# Git Sparse-Index Design Document

The sparse-checkout feature allows users to focus a working directory on a subset of the files at HEAD. The cone mode patterns, enabled by `core.sparseCheckoutCone`, allow for very fast pattern matching to discover which files at HEAD belong in the sparse-checkout cone.

Three important scale dimensions for a Git working directory are:

- `HEAD`: How many files are present at `HEAD`?

- Populated: How many files are within the sparse-checkout cone.

- Modified: How many files has the user modified in the working directory?

We will use big-O notation — O(X) — to denote how expensive certain operations are in terms of these dimensions.

These dimensions are ordered by their magnitude: users (typically) modify fewer files than are populated, and we can only populate files at `HEAD`.

Problems occur if there is an extreme imbalance in these dimensions. For example, if `HEAD` contains millions of paths but the populated set has only tens of thousands, then commands like `git` `status` and `git` `add` can be dominated by operations that require O(`HEAD`) operations instead of O(Populated). Primarily, the cost is in parsing and rewriting the index, which is filled primarily with files at `HEAD` that are marked with the `SKIP_WORKTREE` bit.

The sparse-index intends to take these commands that read and modify the index from O(`HEAD`) to O(Populated). To do this, we need to modify the index format in a significant way: add "sparse directory" entries.

With cone mode patterns, it is possible to detect when an entire directory will have its contents outside of the sparse-checkout definition. Instead of listing all of the files it contains as individual entries, a sparse-index contains an entry with the directory name, referencing the object ID of the tree at `HEAD` and marked with the `SKIP_WORKTREE` bit. If we need to discover the details for paths within that directory, we can parse trees to find that list.

At time of writing, sparse-directory entries violate expectations about the index format and its in-memory data structure. There are many consumers in the codebase that expect to iterate through all of the index entries and see only files. In fact, these loops expect to see a reference to every staged file. One way to handle this is to parse trees to replace a sparse-directory entry with all of the files within that tree as the index is loaded. However, parsing trees is slower than parsing the index format, so that is a slower operation than if we left the index alone. The plan is to make all of these integrations "sparse aware" so this expansion through tree parsing is unnecessary and they use fewer resources than when using a full index.

The implementation plan below follows four phases to slowly integrate with the sparse-index. The intention is to incrementally update Git commands to interact safely with the sparse-index without significant slowdowns. This may not always be possible, but the hope is that the primary commands that users need in their daily work are dramatically improved.

## Phase I: Format and initial speedups

During this phase, Git learns to enable the sparse-index and safely parse one. Protections are put in place so that every consumer of the in-memory data structure can operate with its current assumption of every file at `HEAD`.

At first, every index parse will call a helper method, `ensure_full_index`(), which scans the index for sparse-directory entries (pointing to trees) and replaces them with the full list of paths (with blob contents) by parsing tree objects. This will be slower in all cases. The only noticeable change in behavior will be that the serialized index file contains sparse-directory entries.

To start, we use a new required index extension, `sdir`, to allow inserting sparse-directory entries into indexes with file format versions 2, 3, and 4. This prevents Git versions that do not understand the sparse-index from operating on one, while allowing tools that do not understand the sparse-index to operate on repositories as long as they do not interact with the index. A new format, index v5, will be introduced that includes sparse-directory entries by default. It might also introduce other features that have been considered for improving the index, as well.

Next, consumers of the index will be guarded against operating on a sparse-index by inserting calls to `ensure_full_index`() or `expand_index_to_path`(). If a specific path is requested, then those will be protected from within the `index_file_exists`() and `index_name_pos`() API calls: they will call `ensure_full_index`() if necessary. The intention here is to preserve existing behavior when interacting with a sparse-checkout. We don’t want a change to happen by accident, without tests. Many of these locations may not need any change before removing the guards, but we should not do so without tests to ensure the expected behavior happens.

It may be desirable to change the behavior of some commands in the presence of a sparse index or more generally in any sparse-checkout scenario. In such cases, these should be carefully communicated and tested. No such behavior changes are intended during this phase.

During a scan of the codebase, not every iteration of the cache entries needs an `ensure_full_index`() check. The basic reasons include:

1. The loop is scanning for entries with non-zero stage. These entries are not collapsed into a sparse-directory entry.

2. The loop is scanning for submodules. These entries are not collapsed into a sparse-directory entry.

3. The loop is part of the index API, especially around reading or writing the format.

4. The loop is checking for correct order of cache entries and that is correct if and only if the sparse-directory entries are in the correct location.

5. The loop ignores entries with the `SKIP_WORKTREE` bit set, or is otherwise already aware of sparse directory entries.

6. The sparse-index is disabled at this point when using the split-index feature, so no effort is made to protect the split-index API.

Even after inserting these guards, we will keep expanding sparse-indexes for most Git commands using the `command_requires_full_index` repository setting. This setting will be on by default and disabled one builtin at a time until we have sufficient confidence that all of the index operations are properly guarded.

To complete this phase, the commands `git` `status` and `git` `add` will be integrated with the sparse-index so that they operate with O(Populated) performance. They will be carefully tested for operations within and outside the sparse-checkout definition.

## Phase II: Careful integrations

This phase focuses on ensuring that all index extensions and APIs work well with a sparse-index. This requires significant increases to our test coverage, especially for operations that interact with the working directory outside of the sparse-checkout definition. Some of these behaviors may not be the desirable ones, such as some tests already marked for failure in `t1092-sparse-checkout-compatibility.sh`.

The index extensions that may require special integrations are:

- FS Monitor

- Untracked cache

While integrating with these features, we should look for patterns that might lead to better APIs for interacting with the index. Coalescing common usage patterns into an API call can reduce the number of places where sparse-directories need to be handled carefully.

## Phase III: Important command speedups

At this point, the patterns for testing and implementing sparse-directory logic should be relatively stable. This phase focuses on updating some of the most common builtins that use the index to operate as O(Populated). Here is a potential list of commands that could be valuable to integrate at this point:

- `git` `commit`

- `git` `checkout`

- `git` `merge`

- `git` `rebase`

Hopefully, commands such as `git` `merge` and `git` `rebase` can benefit instead from merge algorithms that do not use the index as a data structure, such as the merge-ORT strategy. As these topics mature, we may enable the ORT strategy by default for repositories using the sparse-index feature.

Along with `git` `status` and `git` `add`, these commands cover the majority of users' interactions with the working directory. In addition, we can integrate with these commands:

- `git` `grep`

- `git` `rm`

These have been proposed as some whose behavior could change when in a repo with a sparse-checkout definition. It would be good to include this behavior automatically when using a sparse-index. Some clarity is needed to make the behavior switch clear to the user.

This phase is the first where parallel work might be possible without too much conflicts between topics.

## Phase IV: The long tail

This last phase is less a "phase" and more "the new normal" after all of the previous work.

To start, the `command_requires_full_index` option could be removed in favor of expanding only when hitting an API guard.

There are many Git commands that could use special attention to operate as O(Populated), while some might be so rare that it is acceptable to leave them with additional overhead when a sparse-index is present.

Here are some commands that might be useful to update:

- `git` `sparse-checkout` `set`

- `git` `am`

- `git` `clean`

- `git` `stash`