[aklug] Re: systemd - was Re: multiple distros coordinate to establish /run directory

On Wed, 6 Apr 2011, Jim Gribbin wrote:

> I am not trying to take sides, I am just trying to figure out what
> systemd is. I need to know what it is before I can figure out where I
> might stand on it.
>
> Correct me if I am headed down the wrong path.
>
> A "socket" is something akin to a socket in the wall that you connect to
> similarly plugging an electrical cord into an electrical outlet, or an
> ethernet cable into an rj59 outlet in the wall.
>
> When I plug a standard extension cord or a lamp cord into what we
> conventionally call an electrical outlet or an Ethernet cable into an
> rj59 outlet, I normally expect to find 115v @ 60Hz or networking signals
> (occasionally pots) from the respective sockets.
>
> If I am following the general idea of systemd, something, not
> necessarily within my control, can decide to change what is available at
> the socket on the fly.
>
> How do I connect to the socket confident that I am receiving expected
> telemetry data from from Joe in ICU and that something hasn't
> re-purposed the socket so that it's feeding data from the hospital's
> HVAC system? I would prefer not to give Joe the "smoke test".
>
> This is also the part of the concept that was making me uncomfortable.
> It smells a lot like self modifying code. I was taught this was
> something to be avoided. It was a long time ago that I was taught that,
> maybe philosophies have changed over the years?

That particular aspect of systemd is more like inetd (but not exactly). In
a nutshell, it knows of all the services installed and it listens on those
sockets itself. If another application connects to it it launches the
applicable service and hands over the waiting client connection. It's
basically on-demand services, but without the stateless quality of inetd,
where the service is terminated after each request. Systemd also adds
additional launch activation triggers, like dbus messages and more.

In contrast to your example above where you're worried about Joe in ICU and
you get HVAC data, each socket has a specific address (port # for inet
sockets, or a path for UNIX domain sockets), and each service uses a unique
socket. So, in practice what you're worried about doesn't normally occur,
even under systemd.

If that's all systemd did, though, I would have no objections. It's that
mingling with automountd functionality, the kernel binary format
augmentation, the replacement of sysvinit responsibilities, virtual console
setup, it's the timer integration (which is likely going to lead to a full
cron replacement), the temp file/dir life cycle management, the XDG session
management and PAM management, kernel module management, and so on. If that
run-on sentence from hell doesn't scare you, it should.

         --Arthur Corliss
           Live Free or Die
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Received on Wed Apr 6 23:02:15 2011