[bug#52698] [RFC PATCH] doc: Add Writing Service Configuration section.

[Xinglu Chen]

Am Dienstag, der 21. Dezember 2021, um 13:21 +032, schrieb Andrew Tropin 
<andrew@trop.in>:

> * guix.texi (Writing Service Configuration): New section.
> ---
> After reading the source code of different system services and implementing a
> few of home services I decided to write down most important tips for
> implementing guix service configurations.  I belive having such a guideline
> can simplify the development of new services and configurations for them, as
> well as keeping those implementations consistent, which will simplify the life
> for users too because they won't need to learn a different configuration
> approaches for different services.
>
> This section is not a final document, but a starting point for discussion and
> further extension of the guideline.  Feel free to raise a question, point to a
> mistake, make a suggestion or propose an idea.

Thanks for working on this!  I left some comments and thoughts as I read
through it (Warning, these is quite a lot :-)).

>  doc/guix.texi | 209 +++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 205 insertions(+), 4 deletions(-)
>
> diff --git a/doc/guix.texi b/doc/guix.texi
> index 333cb4117a..a48fb0e2b7 100644
> --- a/doc/guix.texi
> +++ b/doc/guix.texi
> @@ -35652,10 +35652,11 @@ them in an @code{operating-system} declaration.  
> But how do we define
>  them in the first place?  And what is a service anyway?
>  
>  @menu
> -* Service Composition::         The model for composing services.
> -* Service Types and Services::  Types and services.
> -* Service Reference::           API reference.
> -* Shepherd Services::           A particular type of service.
> +* Service Composition::            The model for composing services.
> +* Service Types and Services::     Types and services.
> +* Service Reference::              API reference.
> +* Shepherd Services::              A particular type of service.
> +* Writing Service Configurations:: A guideline for writing guix services.
>  @end menu
>  
>  @node Service Composition
> @@ -35851,6 +35852,206 @@ There can be only one instance of an extensible 
> service type such as
>  Still here?  The next section provides a reference of the programming
>  interface for services.
>  
> +@node Writing Service Configurations
> +@subsection Writing Service Configurations

The TOC menu says that “Writing Services Configurations” comes after
“Shepherd Services”, but this doesn’t seem to be the case here.

> +There are a lot of system and home services already written, but from
> +time to time it's necessary to write one more.

I would write something like

  Guix already contains a wide variety of system and home services, but
  sometimes users might want to add new services.

> +This section contains
> +tips for simplifying this process, and should help to make service
> +configurations and their implementations more consistent.
> +
> +@quotation Note
> +If you find any exceptions or patterns missing in this section, please
> +send a patch with additions/changes to @email{guix-devel@@gnu.org}
> +mailing list or just start a discussion/ask a question.
> +@end quotation

I don’t think this note is really necessary; there is already a section
on contributing to the project, see “17 Contributing”.

> +@subsubheading Configuration Itself
> +
> +As we know from previous section a guix service can accept a value and
                                   ^ missing comma
s/section/sections/
s/guix/Guix

When you say “service”, you mean a “service type”, right?  Just “value”
sounds a bit vague, maybe

  … a value, usually some kind of configuration
  record (@pxref{RELEVANT NODE(s)})

?

> +be extended with additional values by other services.

Not all services are extendable though, to avoid ambiguity, maybe

  …, and optionally, be extended with additional configurations by other
  services (@pxref{Service Composition}).

> +There are some
> +cases, when the service accepts a list of pairs or some other values for

I suggest:

  When being extended, most services take some kind of configuration
  record or a list thereof, but in some cases a simpler value is all
  that is necessary.

> +example @code{console-font-service-type} accepts list of pairs (tty and
> +font name/file) or @code{etc-service-type} accepts list of lists
> +(resulting file name and file-like object)

It is probably better to link to the service documentation instead of
trying to explain the specification in a few words in brackets.  You can
use Texinfo “anchors” to achieve this, see “5.8 '@anchor': Defining
Arbitrary Cross-reference Targets”.

  For example, @code{console-font-service-type}
  (@pxref{console-font-service-type}) accepts an association list, and
  @code{etc-service-type} (@pxref{etc-service-type}) accepts a list of
  lists.

Also, is should there be any preference for using alists or list of
lists or vice versa?

> +those services are kinda special, they are an intermediate helpers
> +doing auxiliary work.

It is not clear what the last clause means, how do they differ from
other, more “regular” services?

> +However, in most cases a guix service is wrapping some software, which
> +consist of package or a few packages, and configuration file or files.

“…consists of one or more packages and configuration files.”

> +Therefore, the value for such service is quite complicated and it's hard
> +to represent it with a just list or basic data type, in such cases we
> +use a record.  Each such record have -configuration suffix, for example
               ^^ Link to the “Records” page in the Guile manual

@code{-configuration} or maybe @samp{-configuration}

> +@code{docker-configuration} for @code{docker-service-type} and a few
> +different fields helping to customize the software.

I suggest:

  …, for example, the @code{docker-service-type} should accept a record
  type named @code{docker-configuration}, which contains a fields used
  to configure Docker.

> +Configuration
> +records for home services also have a @code{home-} prefix in their name.
                            ^ missing “should”

> +There is a module @code{gnu service configuration}, which contains
> +helpers simplifying configuration definition process.  Take a look at
> +@code{gnu services docker} module or grep for
> +@code{define-configuration} to find usage examples.
> +
> +@c Provide some examples, tips, and rationale behind @code{gnu service
> +@c configuration} module.

Note that I already sent a patch that (at least tries to) document (gnu
service configuration)[1].

One thing that is lacking is when to use (guix records) (which isn’t
documented yet) vs (gnu service configuration).  There should probably
be one or two paragraphs about that.

> +After a configuration record properly named and defined let's discuss
                               ^ “…has been…”

> +how to name and define fields, and which approach to use for
                         ^ missing “the”

> +implementing the serialization code related to them.

“serialization” doesn’t seem to be mentioned anywhere else in the manual
in the context of Guix services, so I think we should avoid using that
term before explaining what it actually means.  Maybe

  …and what approach to use to convert Scheme records into strings, which
  will be put into one or more configuration files.

> +@subsubheading Configuration Record Fields
> +
> +@enumerate
> +@item
> +It's a good idea to have a field/fields for specifying package/packages
> +being installed for this service.  For example
                                                 ^ missing comma
I suggest

  It's a good idea to have one or more fields for specifying the package
  or packages that will be installed by a service. 

> +@code{docker-configuration} has @code{docker}, @code{docker-cli},
> +@code{containerd} fields.

Having a link to the docker service would probably be a good idea.

> +Sometimes it make sense to make a field,
> +which accepts a list of packages for cases, where an arbitrary list of
> +plugins can be passed to the configuration.  There are some services,
> +which provide a field called @code{package} in their configuration,
> +which is ok, but the way it done in @code{docker-configuration} is more
> +flexible and thus preferable.

In what way is it more flexible?  Just naming the field ‘docker’ would
be a bit ambigous; ‘docker-package’ make things more clear.

> +@item
> +Fields for configuration files, should be called the same as target

s/called/named/

“…same as the name of the target configuration file”

> +configuration file name, but in kebab-case: bashrc for bashrc,

Not everyone might familiar with what exactly “kebab-case” means; we
should probably leave a footnote or something.

“…@code{bashrc} for @file{.bashrc}…”

It should also mention that preceding dots should be removed as well.
What should happend with files named ‘file.ext’?  Should the field be
named ‘file-ext’?

> +bash-profile for bash_profile, etc.  The implementation for such fields

“…@code{bash-profile} for @file{.bash_profile}.

Also, many services have an ‘extra-content’, ‘extra-config’, or
‘extra-options’ field.  In most cases these just take a string and
appends it to some configuration file.  Should these instead be named
‘sshd_config’, ‘xserver-conf’, and ‘asound-config’, respectively?

> +@item
> +Other fields in most cases add some boilerplates/reasonable defaults to
               ^ missing “should” maybe?
               
> +configuration files

Do you mean that for some services, there could be a
‘reasonable-defaults?’ field that sets some resonable defaults?

> +turns on/off installation of some packages or provide other custom
> behavior.

“turns on/off” sounds a bit weird; I think “enable/disabled” sounds
better.

> +There is no any special requirements or
> +recommendations here, but it's necessary to make it possible to disable
> +all the effects of such fields to provide a user with an empty
> +configuration and let them generate it from scratch with only field for
> +configuration file.

I don’t really understand what is meant by “let them generate it from
scratch with only field for configuration file”.  

It doesn’t mention if a configuration record should cover all the
configuration options available in a configuration file.  For example,
the current ‘openssh-configuration’ has quite a few options, but these
obviously don’t cover all the options available in /etc/ssh/sshd_config,
which is why there is an “escape hatch”, ‘extra-content’ field.

In some cases a program might have too many configuration fields for us
to map using configuration records, e.g., Git.  In rde, the approach we
took was to use nested lists to represent the INI configuration.  I
think this approach could also be mentioned here.

> +@end enumerate
> +
> +@subsubheading Fields for Configuration Files
> +
> +The field should accept a datastructure (preferably a combination of
                                 ^ missing space
> +simple lists, alists, vectors, gexps and basic data types), which will

There should probably be links to at least ‘vectors’ and ‘gexps’, since
many people probably aren’t too familiar with them.

> +be serialized to target configuration format, in other words it should
                                                missing comma  ^ 
> +provide an alternative lisp syntax, which can be later translated to

Capitalize “lisp”.

> +target one, like SXML for XML.  Such approach is quite flexible and
   ^ missing “a”

You mean “SXML to XML”, right (SXML being the Lisp syntax, and XML being
the target one)?

> +simple, it requires to write serializer once for one configuration
                      ^ “one” or “you”
                      
> +format and can be reused multiple times in different guix services.

Capitalize “guix”.

> +Let's take a look at JSON: we implement serialization function, which
> +converts vectors to arrays, alists to objects (AKA dictionaries or
> +associative arrays), numbers to numbers, gexps to the strings, file-like
> +objects to the strings, which contains the path to the file in the
> +store, @code{#t} to @code{true} and so on, and now we have all apps

“Apps” sounds kind of smartphone-y; “programs” is probably more
appropriate.

There should be a link “file-like object” since it may be unknown for
many.

> +using JSON and YAML as a format for configurations covered.  Maybe some

You only mentioned JSON above; why would YAML also be covered by JSON?

> +fine-tunning will be needed for particular application, but the primary
> +serilalization part is already finished.

“serialization” typo.

> +The pros and cons of such approach is inherited from open-world
> +assumption.  It doesn't matter if underlying applications provides new
                                    ^ “the”
                                    
What do you mean by “open-world assumption”?

> +configuration options, we don't need to change anything in service
                                                             ^ “the”
                                                             
> +configuration and its serialization code, it will work perfectly fine,
                               A full stop should probably be used here ^

> +on the other hand it harder to type check and structure check
                       ^ “is”
> +``compile-time'', and we can end up with configuration, which won't be
   ^ missing “during” or “at”?             ^ “a”
   
> +accepted by target application cause of unexisting, misspelled or
              ^ “the”

s/application/program/ :-)
s/cause/because/

> +wrongly-typed options.  It's possible to add those checks, but we will
> +get the drawbacks of closed-world assumption: we need to keep the
> +service implementation in-sync with app config options, and it will make
> +impossible to use the same service with older/newer package version,
> +which has a slightly different list of available options and will add an
> +excessive maintanence load.
> +
> +However, for some applications with really stable configuration those
> +checks can be helpful and should be implemented if possible, for some
> +other we can implement them only partially.

s/other/others/

> +The alternative approach applied in some exitsting services is to use
> +records for defining the structure of configuration field, it has the
> +same downsides of closed-world assumption and a few more problems:
> +
> +@enumerate
> +@item
> +It has to replicate all the available options for the app (sometimes
> +hundreds or thousands) to allow user express any configuration they
                                  ^ “the”
> +wants.

s/wants/want/

> +@item
> +Having a few records, adds one more layer of abstraction between service
                       ^ spurious comma


> +configuration and resulting app config, including different field
> +casing, new semantic units.

But it means that the syntax for configuring a program is more
Scheme-like.  For example, the Dovecot service provides a very
complicated but Schemeish interface for configuring Dovecot, though as
you have mentioned, it might be missing some fields since the Dovecot
configuration file might have changed since the creation of the service.

> +@c provide examples?
> +@item
> +It harder to implement optional settings, serialization becomes very
> +ad-hoc and hard to reuse among other services with the same target
> +config format.
> +@end enumerate
> +
> +Exceptions can exist, but the overall idea is to provide a lispy syntax
> +for target configuration.  Take a look at sway example configuration

Capitalize “Sway”.

> +(which also can be used for i3).  The following value of @code{config}
> +field of @code{home-sway-configuration}:

‘home-sway-configuration’ isn’t in Guix as of now, so it probably
shouldn’t be mentioned, as least for now.

> +@example
> +`((include ,(local-file "./sway/config"))
> +  (bindsym $mod+Ctrl+Shift+a exec emacsclient -c --eval "'(eshell)'")
> +  (bindsym $mod+Ctrl+Shift+o "[class=\"IceCat\"]" kill)
> +  (input * ((xkb_layout us,ru)
> +            (xkb_variant dvorak,))))
> +@end example
> +
> +would yield something like:
> +
> +@example
> +include /gnu/store/408jwvh6wxxn1j85lj95fniih05gx5xj-config
> +bindsym $mod+Ctrl+Shift+a exec emacsclient -c --eval '(eshell)'
> +bindsym $mod+Ctrl+Shift+o [class="IceCat"] kill
> +input * @{
> +    xkb_layout us,ru
> +    xkb_variant dvorak,
> +@}
> +@end example
> +
> +The mapping between scheme code and resulting configuration is quite

Capitalize “Scheme”.

> +obvious.  The serialization code with some type and structure checks
> +takes less than 70 lines and every possible sway/i3 configuration can be

Not sure if LoC is the best measure, and since ‘home-sway-configuration’
isn’t in Guix proper, users have no idea of where to look if they want
to see the source code.

> +expressed using this field.
> +
> +@subsubheading Let User Escape

I suggest “Escape Hatches” since the term is already mentioned in some
places in the manual.

> +Sometimes user already have a configuration file for an app, make sure
            ^ “a”

s/have/has/
s/app/program/

> +that it is possible to reuse it directly without rewriting.  In the
> +example above, the following snippet allows to include already existing
                       missing “you” or “one” ^          ^ missing “an”
                       
> +config to the newly generated one utilizing @code{include} directive of
> +i3/sway config language:
> +
> +@example
> +(include ,(local-file "./sway/config"))
> +@end example

Use @lisp instead.

> +When building a resulting config the file-like objects are substituted
> +with a path of the file in the store and sway's @code{include} loads
> +this file during startup.  The way file-like objects are treated here
> +also allows to specify paths to plugins or other binary files like:
       ^ missing “you” or “one”
       
> +@code{(load-plugin ,(file-append plugin-package "/share/plugin.so"))}

This should probably be put in its own @lisp block.

> +(the example value for imaginary service configuration config file
> +field).
> +
> +In some cases target configuration language may not have such
                ^ “the”                            missing “an” ^
> +@code{include} directive and can't provide such a functionallity, to
> +workaround it we can do the following trick:
> +
> +@example
> +`(#~(call-with-input-file
> +     #$(local-file "./sway/config")
> +     (@@ (ice-9 textual-ports) get-string-all)))
> +@end example

Use @lisp instead.

Where exactly should something like this be put?

‘@@’ is not a good practice; better to use ‘use-modules’ at the
beginning of the file.

> +G-expressions get serialized to its values, and the example above reads
> +the content of the file-like object and inserts it in the resulting
> +configuration file.

I suggest

  The ‘get-string-all’ procedure will read the contents of the
  @file{./sway/config} file, and return a string containing the
  contents.  Once serialized, the G-expression will thus be turn into
  the contents of the Sway configuration file in @file{./sway/config}.

> +Following these simple rules will help to make a simple, consistent and
                                                  ^ spurious “a”
                                                  
> +maintainable service configurations, will let user express any possible
                                       ^ missing “and”

s/user/users/

[1]: 
<665c4d2070de80af1d3594a268f0f6d3fb596d15.1639839498.git.public@yoctocell.xyz/">https://yhetil.org/guix-patches/665c4d2070de80af1d3594a268f0f6d3fb596d15.1639839498.git.public@yoctocell.xyz/>

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