[arch-general] Forking daemons and systemd

[Leonid Isaev]

On Mon, 5 Nov 2012 11:18:48 -0500
Dave Reisner <d at falconindy.com> wrote:

> On Mon, Nov 05, 2012 at 10:01:11AM -0600, Leonid Isaev wrote:
> > Hi,
> > 
> > 	I was wondering whether there is a guideline regarding using
> > Type=forking daemons in systemd units. For instance, if a daemon supports a
> > cmdline switch to run in foreground isn't it better to use this argument in
> > ExecStart?
> > 	Personally, I was bitten by this with haveged.service which fails
> > on shutdown and whose unit has Type=forking, but I also noticed that ntpd
> > is allowed to fork. Both of them support foreground operation (haveged -F
> > and ntpd -n respectively)?
> 
> Essentially, it comes down to ordering of other daemons.
> 
> It's always going to be some trifling amount faster to start a daemon in
> the foreground because systemd assumes it to be alive as soon as it
> starts. Conversely, a Type=forking daemon is only considered alive only
> once the parent process has exited.
> 
> What this means is that while you might be able to start a daemon which
> normally forks in non-forking mode, you can't guarantee that daemons
> which rely on the non-forking daemon can be reliably started, since
> various listeners or other channels may not be established in time.
> 
> The ideal solution is to implement sd_notify() and use Type=notify, or
> full blown socket activation should it be appropriate for the daemon.
> Both of these cases allow for essentially fire-and-forget style startup
> with guarantees of availability for ordering.
> 
> Of course, if you don't think you ever need to order anything on a given
> daemon, then Type=simple is just fine.
> 
> HTH,
> d

Aha... thanks for the clarification. I'm pretty sure that havege does not
open any sockets/has to be a dependency of anything, but ntpd I'll have to
check.

-- 
Leonid Isaev
GnuPG key: 0x164B5A6D
Fingerprint: C0DF 20D0 C075 C3F1 E1BE  775A A7AE F6CB 164B 5A6D
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