Re: Dependencies between service extensions

[conjaroy <conjaroy@gmail.com>]

Does anyone have feedback on how to implement this type of dependency
correctly in a Guix service?

On Sun, Jun 7, 2020 at 11:43 AM conjaroy <conjaroy@gmail.com> wrote:

> Greetings help-guix,
>
> I've been a casual user of Nix for a couple of years and have decided to
> test the waters with Guix. While I'm looking forward to spending time with
> Lisp after many years away, my biggest impression is that Guix seems to
> have well-documented interfaces in cases where Nix relies more on loose
> conventions.
>
> After reviewing the manual and some of the service definitions, I'd like a
> better understanding of how to implement a common pattern. Let's say that I
> have some application Foo that uses an external system for persistence,
> like a SQL database. Before starting up service Foo I need to ensure both
> that the database service is running and that the database instance for Foo
> has been initialized, because Foo doesn't know how to initialize the
> database on its own.
>
> The first issue (how to ensure that the database service is up) seems to
> be solved by adding a shepherd-root-service-type service extension that
> declares a set of "requirements". And the second issue (performing
> pre-startup initialization) seems to be handled by the
> activation-service-type extension. So far so good.
>
> But I couldn't find documentation on whether service activation scripts
> can safely rely on other services that happen to be declared as
> requirements in the shepherd-root-service-type extension. And while I found
> many activation scripts that do simple things like modifying the
> filesystem, I couldn't see any that interact directly with other services.
> However, I did see some evidence of service extensions relying on the side
> effects of other service extensions: a number of activation scripts call
> "getpwnam" for info on system accounts that could exist only if the
> corresponding account-service-type extension has already been executed.
>
> So my questions are: could someone clarify best practices for initializing
> state in Service A before Service B starts up? And is there anything about
> the ordering/dependencies of a service's extensions that could be better
> documented in the manual?
>
> Thanks for all of your work on this project.
>
> Jason
>