Re: Dependencies between service extensions

[conjaroy <conjaroy@gmail.com>]

Hello Marius,

Thanks a lot for your reply - I agree that the concept of a meta-service to
factor out common startup tasks seems logical.

Based on your example, it sounds like you're suggesting that if I have a
service that extends both activation-service-type and
shepherd-root-service-type, and if the Shepherd extension service defines
its own service dependencies (via shepherd-service-requirement), then the
activation-service-type extension script will run in a context where those
required services are active?

I've spent a few days investigating how the service graph works and here's
what I think is happening:

- Individually, activation-service-type and shepherd-root-service-type
concatenate their respective service extension scripts into a single script.

- These two service types in turn extend boot-service-type, so when
extensions of boot-service-type are processed, the combined
activation-service-type boot script is concatenated with the combined
shepherd-root-service-type boot script (in that order, apparently).

- Therefore, at startup, Guix executes scripts for all
activation-service-type extensions before starting any Shepherd services.

So while the idea of performing initialization as a separate service still
makes sense, I believe that any logic depending on other Shepherd services
needs to run as a shepherd-root-service-type extension instead of an
activation-service-type extension.

If any of the above is incorrect, I'd love to hear more!

Cheers,

Jason

On Tue, Jun 23, 2020 at 5:51 PM Marius Bakke <marius@gnu.org> wrote:

> conjaroy <conjaroy@gmail.com> writes:
>
> > 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?
>
> To encode requirements for an activation script, I think you need to
> declare a service type for it with appropriate requirements, and make
> the start and stop actions "noop".  Then you can have other services
> depend on the "activation service".
>
> I did something similar in a service I'm working on that consists of
> many different daemons.  To avoid having to run essentially the same
> activation script on each, I created a "common" service that all daemons
> depend upon.  It's fairly verbose (you don't need a record type), but
> looks like this:
>
> --8<---------------cut here---------------start------------->8---
> ;; This is a dummy service that all Ganeti daemons depend upon, mainly to
> ;; avoid having the same activation snippet on each.
> (define-record-type* <ganeti-common-configuration>
>   ganeti-common-configuration make-ganeti-common-configuration
>   ganeti-common-configuration?
>   (ganeti ganeti-common-configuration-ganeti            ;<package>
>           (default ganeti))
>   (directories ganeti-common-configuration-directories  ;list of strings
>                (default '("/var/log/ganeti"
>                           "/var/log/ganeti/kvm"
>                           "/var/log/ganeti/os"
>                           "/var/lib/ganeti/rapi"
>                           "/var/lib/ganeti/queue"
>                           "/var/run/ganeti/bdev-cache"
>                           "/var/run/ganeti/socket"
>                           "/var/run/ganeti/instance-disks"
>                           "/var/run/ganeti/instance-reason"
>                           "/var/run/ganeti/livelocks"))))
>
> (define (ganeti-common-activation config)
>   (let ((directories (ganeti-common-configuration-directories config)))
>   #~(begin
>       (use-modules (guix build utils))
>       (for-each mkdir-p '#$directories))))
>
> (define ganeti-common-service
>   (lambda _
>     (list (shepherd-service
>            (documentation "Create the directories required by Ganeti.")
>            (provision '(ganeti-common))
>            (requirement '(file-systems))
>            ;; Do nothing but the activation snippet, at least for now.
>            (start #~(const #t))))))
>
> (define ganeti-common-service-type
>   (service-type (name 'ganeti-common)
>                 (extensions
>                  (list (service-extension activation-service-type
>                                           ganeti-common-activation)
>                        ;; This service also installs Ganeti to the profile
>                        ;; to make gnt-cluster, etc readily available.
>                        (service-extension profile-service-type
>                                           (compose list
>
>  ganeti-common-configuration-ganeti))
>                        (service-extension shepherd-root-service-type
>                                           ganeti-common-service)))
>                 (default-value (ganeti-common-configuration))
>                 (description
>                  "This service creates directories used by other Ganeti
> daemons.")))
> --8<---------------cut here---------------end--------------->8---
>