From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mp2 ([2001:41d0:2:4a6f::]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)) by ms11 with LMTPS id +POOAnN58l7RTwAA0tVLHw (envelope-from ) for ; Tue, 23 Jun 2020 21:51:47 +0000 Received: from aspmx1.migadu.com ([2001:41d0:2:4a6f::]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)) by mp2 with LMTPS id sGH/OXJ58l4efAAAB5/wlQ (envelope-from ) for ; Tue, 23 Jun 2020 21:51:46 +0000 Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by aspmx1.migadu.com (Postfix) with ESMTPS id 788C094038E for ; Tue, 23 Jun 2020 21:51:46 +0000 (UTC) Received: from localhost ([::1]:46458 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jnqpV-00078c-C8 for larch@yhetil.org; Tue, 23 Jun 2020 17:51:45 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:60722) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jnqpO-00078K-4E for help-guix@gnu.org; Tue, 23 Jun 2020 17:51:38 -0400 Received: from fencepost.gnu.org ([2001:470:142:3::e]:40800) by eggs.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jnqpN-0004ye-Rg; Tue, 23 Jun 2020 17:51:37 -0400 Received: from ti0006q161-3115.bb.online.no ([88.95.106.80]:43832 helo=localhost) by fencepost.gnu.org with esmtpsa (TLS1.2:RSA_AES_256_CBC_SHA1:256) (Exim 4.82) (envelope-from ) id 1jnqpN-0000SM-8y; Tue, 23 Jun 2020 17:51:37 -0400 From: Marius Bakke To: conjaroy , help-guix@gnu.org Subject: Re: Dependencies between service extensions In-Reply-To: References: Date: Tue, 23 Jun 2020 23:51:34 +0200 Message-ID: <87k0zx8n89.fsf@gnu.org> MIME-Version: 1.0 Content-Type: multipart/signed; boundary="=-=-="; micalg=pgp-sha512; protocol="application/pgp-signature" X-BeenThere: help-guix@gnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: help-guix-bounces+larch=yhetil.org@gnu.org Sender: "Help-Guix" X-Scanner: scn0 Authentication-Results: aspmx1.migadu.com; dkim=none; dmarc=none; spf=pass (aspmx1.migadu.com: domain of help-guix-bounces@gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=help-guix-bounces@gnu.org X-Spam-Score: -3.11 X-TUID: TMloM2rZvVOD --=-=-= Content-Type: text/plain 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 make-ganeti-common-configuration ganeti-common-configuration? (ganeti ganeti-common-configuration-ganeti ; (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--- --=-=-= Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCgAdFiEEu7At3yzq9qgNHeZDoqBt8qM6VPoFAl7yeWYACgkQoqBt8qM6 VPoWhgf/YpZRXJkunXZVZ2MHZEPPxRG8dGyC2FkaA4PTzGH4rdqouc9g0XF/a9Yi UGXzIfbVPma3EitlZcPnlQByD6sdJSht0THIQUhYusdKj2nGobFOX38mqc3/r13o q/XLrOr/G9Cb9KBiZ1/csFOjn7Kk95vpNuw8mhnWXpDVG4nzxJa/N8cczIzTS5jV hLIoBHHPsvO1o2Ne1y8k1rbMOaCPDGsfErMPfjY2SPq8hrnAfKRlmApz6OIlZlZt nJfwvN4/rllYy1ZfCPpmwXp2eJxILKYw1vc2W3dCEGHZldD3mlR/aGBCP+zsf734 Zu3WY4xfVeC1TsLMInZE03MH1uqfRQ== =XzV9 -----END PGP SIGNATURE----- --=-=-=--