Proposal: auto-instantiate services with a default value

Proposal: auto-instantiate services with a default value

[Ludovic Courtès]

Hello Guix!

Currently on GuixSD, users have to explicitly specify all the services
they need, recursively.  For instance, if you add:

  (service rottlog-service-type)

to your ‘services’ field, you also need to add:

  (service mcron-service-type)

because the former happens to require the latter.  If you don’t do that,
you get an error message (see ‘fold-services’ in (gnu services)), and
then you go back to your config and add that line.

This was intentional, but it’s annoying, especially when considering
higher-level, complex services—imagine a MediaGoblin service: you
wouldn’t want to manually specify a dozen services that, to you, are an
implementation detail you’d rather ignore.

We discussed some time ago the possibility to automatically instantiate
services.  Now that service types have default value, I think it’s a
viable approach: if a service is required, and *if* it provides a
default value, then we can automatically instantiate it.

So in the example above, you’d just specify ‘rottlog-service-type’ and
an ‘mcron-service-type’ would be automatically added if it’s not already
there.

How does that sound?

Ludo’.

Re: Proposal: auto-instantiate services with a default value

[ng0]

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Ludovic Courtès transcribed 1.3K bytes:
> Hello Guix!
> 
> Currently on GuixSD, users have to explicitly specify all the services
> they need, recursively.  For instance, if you add:
> 
>   (service rottlog-service-type)
> 
> to your ‘services’ field, you also need to add:
> 
>   (service mcron-service-type)
> 
> because the former happens to require the latter.  If you don’t do that,
> you get an error message (see ‘fold-services’ in (gnu services)), and
> then you go back to your config and add that line.
> 
> This was intentional, but it’s annoying, especially when considering
> higher-level, complex services—imagine a MediaGoblin service: you
> wouldn’t want to manually specify a dozen services that, to you, are an
> implementation detail you’d rather ignore.
> 
> We discussed some time ago the possibility to automatically instantiate
> services.  Now that service types have default value, I think it’s a
> viable approach: if a service is required, and *if* it provides a
> default value, then we can automatically instantiate it.
> 
> So in the example above, you’d just specify ‘rottlog-service-type’ and
> an ‘mcron-service-type’ would be automatically added if it’s not already
> there.
> 
> How does that sound?
> 
> Ludo’.

Sounds alright to me.

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Re: Proposal: auto-instantiate services with a default value

[Danny Milosavljevic]

Hi Ludo,

On Tue, 09 Jan 2018 10:05:34 +0100
ludo@gnu.org (Ludovic Courtès) wrote:

> We discussed some time ago the possibility to automatically instantiate
> services.  Now that service types have default value, I think it’s a
> viable approach: if a service is required, and *if* it provides a
> default value, then we can automatically instantiate it.
> 
> So in the example above, you’d just specify ‘rottlog-service-type’ and
> an ‘mcron-service-type’ would be automatically added if it’s not already
> there.

Yes, as long as there's a way for the user to easily see what it instantiated (in the running system).

Also, that means that the instantiated mcron-service would automatically get a rottlog entry even though the mcron-service-types's default job list is empty, right ?

In that case it would be nice to be able to (optionally) override this mcron-service by a custom mcron-service in /etc/config.scm but retain the modified configuration with the implicit job in it.  Is that possible?

Re: Proposal: auto-instantiate services with a default value

[Ludovic Courtès]

Hi,

Danny Milosavljevic <dannym@scratchpost.org> skribis:

> On Tue, 09 Jan 2018 10:05:34 +0100
> ludo@gnu.org (Ludovic Courtès) wrote:
>
>> We discussed some time ago the possibility to automatically instantiate
>> services.  Now that service types have default value, I think it’s a
>> viable approach: if a service is required, and *if* it provides a
>> default value, then we can automatically instantiate it.
>> 
>> So in the example above, you’d just specify ‘rottlog-service-type’ and
>> an ‘mcron-service-type’ would be automatically added if it’s not already
>> there.
>
> Yes, as long as there's a way for the user to easily see what it instantiated (in the running system).

‘operating-system-services’ would still only lists services that have
been explicitly specified, but ‘guix system extension-graph’ would show
everything.

> Also, that means that the instantiated mcron-service would automatically get a rottlog entry even though the mcron-service-types's default job list is empty, right ?

Correct.

> In that case it would be nice to be able to (optionally) override this mcron-service by a custom mcron-service in /etc/config.scm but retain the modified configuration with the implicit job in it.  Is that possible?

Sure, the current way to do things (which is to explicitly list rottlog
*and* mcron) would remain valid, and you could pass your own mcron
config this way.

Ludo’.

Re: Proposal: auto-instantiate services with a default value

[Ricardo Wurmus]

Ludovic Courtès <ludo@gnu.org> writes:

>> Also, that means that the instantiated mcron-service would automatically get a rottlog entry even though the mcron-service-types's default job list is empty, right ?
>
> Correct.
>
>> In that case it would be nice to be able to (optionally) override this mcron-service by a custom mcron-service in /etc/config.scm but retain the modified configuration with the implicit job in it.  Is that possible?
>
> Sure, the current way to do things (which is to explicitly list rottlog
> *and* mcron) would remain valid, and you could pass your own mcron
> config this way.

If a user already had an mcron service with a custom configuration,
it would still be extended as needed by adding the rottlog-service — or
am I mistaken?

I think auto-instantiation is great when it all shows up with “guix
system extension-graph” and users can thus figure out why certain
services were instantiated unexpectedly.

--
Ricardo

GPG: BCA6 89B6 3655 3801 C3C6  2150 197A 5888 235F ACAC
https://elephly.net

Re: Proposal: auto-instantiate services with a default value

[Ludovic Courtès]

Ricardo Wurmus <rekado@elephly.net> skribis:

> Ludovic Courtès <ludo@gnu.org> writes:
>
>>> Also, that means that the instantiated mcron-service would automatically get a rottlog entry even though the mcron-service-types's default job list is empty, right ?
>>
>> Correct.
>>
>>> In that case it would be nice to be able to (optionally) override this mcron-service by a custom mcron-service in /etc/config.scm but retain the modified configuration with the implicit job in it.  Is that possible?
>>
>> Sure, the current way to do things (which is to explicitly list rottlog
>> *and* mcron) would remain valid, and you could pass your own mcron
>> config this way.
>
> If a user already had an mcron service with a custom configuration,
> it would still be extended as needed by adding the rottlog-service — or
> am I mistaken?

Yes.  Basically an existing currently-working configuration will still
work exactly the same.

The novelty is that one would now be able to omit services that are
indirectly needed.

> I think auto-instantiation is great when it all shows up with “guix
> system extension-graph” and users can thus figure out why certain
> services were instantiated unexpectedly.

Yes.  Speaking of which, it’d be great to have an interactive view of
service folding (d3.js?)…

Ludo’.

Re: Proposal: auto-instantiate services with a default value

[Chris Marusich]

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ludo@gnu.org (Ludovic Courtès) writes:

> Hello Guix!
>
> Currently on GuixSD, users have to explicitly specify all the services
> they need, recursively.  For instance, if you add:
>
>   (service rottlog-service-type)
>
> to your ‘services’ field, you also need to add:
>
>   (service mcron-service-type)
>
> because the former happens to require the latter.  If you don’t do that,
> you get an error message (see ‘fold-services’ in (gnu services)), and
> then you go back to your config and add that line.
>
> This was intentional, but it’s annoying, especially when considering
> higher-level, complex services—imagine a MediaGoblin service: you
> wouldn’t want to manually specify a dozen services that, to you, are an
> implementation detail you’d rather ignore.
>
> We discussed some time ago the possibility to automatically instantiate
> services.  Now that service types have default value, I think it’s a
> viable approach: if a service is required, and *if* it provides a
> default value, then we can automatically instantiate it.
>
> So in the example above, you’d just specify ‘rottlog-service-type’ and
> an ‘mcron-service-type’ would be automatically added if it’s not already
> there.
>
> How does that sound?

This sounds great to me!  It would be quite convenient.

What will we do in the situation where two services S and T both require
a service U, but S and T require different configurations of U?  In this
case, it's clear that either S or T (or both) requires a non-default
configuration of U, so maybe this is outside the scope of what you've
proposed.

Is it intended that if I wish to modify a service's dependent services,
I would continue to do so via the usual methods (e.g., modify-services)?

-- 
Chris

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Re: Proposal: auto-instantiate services with a default value

[Ludovic Courtès]

Hello!

ludo@gnu.org (Ludovic Courtès) skribis:

> We discussed some time ago the possibility to automatically instantiate
> services.  Now that service types have default value, I think it’s a
> viable approach: if a service is required, and *if* it provides a
> default value, then we can automatically instantiate it.

I’ve implemented this here:

  https://git.savannah.gnu.org/cgit/guix.git/commit/?id=d466b1fc8221a6224fe7ded53a828f9c29ed9457

Feedback welcome!

Ludo’.