* 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