to you, is list all the directives in a service file and
>rate their conversion difficulty, so you can then evaluate your own
>service files and assess the feasability of an automated conversion tool.

  It took way longer than expected, and was a harrowing task, but it's
finally done - and I'm glad I did it because it was a fascinating dive
into the systemd worldview, and I'm all the more knowledgeable for it.
(And a little dumber, too.)

  The results can be found here:
  https://skarnet.org/software/s6/unit-conversion.html

  I could probably have written a comment - and, sometimes, even an
informative and non-snarky one! - for every single directive I listed,
but I had to restrain myself in order to finish the document before the
end of the year.

  Enjoy.

--
  Laurent


^ permalink raw reply	[flat|nested] 4+ messages in thread

end of thread, other threads:[~2023-04-24  8:06 UTC | newest]

Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
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2023-02-27 16:25 Systemd unit parser and s6 generator Oliver Schad
2023-02-28  2:02 ` Laurent Bercot
2023-04-24  8:06   ` Laurent Bercot
2023-04-09  7:43 ` Jonathan de Boyne Pollard

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