Subject: Re: RFC: Modernizing the contept of plumbing v.s. porcelain

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Thu, 10 Dec 2020 15:14:58 +0100

On Wed, Dec 09 2020, Junio C Hamano wrote:

> Ævar Arnfjörð Bjarmason <[email-masked]> writes:
>
>> A lot of external guides and people's mental models of /usr/bin/git as a
>> scriptable client reference the concept of plumbing & porcelain. Just
>> one such example [1] prompted me to write this E-Mail.
>>
>> I've wondered if we shouldn't be updating this concept to reflect the
>> reality on the ground in the git command ecosystem.
>
> The example tells me that they are trying to be a good ecosystem
> citizen by sticking to the plumbing and refraining from using
> Porcelain command when writing their script.  The practice gives
> them assurance that we won't unilaterally break them, and gives us
> the freedom to improve Porcelain for human consumption.
>
>> I.e. if you look at "git help git"'s list of plumbing v.s. porcelain it
>> makes no mention or distinction between those commands & functionalities
>> that are truly transitory "porcelain". E.g. the specific error message a
>> command might return, and those that are effectively plumbing. E.g. some
>> "git config" functionality, "git init", the pretty formats in "git log"
>> etc.
>>
>> I'm not quite sure what I'm proposing if anything, just putting out
>> feelers to see if others think this documentary status quo has drifted
>> from reality.
>>
>> One potential change would be to mostly/entirely remove the
>> "porcelain/ancillary/plumbing" distinction in "git help git" (except
>> maybe e.g. "hash-object") and instead make a mention of the status of
>> the command at the top of its own manpage, which could then also
>> (e.g. in the case of "git log") document the API reliability of its
>> various sub-features.
>>
>> 1. https://gitlab.com/gitlab-org/gitaly/-/blob/afc90e3c2/doc/serverside_git_usage.md#L11-17
>
> I am not sure what you want to propose as a solution, but even
> before that, what problem you are perceiving.  Are you wondering if
> it may be a better general direction for us to tell "no, no, there
> is no value in sticking to the plumbing because we will break you
> anyway in the future" to those who wrote [1]?

I was updating that documentation, and ended up going for:

    +#### Plumbing v.s. porcelain
    +
     `git(1)` is the default choice to access repositories for Gitaly. Not all
    -commands that are available should be used in the Gitaly code base. Git makes
    -a distinction between porcelain and plumbing commands. Porcelain commands are
    -intended for the end-user and are the user-interface of the default `git`
    -client, where plumbing commands are intended for scripted use or to build
    -another porcelain. Gitaly should only use plumbing commands. `man 1 git`
    -contains a section on the low level plumbing.
    +commands that are available should be used in the Gitaly code base.
    +
    +Git makes a distinction between porcelain and plumbing
    +commands. Porcelain commands are intended for the end-user and are the
    +user-interface of the default `git` client, where plumbing commands
    +are intended for scripted use or to build another porcelain.
    +
    +Generally speaking, Gitaly should only use plumbing commands. `man 1
    +git` contains a section on the low level plumbing. However, a lot of
    +git's plumbing-like functionality is exposed as commands not marked as
    +plumbing, but whose API reliability can be considered the
    +same. E.g. `git log`'s `--pretty=` formats, `git config -l -z`, the
    +documented exit codes of `git remote` etc..
    +
    +We should use good judgement when choosing what commands and command
    +functionality to use, with the aim of not having gitaly break due to
    +e.g. an error message being rephrased or functionality the upstream
    +`git` maintainers don't consider plumbing-like being removed or
    +altered.
    +
    +#### Executing Git commands

I.e. the existing advice was to say "just use plumbing", now it takes a
more nuanced view of e.g. pointing out that certin porcelain commands
have "-z" options that can be considered as reliable as things
explicitly marked as plumbing.

It's widespread traditional wisdom when discussing git that there's
plumbing and porcelain, but I think ever since all the builtins were
shellscripts way-back-when this distinction has blurred.

We now have some plumbing tools whole entire functionality is considered
a stable API, some porcelain tools you shouldn't rely on at all for
that, but a large set of tools that are in-between somewhere. E.g. maybe
their output format(s) are "plumbing-like", or some options (such as
"-z") but not others.

I think updating our documentation to reflect this would be a good idea,
and I'm willing to write those patches. Just thought I'd weather-balloon
what others thought about it first.

I think a good way forward there would be:

 1. Keep the high-level/low-level list in "man git", but not mention
    plumbing/porcelain so prominently.

    Some of that's ... a bit suspect, e.g. "its low-level commands are
    sufficient to support development of alternative porcelains", but
    then git-config(1) is porcelain. These days you're not getting very
    far in developing your own alternate porcelain with just the
    plumbing commands.

 2. Either inline at the bottom, or probably better in a new
    gitplumbing(5) (or gitapi(5) or something) explain the nuance, that
    some commads are pure-plumbing, some pure-porcelain, and some are
    hybrids.

    That you can follow some general rules (does it have "-z" output,
    can probably be relied on) to determine "plumbing-like", or
    porcelain-like (is it stdout/stderr output in English, does it
    change under i18n?), that not all manpages might explicitly call out
    plumbing / porcelain, and that when in doubt ask the list.