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1Scalar
2======
3
4Scalar is a repository management tool that optimizes Git for use in large
5repositories. It accomplishes this by helping users to take advantage of
6advanced performance features in Git. Unlike most other Git built-in commands,
7Scalar is not executed as a subcommand of 'git'; rather, it is built as a
8separate executable containing its own series of subcommands.
9
10Background
11----------
12
13Scalar was originally designed as an add-on to Git and implemented as a .NET
14Core application. It was created based on the learnings from the VFS for Git
15project (another application aimed at improving the experience of working with
16large repositories). As part of its initial implementation, Scalar relied on
17custom features in the Microsoft fork of Git that have since been integrated
18into core Git:
19
20* partial clone,
21* commit graphs,
22* multi-pack index,
23* sparse checkout (cone mode),
24* scheduled background maintenance,
25* etc
26
27With the requisite Git functionality in place and a desire to bring the benefits
28of Scalar to the larger Git community, the Scalar application itself was ported
29from C# to C and integrated upstream.
30
31Features
32--------
33
34Scalar is comprised of two major pieces of functionality: automatically
35configuring built-in Git performance features and managing repository
36enlistments.
37
38The Git performance features configured by Scalar (see "Background" for
39examples) confer substantial performance benefits to large repositories, but are
40either too experimental to enable for all of Git yet, or only benefit large
41repositories. As new features are introduced, Scalar should be updated
42accordingly to incorporate them. This will prevent the tool from becoming stale
43while also providing a path for more easily bringing features to the appropriate
44users.
45
46Enlistments are how Scalar knows which repositories on a user's system should
47utilize Scalar-configured features. This allows it to update performance
48settings when new ones are added to the tool, as well as centrally manage
49repository maintenance. The enlistment structure - a root directory with a
50`src/` subdirectory containing the cloned repository itself - is designed to
51encourage users to route build outputs outside of the repository to avoid the
52performance-limiting overhead of ignoring those files in Git.
53
54Design
55------
56
57Scalar is implemented in C and interacts with Git via a mix of child process
58invocations of Git and direct usage of `libgit.a`. Internally, it is structured
59much like other built-ins with subcommands (e.g., `git stash`), containing a
60`cmd_<subcommand>()` function for each subcommand, routed through a `cmd_main()`
61function. Most options are unique to each subcommand, with `scalar` respecting
62some "global" `git` options (e.g., `-c` and `-C`).
63
64Because `scalar` is not invoked as a Git subcommand (like `git scalar`), it is
65built and installed as its own executable in the `bin/` directory, alongside
66`git`, `git-gui`, etc.