git/Documentation/technical/scalar.txt

Scalar
======

Scalar is a repository management tool that optimizes Git for use in large
repositories. It accomplishes this by helping users to take advantage of
advanced performance features in Git. Unlike most other Git built-in commands,
Scalar is not executed as a subcommand of 'git'; rather, it is built as a
separate executable containing its own series of subcommands.

Background
----------

Scalar was originally designed as an add-on to Git and implemented as a .NET
Core application. It was created based on the learnings from the VFS for Git
project (another application aimed at improving the experience of working with
large repositories). As part of its initial implementation, Scalar relied on
custom features in the Microsoft fork of Git that have since been integrated
into core Git:

* partial clone,
* commit graphs,
* multi-pack index,
* sparse checkout (cone mode),
* scheduled background maintenance,
* etc

With the requisite Git functionality in place and a desire to bring the benefits
of Scalar to the larger Git community, the Scalar application itself was ported
from C# to C and integrated upstream.

Features
--------

Scalar is comprised of two major pieces of functionality: automatically
configuring built-in Git performance features and managing repository
enlistments.

The Git performance features configured by Scalar (see "Background" for
examples) confer substantial performance benefits to large repositories, but are
either too experimental to enable for all of Git yet, or only benefit large
repositories. As new features are introduced, Scalar should be updated
accordingly to incorporate them. This will prevent the tool from becoming stale
while also providing a path for more easily bringing features to the appropriate
users.

Enlistments are how Scalar knows which repositories on a user's system should
utilize Scalar-configured features. This allows it to update performance
settings when new ones are added to the tool, as well as centrally manage
repository maintenance. The enlistment structure - a root directory with a
`src/` subdirectory containing the cloned repository itself - is designed to
encourage users to route build outputs outside of the repository to avoid the
performance-limiting overhead of ignoring those files in Git.

Design
------

Scalar is implemented in C and interacts with Git via a mix of child process
invocations of Git and direct usage of `libgit.a`. Internally, it is structured
much like other built-ins with subcommands (e.g., `git stash`), containing a
`cmd_<subcommand>()` function for each subcommand, routed through a `cmd_main()`
function. Most options are unique to each subcommand, with `scalar` respecting
some "global" `git` options (e.g., `-c` and `-C`).

Because `scalar` is not invoked as a Git subcommand (like `git scalar`), it is
built and installed as its own executable in the `bin/` directory, alongside
`git`, `git-gui`, etc.

Roadmap
-------

NOTE: this section will be removed once the remaining tasks outlined in this
roadmap are complete.

Scalar is a large enough project that it is being upstreamed incrementally,
living in `contrib/` until it is feature-complete. So far, the following patch
series have been accepted:

- `scalar-the-beginning`: The initial patch series which sets up
  `contrib/scalar/` and populates it with a minimal `scalar` command that
  demonstrates the fundamental ideas.

- `scalar-c-and-C`: The `scalar` command learns about two options that can be
  specified before the command, `-c <key>=<value>` and `-C <directory>`.

- `scalar-diagnose`: The `scalar` command is taught the `diagnose` subcommand.

- `scalar-generalize-diagnose`: Move the functionality of `scalar diagnose`
  into `git diagnose` and `git bugreport --diagnose`.

Roughly speaking (and subject to change), the following series are needed to
"finish" this initial version of Scalar:

- Finish Scalar features: Enable the built-in FSMonitor in Scalar enlistments
  and implement `scalar help`. At the end of this series, Scalar should be
  feature-complete from the perspective of a user.

- Move Scalar to toplevel: Move Scalar out of `contrib/` and into the root of
  `git`, including updates to build and install it with the rest of Git. This
  change will incorporate Scalar into the Git CI and test framework, as well as
  expand regression and performance testing to ensure the tool is stable.

Finally, there are two additional patch series that exist in Microsoft's fork of
Git, but there is no current plan to upstream them. There are some interesting
ideas there, but the implementation is too specific to Azure Repos and/or VFS
for Git to be of much help in general.

These still exist mainly because the GVFS protocol is what Azure Repos has
instead of partial clone, while Git is focused on improving partial clone:

- `scalar-with-gvfs`: The primary purpose of this patch series is to support
  existing Scalar users whose repositories are hosted in Azure Repos (which does
  not support Git's partial clones, but supports its predecessor, the GVFS
  protocol, which is used by Scalar to emulate the partial clone).

  Since the GVFS protocol will never be supported by core Git, this patch series
  will remain in Microsoft's fork of Git.

- `run-scalar-functional-tests`: The Scalar project developed a quite
  comprehensive set of integration tests (or, "Functional Tests"). They are the
  sole remaining part of the original C#-based Scalar project, and this patch
  adds a GitHub workflow that runs them all.

  Since the tests partially depend on features that are only provided in the
  `scalar-with-gvfs` patch series, this patch cannot be upstreamed.