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scalar: add a README with a roadmap

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The Scalar command will be contributed incrementally, over a bunch of
patch series. Let's document what Scalar is about, and then describe the
patch series that are planned.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Johannes Schindelin 2021-12-03 13:34:15 +00:00 committed by Junio C Hamano
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# Scalar - an opinionated repository management tool
Scalar is an add-on to Git that helps users take advantage of advanced
performance features in Git. Originally implemented in C# using .NET Core,
based on the learnings from the VFS for Git project, most of the techniques
developed by the Scalar project have been integrated into core Git already:
* partial clone,
* commit graphs,
* multi-pack index,
* sparse checkout (cone mode),
* scheduled background maintenance,
* etc
This directory contains the remaining parts of Scalar that are not (yet) in
core Git.
## Roadmap
The idea is to populate this directory via incremental patch series and
eventually move to a top-level directory next to `gitk-git/` and to `git-gui/`. The
current plan involves the following patch series:
- `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-and-builtin-fsmonitor`: The built-in FSMonitor is enabled in `scalar
register` and in `scalar clone`, for an enormous performance boost when
working in large worktrees. This patch series necessarily depends on Jeff
Hostetler's FSMonitor patch series to be integrated into Git.
- `scalar-gentler-config-locking`: Scalar enlistments are registered in the
user's Git config. This usually does not represent any problem because it is
rare for a user to register an enlistment. However, in Scalar's functional
tests, Scalar enlistments are created galore, and in parallel, which can lead
to lock contention. This patch series works around that problem by re-trying
to lock the config file in a gentle fashion.
- `scalar-extra-docs`: Add some extensive documentation that has been written
in the original Scalar project (all subject to discussion, of course).
- `optionally-install-scalar`: Now that Scalar is feature (and documentation)
complete and is verified in CI builds, let's offer to install it.
- `move-scalar-to-toplevel`: Now that Scalar is complete, let's move it next to
`gitk-git/` and to `git-gui/`, making it a top-level command.
The following two patch series exist in Microsoft's fork of Git and are
publicly available. There is no current plan to upstream them, not because I
want to withhold these patches, but because I don't think the Git community is
interested in these patches.
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 (and also: my
colleagues tried to upstream some patches already and the enthusiasm for
integrating things related to Azure Repos and VFS for Git can be summarized in
very, very few words).
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.