[systemd-devel] Is restart gracefull?

[Cecil Westerhof]

On 11/29/2013 08:59 AM, David Timothy Strauss wrote:
> On Fri, Nov 29, 2013 at 12:53 PM, Lennart Poettering
> <lennart at poettering.net> wrote:
>> On Fri, 29.11.13 00:11, Cecil Westerhof (Cecil.Westerhof at Snow.nl) wrote:
>>> I have done a trial presentation about systemd. One of the questions
>>> there was: when a restart of for example apache is doen, is this
>>> restart done graceful?
>> For systemd a "restart" is actually exactly what the name says: a stop
>> plus a start. "reload" otoh is up to the implementer of the unit file,
>> whatever he chooses.
>
> Just to clarify, "stop" (and, by extension "restart") is also up to
> the implementer of the unit file. There just happens to be a default,
> unlike with "reload." See the service and exec man pages for details.
> Regardless, "restart" and "stop" usually both involve a full shutdown
> of every process running the daemon. I think it's possible to override
> this behavior with certain options, but you shouldn't.

There is of-course an important difference. As I understood it, Apache 
is notorious for processes that escape. (That is one of the reasons that 
cgroups usage of systemd is such an improvement.) In the old situation 
nothing happened to those processes. (With potential nasty results.) 
What happens to those processes under systemd?


> If you want to explore much deeper, you may also be interested in
> socket activation, which allows a server socket to remain open even
> while a daemon is fully shut down (whether because it hasn't started
> yet, is restarting, or has been stopped). It takes graceful
> restarts/reloads up to the systemd level.

I certainly would. I thinks it is a very important part of systemd. And 
it is also clearly mentioned in my presentation. But I have a ‘few’ 
restrictions:
- I have to give the presentation on December 5
- I am quite new to systemd and only scanned the surface I am afraid
- There are a lot of other things I need to know more about
- I am not an administrator, but a developer, so most is Terra Incognita 
to me

How much time would I need to understand these levels?