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Add to glossary and clarify garbage collection

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While trying to understand garbage collection it was not immediately
clear that only the runtime dependency closure of output paths
would be kept (instead of the build-time dependency closure).
This commit attempts to clarify this by expanding some of the
glossary definitions and extending the Garbage Collection
section.
This commit is contained in:
Sean Seefried 2018-04-12 19:23:24 +10:00
parent dc0a542c9f
commit 2ef8f0608c
3 changed files with 44 additions and 16 deletions
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49
doc/manual/glossary/glossary.xml
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@ -85,29 +85,48 @@
<glossentry xml:id="gloss-reference"><glossterm>reference</glossterm>
<glossdef><para>A store path <varname>P</varname> is said to have a
reference to a store path <varname>Q</varname> if the store object
at <varname>P</varname> contains the path <varname>Q</varname>
somewhere. This implies than an execution involving
<varname>P</varname> potentially needs <varname>Q</varname> to be
present. The <emphasis>references</emphasis> of a store path are
the set of store paths to which it has a reference.</para></glossdef>
<glossdef>
<para>A store path <varname>P</varname> is said to have a
reference to a store path <varname>Q</varname> if the store object
at <varname>P</varname> contains the path <varname>Q</varname>
somewhere. The <emphasis>references</emphasis> of a store path are
the set of store paths to which it has a reference.
</para>
<para>A derivation can reference other derivations and sources
(but not output paths), whereas an output path only references other
output paths.
</para>
</glossdef>
</glossentry>
<glossentry xml:id="gloss-reachable"><glossterm>reachable</glossterm>
<glossdef><para>A store path <varname>Q</varname> is reachable from
another store path <varname>P</varname> if <varname>Q</varname> is in the
<link linked="gloss-closure">closure</link> of the
<link linkend="gloss-reference">references</link> relation.
</para></glossdef>
</glossentry>
<glossentry xml:id="gloss-closure"><glossterm>closure</glossterm>
<glossdef><para>The closure of a store path is the set of store
paths that are directly or indirectly “reachable” from that store
path; that is, it’s the closure of the path under the <link
linkend="gloss-reference">references</link> relation. For instance,
if the store object at path <varname>P</varname> contains a
reference to path <varname>Q</varname>, then <varname>Q</varname> is
in the closure of <varname>P</varname>. For correct deployment it
is necessary to deploy whole closures, since otherwise at runtime
files could be missing. The command <command>nix-store
-qR</command> prints out closures of store paths.</para></glossdef>
linkend="gloss-reference">references</link> relation. For a package, the
closure of its derivation is equivalent to the build-time
dependencies, while the closure of its output path is equivalent to its
runtime dependencies. For correct deployment it is necessary to deploy whole
closures, since otherwise at runtime files could be missing. The command
<command>nix-store -qR</command> prints out closures of store paths.
</para>
<para>As an example, if the store object at path <varname>P</varname> contains
a reference to path <varname>Q</varname>, then <varname>Q</varname> is
in the closure of <varname>P</varname>. Further, if <varname>Q</varname>
references <varname>R</varname> then <varname>R</varname> is also in
the closure of <varname>P</varname>.
</para></glossdef>
</glossentry>
@ -147,7 +166,7 @@
linkend="sec-profiles" />.</para>
</glossdef>
</glossentry>