bug#53580: shepherd's architecture

[Csepp]

Ludovic Courtès <ludo@gnu.org> writes:

> Hi Attila,
>
> Attila Lendvai <attila@lendvai.name> skribis:
>
>> [forked from: bug#53580: /var/run/shepherd/socket is missing on an otherwise 
>> functional system]
>>
>>> So I think we’re mostly okay now. The one thing we could do is load
>>> the whole config file in a separate fiber, and maybe it’s fine to keep
>>> going even when there’s an error during config file evaluation?
>>>
>>> WDYT?
>>
>>
>> i think there's a fundamental issue to be resolved here, and
>> addressing that would implicitly resolve the entire class of issues
>> that this one belongs to.
>>
>> guile (shepherd) is run as the init process, and because of that it
>> may not exit or be respawn. but at the same time when we reconfigure
>> a guix system, then shepherd's config should not only be reloaded,
>> but its internal state merged with the new config, and potentially
>> even with an evolved shepherd codebase.
>
> Sorry to be direct: is there a concrete bug you’re reporting here?
>
>> i still lack a proper mental model of all this to succesfully
>> predict what will happen when i `guix system reconfigure` after i
>> `guix pull`-ed my service code, and/or changed the config of my
>> services.
>
> What happens is that ‘guix system reconfigure’ loads new services into
> the running shepherd.  New services simply get started; services for
> which a same-named service is already running instead get registered as
> a “replacement”, meaning that the new version of the service only gets
> started when the user explicitly runs ‘herd restart SERVICE’.
>
> Non-stop upgrades is ideal, but shepherd alone cannot do that.  For
> instance, nginx supports that, and no init system could implement that
> on its behalf.
>
> Ludo’.

Do services get a reference to their previously running version?
The Minix project was experimenting with supporting something like
supervisor trees for high uptime, and one way they were trying to
achieve that was by giving services the memory of their previous
version, so they could read their state and migrate it to their own
memory.