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Re: Dependencies between service extensions

From: Marius Bakke
Subject: Re: Dependencies between service extensions
Date: Tue, 23 Jun 2020 23:51:34 +0200

conjaroy <> 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 ganeti-common-configuration-ganeti            ;<package>
          (default ganeti))
  (directories ganeti-common-configuration-directories  ;list of strings
               (default '("/var/log/ganeti"

(define (ganeti-common-activation config)
  (let ((directories (ganeti-common-configuration-directories config)))
      (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)
                 (list (service-extension activation-service-type
                       ;; This service also installs Ganeti to the profile
                       ;; to make gnt-cluster, etc readily available.
                       (service-extension profile-service-type
                                          (compose list
                       (service-extension shepherd-root-service-type
                (default-value (ganeti-common-configuration))
                 "This service creates directories used by other Ganeti
--8<---------------cut here---------------end--------------->8---

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