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 <[email protected]> wrote: > conjaroy <[email protected]> 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--- >
