blob: 572a3bec14a87bdd941f910d7c5fff358c9c5fe9 [file] [log] [blame]
Junio C Hamanobb778e72022-09-19 22:05:531<?xml version="1.0" encoding="UTF-8"?>
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735<body class="article">
736<div id="header">
737<h1>Scalar</h1>
Junio C Hamanoaedeeae2023-07-18 15:55:30738<span id="revdate">2023-07-18</span>
Junio C Hamanobb778e72022-09-19 22:05:53739</div>
740<div id="content">
741<div id="preamble">
742<div class="sectionbody">
743<div class="paragraph"><p>Scalar is a repository management tool that optimizes Git for use in large
744repositories. It accomplishes this by helping users to take advantage of
745advanced performance features in Git. Unlike most other Git built-in commands,
746Scalar is not executed as a subcommand of <em>git</em>; rather, it is built as a
747separate executable containing its own series of subcommands.</p></div>
748</div>
749</div>
750<div class="sect1">
751<h2 id="_background">Background</h2>
752<div class="sectionbody">
753<div class="paragraph"><p>Scalar was originally designed as an add-on to Git and implemented as a .NET
754Core application. It was created based on the learnings from the VFS for Git
755project (another application aimed at improving the experience of working with
756large repositories). As part of its initial implementation, Scalar relied on
757custom features in the Microsoft fork of Git that have since been integrated
758into core Git:</p></div>
759<div class="ulist"><ul>
760<li>
761<p>
762partial clone,
763</p>
764</li>
765<li>
766<p>
767commit graphs,
768</p>
769</li>
770<li>
771<p>
772multi-pack index,
773</p>
774</li>
775<li>
776<p>
777sparse checkout (cone mode),
778</p>
779</li>
780<li>
781<p>
782scheduled background maintenance,
783</p>
784</li>
785<li>
786<p>
787etc
788</p>
789</li>
790</ul></div>
791<div class="paragraph"><p>With the requisite Git functionality in place and a desire to bring the benefits
792of Scalar to the larger Git community, the Scalar application itself was ported
793from C# to C and integrated upstream.</p></div>
794</div>
795</div>
796<div class="sect1">
797<h2 id="_features">Features</h2>
798<div class="sectionbody">
799<div class="paragraph"><p>Scalar is comprised of two major pieces of functionality: automatically
800configuring built-in Git performance features and managing repository
801enlistments.</p></div>
802<div class="paragraph"><p>The Git performance features configured by Scalar (see "Background" for
803examples) confer substantial performance benefits to large repositories, but are
804either too experimental to enable for all of Git yet, or only benefit large
805repositories. As new features are introduced, Scalar should be updated
806accordingly to incorporate them. This will prevent the tool from becoming stale
807while also providing a path for more easily bringing features to the appropriate
808users.</p></div>
809<div class="paragraph"><p>Enlistments are how Scalar knows which repositories on a user&#8217;s system should
810utilize Scalar-configured features. This allows it to update performance
811settings when new ones are added to the tool, as well as centrally manage
812repository maintenance. The enlistment structure - a root directory with a
813<code>src/</code> subdirectory containing the cloned repository itself - is designed to
814encourage users to route build outputs outside of the repository to avoid the
815performance-limiting overhead of ignoring those files in Git.</p></div>
816</div>
817</div>
818<div class="sect1">
819<h2 id="_design">Design</h2>
820<div class="sectionbody">
821<div class="paragraph"><p>Scalar is implemented in C and interacts with Git via a mix of child process
822invocations of Git and direct usage of <code>libgit.a</code>. Internally, it is structured
823much like other built-ins with subcommands (e.g., <code>git stash</code>), containing a
824<code>cmd_&lt;subcommand&gt;()</code> function for each subcommand, routed through a <code>cmd_main()</code>
825function. Most options are unique to each subcommand, with <code>scalar</code> respecting
826some "global" <code>git</code> options (e.g., <code>-c</code> and <code>-C</code>).</p></div>
827<div class="paragraph"><p>Because <code>scalar</code> is not invoked as a Git subcommand (like <code>git scalar</code>), it is
828built and installed as its own executable in the <code>bin/</code> directory, alongside
829<code>git</code>, <code>git-gui</code>, etc.</p></div>
830</div>
831</div>
832</div>
833<div id="footnotes"><hr /></div>
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836Last updated
Junio C Hamanoa7b2c102023-06-13 21:00:15837 2022-09-19 15:03:44 PDT
Junio C Hamanobb778e72022-09-19 22:05:53838</div>
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