From mboxrd@z Thu Jan  1 00:00:00 1970
From: Lukas Gradl <lgradl@openmailbox.org>
Subject: Re: inputs vs. native-inputs vs. propagated-inputs
Date: Sat, 18 Jun 2016 22:57:56 -0500
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In-Reply-To: <87vb161esp.fsf@gnu.org> ("Ludovic =?utf-8?Q?Court=C3=A8s=22'?=
	=?utf-8?Q?s?= message of "Sat, 18 Jun 2016 21:24:06 +0200")
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To: Ludovic =?utf-8?Q?Court=C3=A8s?= <ludo@gnu.org>
Cc: Hartmut Goebel <h.goebel@crazy-compilers.com>, help-guix <help-guix@gnu.org>

ludo@gnu.org (Ludovic Court=C3=A8s) writes:

> Leo Famulari <leo@famulari.name> skribis:
>
>> I _think_ that `gc --references` works by querying the database for a
>> list of references in the store item you give it as an argument. I think
>> that the information in the database is created after building, by
>> scanning the files of the package's output in /gnu/store, looking for
>> strings that appear to be paths in /gnu/store. I say "I think" because I
>> am not sure. I don't understand that part of the code very well yet.
>
> That=E2=80=99s 100% correct!  :-)  Namely, =E2=80=98guix gc --references=
=E2=80=99 makes an RPC to
> the daemon, which then looks things up in the database (see the =E2=80=98=
Refs=E2=80=99
> table in nix/libstore/schema.sql.)
>
> Scanning for references indeed happens at the end of a successful build,
> in =E2=80=98scanForReferences=E2=80=99 in libstore/references.cc.  Since =
scanning is
> expensive (I/O-intensive), the result is stored in the database.

Sorry, this might be a dumb question, I don't quite understand the
concept of these references.  Why are references important?  Shouldn't
every store item know which other store-items are related to it from the
"inputs"-field in its definition?  Why is it necessary to keep track of
the references?

Thank you!