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authorLudovic Courtès <ludo@gnu.org>2015-09-17 23:44:26 +0200
committerLudovic Courtès <ludo@gnu.org>2015-10-10 22:55:15 +0200
commit0adfe95a3eee335847c3127edde3de550e692440 (patch)
tree1c5a059d8f261f09254c0e420e61e1344c9edb45 /doc
parente79467f63a06811ba5dd8c8b0cc79553c5dd4e3a (diff)
downloadguix-0adfe95a3eee335847c3127edde3de550e692440.tar.gz
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services: Introduce extensible services.
This patch rewrites GuixSD services to make them extensible. * gnu-system.am (GNU_SYSTEM_MODULES): Add gnu/services/dbus.scm. * gnu/services.scm (<service>): Replace with new record type. (<service-extension>, <service-type>): New record types. (write-service-type, compute-boot-script, second-argument): New procedures. (%boot-service, boot-service-type): New variables. (file-union, directory-union, modprobe-wrapper, activation-service->script, activation-script, gexps->activation-gexp): New procedures. (activation-service-type, %activation-service): New variables. (etc-directory, files->etc-directory, etc-service): New procedures. (etc-service-type, setuid-program-service, firmware-service-type): New variables. (firmware->activation-gexp): New procedure. (&service-error, &missing-target-service-error, &ambiguous-target-service-error): New condition types. (service-back-edges, fold-services): New procedures. * gnu/services/avahi.scm (<avahi-configuration>): New record type. (configuration-file): Replace keyword parameters with a single 'config' parameter. (%avahi-accounts, %avahi-activation, avahi-service-type): New variables. (avahi-dmd-service): New procedure. (avahi-service): Rewrite using 'service' and 'avahi-configuration'. * gnu/services/base.scm (%root-file-system-dmd-service, root-file-system-service-type): New variables. (root-file-system-service): Use them. (file-system->dmd-service-name): New procedure. (file-system-service-type): New variable. (file-system-service): Use it. Replace keyword parameters with a single 'file-system' object. (user-unmount-service-type): New variable. (user-unmount-service): Use it. (user-processes-service-type): New variable. (user-processes-service): Use it. (host-name-service-type): New variable. (host-name-service): Use it. (console-keymap-service-type): New variable. (console-keymap-service): Use it. (console-font-service-type): New variable. (console-font-service): Use it. (mingetty-pam-service, mingetty-dmd-service): New procedures. (mingetty-service-type): New variable. (mingetty-service): Use it. (nscd-dmd-service): New procedure. (nscd-activation, nscd-service-type): New variables. (nscd-service): Use the latter. (syslog-service-type): New variable. (syslog-service): Use it. (<guix-configuration>): New record type. (%default-guix-configuration): New variable. (guix-dmd-service, guix-accounts, guix-activation): New procedures. (guix-service-type): New variable. (guix-service): Replace list of keyword parameters with a single 'config' parameter. Rewrite using 'service'. (<udev-configuration>): New record type. (udev-dmd-service): New procedure. (udev-service-type): New variable. (udev-service): Use it. (device-mapping-service-type): New variable. (device-mapping-service): Use it. (swap-service-type): New variable. (swap-service): Use it. * gnu/services/databases.scm (<postgresql-configuration>): New record type. (%postgresql-accounts, postgresql-activation): New variables. (postgresql-dmd-service): New procedure. (postgresql-service): Rewrite using 'service' and 'postgresql-configuration'. * gnu/services/dbus.scm: New file. * gnu/services/desktop.scm (dbus-configuration-directory, dbus-service): Remove. (wrapped-dbus-service): New procedure. (<upower-configuration>): New record type. (upower-configuration-file): Replace keyword parameters with single <upower-configuration> parameter. (%upower-accounts, %upower-activation): New variables. (upower-dbus-service, upower-dmd-service): New procedures. (upower-service-type): New variable. (upower-service): Rewrite using 'service' and 'upower-configuration'. (%colord-activation, %colord-accounts): New variables. (colord-dmd-service): New procedure. (colord-service-type): New variable. (colord-service): Rewrite using 'service'. (<geoclue-configuration>): New record type. (geoclue-configuration-file): Replace keyword parameters with a single 'config' parameter. (geoclue-dbus-service, geoclue-dmd-service): New procedures. (%geoclue-accounts, geoclue-service-type): New variables. (geoclue-service): Rewrite using 'service' and 'geoclue-configuration'. (%polkit-accounts, %polkit-pam-services, polkit-service-type): New variables. (polkit-dmd-service): New procedure. (polkit-service): Rewrite using 'service'. (<elogind-configuration>)[elogind]: New field. (elogind-dmd-service): New procedure. (elogind-service-type): New variable. (elogind-service): Rewrite using 'service'. (%desktop-services): Remove argument to 'dbus-service'. Remove 'map' over %BASE-SERVICES. * gnu/services/dmd.scm (dmd-boot-gexp): New procedure. (dmd-root-service-type, %dmd-root-service): New variables. (dmd-service-type): New macro. (<dmd-service>): New record type. * gnu/services/lirc.scm (<lirc-configuration>): New record type. (%lirc-activation): New variable. (lirc-dmd-service): New procedure. (lirc-service-type): New variable. (lirc-service): Rewrite using 'service' and 'lirc-configuration'. * gnu/services/networking.scm (<static-networking>): New record type. (static-networking-service-type): New variable. (static-networking-service): Rewrite using 'service' and 'static-networking'. (dhcp-client-service-type): New variable. (dhcp-client-service): Rewrite using 'service'. (<ntp-configuration>): New record type. (ntp-dmd-service): New procedure. (ntp-service-type): New variable. (ntp-service): New procedure. (%tor-accounts, tor-service-type): New variable. (tor-dmd-service): New procedure. (tor-service): Rewrite using 'service'. (<bitlbee-configuration>): New record type. (bitlbee-dmd-service): New procedure. (%bitlbee-accounts, %bitlbee-activation, bitlbee-service-type): New variables. (bitlbee-service): Rewrite using 'service'. (%wicd-activation): New variable. (wicd-dmd-service): New procedure. (wicd-service-type): New variable. (wicd-service): Rewrite using 'service'. * gnu/services/ssh.scm (<lsh-configuration>): New record type. (activation): Rename to... (lsh-initialization): ... this. (lsh-activation, lsh-dmd-service, lsh-pam-services): New procedures. (lsh-service-type): New variable. (lsh-service): Rewrite using 'service' and 'lsh-configuration'. * gnu/services/web.scm (<nginx-configuration>): New record type. (%nginx-accounts): New variable. (nginx-activation, nginx-dmd-service): New procedures. (nginx-service-type): New variable. (nginx-service): Rewrite using 'service' and 'nginx-configuration'. * gnu/services/xorg.scm (<slim-configuration>): New record type. (slim-pam-service, slim-dmd-service): New procedures. (slim-service-type): New variable. (slim-service): Rewrite using 'service' and 'slim-configuration'. * gnu/system.scm (file-union): Remove. (other-file-system-services): Adjust to new 'file-system-service' signature. (essential-services): Add #:container? parameter. Add %DMD-ROOT-SERVICE, %ACTIVATION-SERVICE, and calls to 'pam-root-service', 'account-service', 'operating-system-etc-service', and a SETUID-PROGRAM-SERVICE instance. (operating-system-services): Pass #:container? to 'essential-services. (etc-directory): Remove. (operating-system-etc-service): New procedure. Rewrite as a call to 'etc-service'. (operating-system-accounts): Change to not return accounts required by services. (operating-system-etc-directory): Rewrite as a call to 'fold-services' and 'etc-directory'. (user-group->gexp, user-account->gexp, modprobe-wrapper): Remove. (operating-system-activation-script): Rewrite as a call to 'fold-services' and 'activation-service->script'. (operating-system-boot-script): Likewise. (operating-system-derivation): Add call to 'lower-object'. (emacs-site-file, emacs-site-directory, shells-file): Change to use 'computed-file' and 'scheme-file' instead of the monadic procedures. * gnu/system/install.scm (cow-store-service-type): New variable. (cow-store-service): Rewrite using 'service'. (/etc/configuration-files): New procedure. (configuration-template-service-type, %configuration-template-service): New variables. (configuration-template-service): Remove. (installation-services): Adjust accordingly. Adjust argument to 'guix-service'. * gnu/system/linux.scm (/etc-entry, pam-root-service): New procedures. (pam-root-service-type): New variable. * gnu/system/shadow.scm (user-group->gexp, user-account->gexp, account-activation, etc-skel, account-service): New procedures. (account-service-type): New variable. * tests/services.scm: New file. * doc/guix.texi (Base Services, Desktop Services): Adjust accordingly. (Defining Services): Rewrite. * doc/images/service-graph.dot: New file. * doc.am (DOT_FILES): Add it. * po/guix/POTFILES.in: Add gnu/services.scm.
Diffstat (limited to 'doc')
-rw-r--r--doc/guix.texi471
-rw-r--r--doc/images/service-graph.dot35
2 files changed, 444 insertions, 62 deletions
diff --git a/doc/guix.texi b/doc/guix.texi
index 1b25bd938f..a3fa989d48 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -182,6 +182,13 @@ Services
* Web Services:: Web servers.
* Various Services:: Other services.
+Defining Services
+
+* Service Composition:: The model for composing services.
+* Service Types and Services:: Types and services.
+* Service Reference:: API reference.
+* dmd Services:: A particular type of service.
+
Packaging Guidelines
* Software Freedom:: What may go into the distribution.
@@ -5899,23 +5906,41 @@ Return a service that runs @code{syslogd}. If configuration file name
settings.
@end deffn
-@deffn {Scheme Procedure} guix-service [#:guix guix] @
- [#:builder-group "guixbuild"] [#:build-accounts 10] @
- [#:authorize-hydra-key? #t] [#:use-substitutes? #t] @
- [#:extra-options '()]
-Return a service that runs the build daemon from @var{guix}, and has
-@var{build-accounts} user accounts available under @var{builder-group}.
+@anchor{guix-configuration-type}
+@deftp {Data Type} guix-configuration
+This data type represents the configuration of the Guix build daemon.
+@xref{Invoking guix-daemon}, for more information.
+
+@table @asis
+@item @code{guix} (default: @var{guix})
+The Guix package to use.
-When @var{authorize-hydra-key?} is true, the @code{hydra.gnu.org} public key
-provided by @var{guix} is authorized upon activation, meaning that substitutes
-from @code{hydra.gnu.org} are used by default.
+@item @code{build-group} (default: @code{"guixbuild"})
+Name of the group for build user accounts.
-If @var{use-substitutes?} is false, the daemon is run with
-@option{--no-substitutes} (@pxref{Invoking guix-daemon,
-@option{--no-substitutes}}).
+@item @code{build-accounts} (default: @code{10})
+Number of build user accounts to create.
-Finally, @var{extra-options} is a list of additional command-line options
-passed to @command{guix-daemon}.
+@item @code{authorize-key?} (default: @code{#t})
+Whether to authorize the substitute key for @code{hydra.gnu.org}
+(@pxref{Substitutes}).
+
+@item @code{use-substitutes?} (default: @code{#t})
+Whether to use substitutes.
+
+@item @code{extra-options} (default: @code{'()})
+List of extra command-line options for @command{guix-daemon}.
+
+@item @code{lsof} (default: @var{lsof})
+@itemx @code{lsh} (default: @var{lsh})
+The lsof and lsh packages to use.
+
+@end table
+@end deftp
+
+@deffn {Scheme Procedure} guix-service @var{config}
+Return a service that runs the Guix build daemon according to
+@var{config}.
@end deffn
@deffn {Scheme Procedure} udev-service [#:udev udev]
@@ -6179,11 +6204,10 @@ The @var{%desktop-services} variable can be used as the @code{services}
field of an @code{operating-system} declaration (@pxref{operating-system
Reference, @code{services}}).
-The actual service definitions provided by @code{(gnu services desktop)}
-are described below.
+The actual service definitions provided by @code{(gnu services dbus)}
+and @code{(gnu services desktop)} are described below.
-@deffn {Scheme Procedure} dbus-service @var{services} @
- [#:dbus @var{dbus}]
+@deffn {Scheme Procedure} dbus-service [#:dbus @var{dbus}] [#:services '()]
Return a service that runs the ``system bus'', using @var{dbus}, with
support for @var{services}.
@@ -6197,8 +6221,7 @@ and policy files. For example, to allow avahi-daemon to use the system bus,
@var{services} must be equal to @code{(list avahi)}.
@end deffn
-@deffn {Scheme Procedure} elogind-service @
- [#:elogind @var{elogind}] [#:config @var{config}]
+@deffn {Scheme Procedure} elogind-service [#:config @var{config}]
Return a service that runs the @code{elogind} login and
seat management daemon. @uref{https://github.com/andywingo/elogind,
Elogind} exposes a D-Bus interface that can be used to know which users
@@ -6957,54 +6980,378 @@ build users.
@node Defining Services
@subsection Defining Services
-The @code{(gnu services @dots{})} modules define several procedures that allow
-users to declare the operating system's services (@pxref{Using the
-Configuration System}). These procedures are @emph{monadic
-procedures}---i.e., procedures that return a monadic value in the store
-monad (@pxref{The Store Monad}). For examples of such procedures,
-@xref{Services}.
-
-@cindex service definition
-The monadic value returned by those procedures is a @dfn{service
-definition}---a structure as returned by the @code{service} form.
-Service definitions specifies the inputs the service depends on, and an
-expression to start and stop the service. Behind the scenes, service
-definitions are ``translated'' into the form suitable for the
-configuration file of dmd, the init system (@pxref{Services,,, dmd, GNU
-dmd Manual}).
-
-As an example, here is what the @code{nscd-service} procedure looks
-like:
+The previous sections how the available services and how one can combine
+them in an @code{operating-system} declaration. But how do we define
+them in the first place? And what is a service anyway?
-@lisp
-(define (nscd-service)
- (with-monad %store-monad
- (return (service
- (documentation "Run libc's name service cache daemon.")
- (provision '(nscd))
- (activate #~(begin
- (use-modules (guix build utils))
- (mkdir-p "/var/run/nscd")))
- (start #~(make-forkexec-constructor
- (string-append #$glibc "/sbin/nscd")
- "-f" "/dev/null" "--foreground"))
- (stop #~(make-kill-destructor))
- (respawn? #f)))))
-@end lisp
+@menu
+* Service Composition:: The model for composing services.
+* Service Types and Services:: Types and services.
+* Service Reference:: API reference.
+* dmd Services:: A particular type of service.
+@end menu
+
+@node Service Composition
+@subsubsection Service Composition
+
+@cindex services
+@cindex daemons
+Here we define a @dfn{service} as, broadly, something that extends the
+operating system's functionality. Often a service is a process---a
+@dfn{daemon}---started when the system boots: a secure shell server, a
+Web server, the Guix build daemon, etc. Sometimes a service is a daemon
+whose execution can be triggered by another daemon---e.g., an FTP server
+started by @command{inetd} or a D-Bus service activated by
+@command{dbus-daemon}. Occasionally, a service does not map to a
+daemon. For instance, the ``account'' service collects user accounts
+and makes sure they exist when the system runs; the ``udev'' service
+collects device management rules and makes them available to the eudev
+daemon; the @file{/etc} service populates the system's @file{/etc}
+directory.
+
+GuixSD services are connected by @dfn{extensions}. For instance, the
+secure shell service @emph{extends} dmd---GuixSD's initialization system,
+running as PID@tie{}1---by giving it the command lines to start and stop
+the secure shell daemon (@pxref{Networking Services,
+@code{lsh-service}}); the UPower service extends the D-Bus service by
+passing it its @file{.service} specification, and extends the udev
+service by passing it device management rules (@pxref{Desktop Services,
+@code{upower-service}}); the Guix daemon service extends dmd by passing
+it the command lines to start and stop the daemon, and extends the
+account service by passing it a list of required build user accounts
+(@pxref{Base Services}).
+
+All in all, services and their ``extends'' relations form a directed
+acyclic graph (DAG). If we represent services as boxes and extensions
+as arrows, a typical system might provide something like this:
+
+@image{images/service-graph,,5in,Typical service extension graph.}
+
+At the bottom, we see the @dfn{boot service}, which produces the boot
+script that is executed at boot time from the initial RAM disk.
+
+@cindex service types
+Technically, developers can define @dfn{service types} to express these
+relations. There can be any number of services of a given type on the
+system---for instance, a system running two instances of the GNU secure
+shell server (lsh) has two instances of @var{lsh-service-type}, with
+different parameters.
+
+The following section describes the programming interface for service
+types and services.
+
+@node Service Types and Services
+@subsubsection Service Types and Services
+
+A @dfn{service type} is a node in the DAG described above. Let us start
+with a simple example, the service type for the Guix build daemon
+(@pxref{Invoking guix-daemon}):
+
+@example
+(define guix-service-type
+ (service-type
+ (name 'guix)
+ (extensions
+ (list (service-extension dmd-root-service-type guix-dmd-service)
+ (service-extension account-service-type guix-accounts)
+ (service-extension activation-service-type guix-activation)))))
+@end example
@noindent
-The @code{activate}, @code{start}, and @code{stop} fields are G-expressions
-(@pxref{G-Expressions}). The @code{activate} field contains a script to
-run at ``activation'' time; it makes sure that the @file{/var/run/nscd}
-directory exists before @command{nscd} is started.
+It defines a two things:
+
+@enumerate
+@item
+A name, whose sole purpose is to make inspection and debugging easier.
+
+@item
+A list of @dfn{service extensions}, where each extension designates the
+target service type and a procedure that, given the service's
+parameters, returns a list of object to extend the service of that type.
+
+Every service type has at least one service extension. The only
+exception is the @dfn{boot service type}, which is the ultimate service.
+@end enumerate
+
+In this example, @var{guix-service-type} extends three services:
+
+@table @var
+@item dmd-root-service-type
+The @var{guix-dmd-service} procedure defines how the dmd service is
+extended. Namely, it returns a @code{<dmd-service>} object that defines
+how @command{guix-daemon} is started and stopped (@pxref{dmd Services}).
+
+@item account-service-type
+This extension for this service is computed by @var{guix-accounts},
+which returns a list of @code{user-group} and @code{user-account}
+objects representing the build user accounts (@pxref{Invoking
+guix-daemon}).
+
+@item activation-service-type
+Here @var{guix-activation} is a procedure that returns a gexp, which is
+a code snippet to run at ``activation time''---e.g., when the service is
+booted.
+@end table
+
+A service of this type is instantiated like this:
+
+@example
+(service guix-service-type
+ (guix-configuration
+ (build-accounts 5)
+ (use-substitutes? #f)))
+@end example
+
+The second argument to the @code{service} form is a value representing
+the parameters of this specific service instance.
+@xref{guix-configuration-type, @code{guix-configuration}}, for
+information about the @code{guix-configuration} data type.
+
+@var{guix-service-type} is quite simple because it extends other
+services but is not extensible itself.
+
+@c @subsubsubsection Extensible Service Types
+
+The service type for an @emph{extensible} service looks like this:
+
+@example
+(define udev-service-type
+ (service-type (name 'udev)
+ (extensions
+ (list (service-extension dmd-root-service-type
+ udev-dmd-service)))
+
+ (compose concatenate) ;concatenate the list of rules
+ (extend (lambda (config rules)
+ (match config
+ (($ <udev-configuration> udev initial-rules)
+ (udev-configuration
+ (udev udev) ;the udev package to use
+ (rules (append initial-rules rules)))))))))
+@end example
+
+This is the service type for the
+@uref{https://wiki.gentoo.org/wiki/Project:Eudev, eudev device
+management daemon}. Compared to the previous example, in addition to an
+extension of @var{dmd-root-service-type}, we see two new fields:
+
+@table @code
+@item compose
+This is the procedure to @dfn{compose} the list of extensions to
+services of this type.
+
+Services can extend the udev service by passing it lists of rules; we
+compose those extensions simply by concatenating them.
+
+@item extend
+This procedure defines how the service's value is @dfn{extended} with
+the composition of the extensions.
+
+Udev extensions are composed into a list of rules, but the udev service
+value is itself a @code{<udev-configuration>} record. So here, we
+extend that record by appending the list of rules is contains to the
+list of contributed rules.
+@end table
+
+There can be only one instance of an extensible service type such as
+@var{udev-service-type}. If there were more, the
+@code{service-extension} specifications would be ambiguous.
+
+Still here? The next section provides a reference of the programming
+interface for services.
+
+@node Service Reference
+@subsubsection Service Reference
+
+We have seen an overview of service types (@pxref{Service Types and
+Services}). This section provides a reference on how to manipulate
+services and service types. This interface is provided by the
+@code{(gnu services)} module.
+
+@deffn {Scheme Procedure} service @var{type} @var{value}
+Return a new service of @var{type}, a @code{<service-type>} object (see
+below.) @var{value} can be any object; it represents the parameters of
+this particular service instance.
+@end deffn
+
+@deffn {Scheme Procedure} service? @var{obj}
+Return true if @var{obj} is a service.
+@end deffn
+
+@deffn {Scheme Procedure} service-kind @var{service}
+Return the type of @var{service}---i.e., a @code{<service-type>} object.
+@end deffn
+
+@deffn {Scheme Procedure} service-parameters @var{service}
+Return the value associated with @var{service}. It represents its
+parameters.
+@end deffn
+
+Here is an example of how a service is created and manipulated:
+
+@example
+(define s
+ (service nginx-service-type
+ (nginx-configuration
+ (nginx nginx)
+ (log-directory log-directory)
+ (run-directory run-directory)
+ (file config-file))))
+
+(service? s)
+@result{} #t
+
+(eq? (service-kind s) nginx-service-type)
+@result{} #t
+@end example
+
+@deftp {Data Type} service-type
+@cindex service type
+This is the representation of a @dfn{service type} (@pxref{Service Types
+and Services}).
+
+@table @asis
+@item @code{name}
+This is a symbol, used only to simplify inspection and debugging.
+
+@item @code{extensions}
+A non-empty list of @code{<service-extension>} objects (see below.)
+
+@item @code{compose} (default: @code{#f})
+If this is @code{#f}, then the service type denotes services that cannot
+be extended---i.e., services that do not receive ``values'' from other
+services.
+
+Otherwise, it must be a one-argument procedure. The procedure is called
+by @code{fold-services} and is passed a list of values collected from
+extensions. It must return a value that is a valid parameter value for
+the service instance.
+
+@item @code{extend} (default: @code{#f})
+If this is @code{#f}, services of this type cannot be extended.
+
+Otherwise, it must be a two-argument procedure: @code{fold-services}
+calls it, passing it the service's initial value as the first argument
+and the result of applying @code{compose} to the extension values as the
+second argument.
+@end table
+
+@xref{Service Types and Services}, for examples.
+@end deftp
+
+@deffn {Scheme Procedure} service-extension @var{target-type} @
+ @var{compute}
+Return a new extension for services of type @var{target-type}.
+@var{compute} must be a one-argument procedure: @code{fold-services}
+calls it, passing it the value associated with the service that provides
+the extension; it must return a valid value for the target service.
+@end deffn
+
+@deffn {Scheme Procedure} service-extension? @var{obj}
+Return true if @var{obj} is a service extension.
+@end deffn
+
+At the core of the service abstraction lies the @code{fold-services}
+procedure, which is responsible for ``compiling'' a list of services
+down to a single boot script. In essence, it propagates service
+extensions down the service graph, updating each node parameters on the
+way, until it reaches the root node.
+
+@deffn {Scheme Procedure} fold-services @var{services} @
+ [#:target-type @var{boot-service-type}]
+Fold @var{services} by propagating their extensions down to the root of
+type @var{target-type}; return the root service adjusted accordingly.
+@end deffn
+
+Lastly, the @code{(gnu services)} module also defines several essential
+service types, some of which are listed below.
+
+@defvr {Scheme Variable} boot-service-type
+The type of the ``boot service'', which is the root of the service
+graph.
+@end defvr
+
+@defvr {Scheme Variable} etc-service-type
+The type of the @file{/etc} service. This service can be extended by
+passing it name/file tuples such as:
+
+@example
+(list `("issue" ,(plain-file "issue" "Welcome!\n")))
+@end example
+
+In this example, the effect would be to add an @file{/etc/issue} file
+pointing to the given file.
+@end defvr
+
+@defvr {Scheme Variable} setuid-program-service-type
+Type for the ``setuid-program service''. This service collects lists of
+executable file names, passed as gexps, and adds them to the set of
+setuid-root programs on the system (@pxref{Setuid Programs}).
+@end defvr
+
+
+@node dmd Services
+@subsubsection dmd Services
+
+@cindex PID 1
+@cindex init system
+The @code{(gnu services dmd)} provides a way to define services managed
+by GNU@tie{}dmd, which is GuixSD initialization system---the first
+process that is started when the system boots, aka. PID@tie{}1
+(@pxref{Introduction,,, dmd, GNU dmd Manual}). The
+@var{%dmd-root-service} represents PID@tie{}1, of type
+@var{dmd-root-service-type}; it can be extended by passing it lists of
+@code{<dmd-service>} objects.
+
+@deftp {Data Type} dmd-service
+The data type representing a service managed by dmd.
+
+@table @asis
+@item @code{provision}
+This is a list of symbols denoting what the service provides.
+These are the names that may be passed to @command{deco start},
+@command{deco status}, and similar commands (@pxref{Invoking deco,,,
+dmd, GNU dmd Manual}). @xref{Slots of services, the @code{provides}
+slot,, dmd, GNU dmd Manual}, for details.
+
+@item @code{requirements} (default: @code{'()})
+List of symbols denoting the dmd services this one depends on.
+
+@item @code{respawn?} (default: @code{#t})
+Whether to restart the service when it stops, for instance when the
+underlying process dies.
+
+@item @code{start}
+@itemx @code{stop} (default: @code{#~(const #f)})
The @code{start} and @code{stop} fields refer to dmd's facilities to
start and stop processes (@pxref{Service De- and Constructors,,, dmd,
-GNU dmd Manual}). The @code{provision} field specifies the name under
-which this service is known to dmd, and @code{documentation} specifies
-on-line documentation. Thus, the commands @command{deco start ncsd},
-@command{deco stop nscd}, and @command{deco doc nscd} will do what you
-would expect (@pxref{Invoking deco,,, dmd, GNU dmd Manual}).
+GNU dmd Manual}). They are given as G-expressions that get expanded in
+the dmd configuration file (@pxref{G-Expressions}).
+
+@item @code{documentation}
+A documentation string, as shown when running:
+
+@example
+deco doc @var{service-name}
+@end example
+
+where @var{service-name} is one of the symbols in @var{provision}
+(@pxref{Invoking deco,,, dmd, GNU dmd Manual}).
+@end table
+@end deftp
+
+@defvr {Scheme Variable} dmd-root-service-type
+The service type for the dmd ``root service''---i.e., PID@tie{}1.
+
+This is the service type that extensions target when they want to create
+dmd services (@pxref{Service Types and Services}, for an example). Each
+extension must pass a list of @code{<dmd-service>}.
+@end defvr
+
+@defvr {Scheme Variable} %dmd-root-service
+This service represents PID@tie{}1.
+@end defvr
@node Installing Debugging Files
diff --git a/doc/images/service-graph.dot b/doc/images/service-graph.dot
new file mode 100644
index 0000000000..3397b878e9
--- /dev/null
+++ b/doc/images/service-graph.dot
@@ -0,0 +1,35 @@
+digraph "Service Type Dependencies" {
+ dmd [shape = box, fontname = Helvetica];
+ pam [shape = box, fontname = Helvetica];
+ etc [shape = box, fontname = Helvetica];
+ accounts [shape = box, fontname = Helvetica];
+ activation [shape = box, fontname = Helvetica];
+ boot [shape = house, fontname = Helvetica];
+ lshd -> dmd;
+ lshd -> pam;
+ udev -> dmd;
+ nscd -> dmd [label = "extends"];
+ "nss-mdns" -> nscd;
+ "kvm-rules" -> udev;
+ colord -> udev;
+ dbus -> dmd;
+ colord -> dbus;
+ upower -> udev;
+ upower -> dbus;
+ polkit -> dbus;
+ polkit -> pam;
+ elogind -> dbus;
+ elogind -> udev;
+ elogind -> polkit [label = "extends"];
+ dmd -> boot;
+ colord -> accounts;
+ accounts -> activation;
+ accounts -> etc;
+ etc -> activation;
+ activation -> boot;
+ pam -> etc;
+ elogind -> pam;
+ guix -> dmd;
+ guix -> activation;
+ guix -> accounts;
+}