[systemd-devel] Socket based activation/deactivation

[Umut Tezduyar Lindskog]

On Wed, Aug 27, 2014 at 3:24 AM, Zbigniew Jędrzejewski-Szmek
<zbyszek at in.waw.pl> wrote:
> On Tue, Aug 26, 2014 at 08:49:29PM -0400, Gary Mort wrote:
>> Socket based activation for systemd is a really cool feature for my
>> dev system.
>>
>> Prior to using socket based activation, on my dev system I would
>> have 2 choices:
>>
>> 1) Automatically start mysql, apache, php fastcgi, memcached,
>> mongodb and a dozen other services just in case I happen to be
>> working with them on this particular bootup.
>>
>> 2) Manually start them when needed....usually /after/ trying to test
>> something and discovering the service isn't running.
>>
>> With socket based activation, I can start up dozens of servers on
>> different sockets and they won't bog my system down - they only
>> truely activate if they are actually used.
>>
>> However, there is still a missing item to this feature[or it's not
>> missing and I am simply unaware of it] - there is no way to do the
>> reverse.  IE if apache is running on my server there is no way to
>> configure things so if there are no active socket connections for 30
>> minutes, have it automatically shutdown and pass the socket control
>> back.
> This has to be implemented in the service itself. After the service
> has been started systemd does not monitor the socket and simply
> does not know when the service can be safely stopped. In addition
> to the socket passed by systemd the service might have opened
> additional channels of communication itself, or might be doing
> some processing that systemd knows nothing about..., so only the
> service knows when it has idle and how long it has been idle.
> The service can close the socket and shut down. This is race free,
> because if a connection comes on the socket in the meantime, it will
> be queued by the kernel, and then when the service has shut down,
> systemd will start it again.
>
> Various systemd services like systemd-hostnamed do this kind of thing
> already.

I really think this can be achieved with notify socket and the
advantage would be having a system wide stop on idle value.

Note: I always mixup StopWhenUnneeded=. The name makes me believe that....

Umut

>
>> A lot of server admins don't
>> like the idea of socket activation because it means that when they
>> boot a server, they won't know if there is some blocker for a server
>> to run[corrupted database files for example] until it is actually
>> used.
> Of course they can configure the services to start uncondtionally
> by adding them to an appropriate target, so this is a really poor
> argument.
>
>> ...
>> So I'm curious if this feature is somewhere in socket activation I'm
>> not aware of[it might simply be needing to write the proper systemd
>> configuration files] or if it is something that would need to be
>> added?
> It needs to be added in the daemon.
>
> Zbyszek
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