doc: add an explanation of Git's data model

Git very often uses the terms "object", "reference", or "index" in its
documentation.

However, it's hard to find a clear explanation of these terms and how
they relate to each other in the documentation. The closest candidates
currently are:

1. `gitglossary`. This makes a good effort, but it's an alphabetically
    ordered dictionary and a dictionary is not a good way to learn
    concepts. You have to jump around too much and it's not possible to
    present the concepts in the order that they should be explained.
2. `gitcore-tutorial`. This explains how to use the "core" Git commands.
   This is a nice document to have, but it's not necessary to learn how
   `update-index` works to understand Git's data model, and we should
   not be requiring users to learn how to use the "plumbing" commands
   if they want to learn what the term "index" or "object" means.
3. `gitrepository-layout`. This is a great resource, but it includes a
   lot of information about configuration and internal implementation
   details which are not related to the data model. It also does
   not explain how commits work.

The result of this is that Git users (even users who have been using
Git for 15+ years) struggle to read the documentation because they don't
know what the core terms mean, and it's not possible to add links
to help them learn more.

Add an explanation of Git's data model. Some choices I've made in
deciding what "core data model" means:

1. Omit pseudorefs like `FETCH_HEAD`, because it's not clear to me
   if those are intended to be user facing or if they're more like
   internal implementation details.
2. Don't talk about submodules other than by mentioning how they
   relate to trees. This is because Git has a lot of special features,
   and explaining how they all work exhaustively could quickly go
   down a rabbit hole which would make this document less useful for
   understanding Git's core behaviour.
3. Don't discuss the structure of a commit message
   (first line, trailers etc).
4. Don't mention configuration.
5. Don't mention the `.git` directory, to avoid getting too much into
   implementation details

Signed-off-by: Julia Evans <julia@jvns.ca>

Author: jvns

Documentation/Makefile

@@ -52,6 +52,7 @@ MAN7_TXT += gitcli.adoc
 MAN7_TXT += gitcore-tutorial.adoc
 MAN7_TXT += gitcredentials.adoc
 MAN7_TXT += gitcvs-migration.adoc
+MAN7_TXT += gitdatamodel.adoc
 MAN7_TXT += gitdiffcore.adoc
 MAN7_TXT += giteveryday.adoc
 MAN7_TXT += gitfaq.adoc

Documentation/gitdatamodel.adoc

@@ -0,0 +1,296 @@
+gitdatamodel(7)
+===============
+
+NAME
+----
+gitdatamodel - Git's core data model
+
+SYNOPSIS
+--------
+gitdatamodel
+
+DESCRIPTION
+-----------
+
+It's not necessary to understand Git's data model to use Git, but it's
+very helpful when reading Git's documentation so that you know what it
+means when the documentation says "object", "reference" or "index".
+
+Git's core operations use 4 kinds of data:
+
+1. <<objects,Objects>>: commits, trees, blobs, and tag objects
+2. <<references,References>>: branches, tags,
+   remote-tracking branches, etc
+3. <<index,The index>>, also known as the staging area
+4. <<reflogs,Reflogs>>: logs of changes to references ("ref log")
+
+[[objects]]
+OBJECTS
+-------
+
+All of the commits and files in a Git repository are stored as "Git objects".
+Git objects never change after they're created, and every object has an ID,
+like `1b61de420a21a2f1aaef93e38ecd0e45e8bc9f0a`.
+
+This means that if you have an object's ID, you can always recover its
+exact contents as long as the object hasn't been deleted.
+
+Every object has:
+
+[[object-id]]
+1. an *ID* (aka "object name"), which is a cryptographic hash of its
+  type and contents.
+  It's fast to look up a Git object using its ID.
+  This is usually represented in hexadecimal, like
+  `1b61de420a21a2f1aaef93e38ecd0e45e8bc9f0a`.
+2. a *type*. There are 4 types of objects:
+   <<commit,commits>>, <<tree,trees>>, <<blob,blobs>>,
+   and <<tag-object,tag objects>>.
+3. *contents*. The structure of the contents depends on the type.
+
+Here's how each type of object is structured:
+
+[[commit]]
+commit::
+    A commit contains the full directory structure of every file
+    in that version of the repository and each file's contents.
+    It has these required fields
+    (though there are other optional fields):
++
+1. The *files* in the commit, stored as the *<<tree,tree>>* ID
+   of the commit's base directory.
+2. Its *parent commit ID(s)*. The first commit in a repository has 0 parents,
+  regular commits have 1 parent, merge commits have 2 or more parents
+3. An *author* and the time the commit was authored
+4. A *committer* and the time the commit was committed.
+5. A *commit message*
++
+Here's how an example commit is stored:
++
+----
+tree 1b61de420a21a2f1aaef93e38ecd0e45e8bc9f0a
+parent 4ccb6d7b8869a86aae2e84c56523f8705b50c647
+author Maya <maya@example.com> 1759173425 -0400
+committer Maya <maya@example.com> 1759173425 -0400
+
+Add README
+----
++
+Like all other objects, commits can never be changed after they're created.
+For example, "amending" a commit with `git commit --amend` creates a new
+commit with the same parent.
++
+Git does not store the diff for a commit: when you ask Git to show
+the commit with linkgit:git-show[1], it calculates the diff from its
+parent on the fly.
+
+[[tree]]
+tree::
+    A tree is how Git represents a directory.
+    It can contain files or other trees (which are subdirectories).
+    It lists, for each item in the tree:
++
+1. The *filename*, for example `hello.py`
+2. The *file mode*. Git has these file modes. which are only
+   spiritually related to Unix file modes:
++
+  - `100644`: regular file (with <<object,object type>> `blob`)
+  - `100755`: executable file (with type `blob`)
+  - `120000`: symbolic link (with type `blob`)
+  - `040000`: directory (with type `tree`)
+  - `160000`: gitlink, for use with submodules (with type `commit`)
+
+3. The <<object-id,*object ID*>> with the contents of the file or directory
++
+For example, this is how a tree containing one directory (`src`) and one file
+(`README.md`) is stored:
++
+----
+100644 blob 8728a858d9d21a8c78488c8b4e70e531b659141f README.md
+040000 tree 89b1d2e0495f66d6929f4ff76ff1bb07fc41947d src
+----
+
+[[blob]]
+blob::
+    A blob object contains a file's contents.
++
+When you make a commit, Git stores the full contents of each file that
+you changed as a blob.
+For example, if you have a commit that changes 2 files in a repository
+with 1000 files, that commit will create 2 new blobs, and use the
+previous blob ID for the other 998 files.
+This means that commits can use relatively little disk space even in a
+very large repository.
+
+[[tag-object]]
+tag object::
+    Tag objects contain these required fields
+    (though there are other optional fields):
++
+1. The object *ID* it references
+2. The object *type*
+3. The *tagger* and tag date
+4. A *tag message*, similar to a commit message
+
+Here's how an example tag object is stored:
+
+----
+object 750b4ead9c87ceb3ddb7a390e6c7074521797fb3
+type commit
+tag v1.0.0
+tagger Maya <maya@example.com> 1759927359 -0400
+
+Release version 1.0.0
+----
+
+NOTE: All of the examples in this section were generated with
+`git cat-file -p <object-id>`.
+
+[[references]]
+REFERENCES
+----------
+
+References are a way to give a name to a commit.
+It's easier to remember "the changes I'm working on are on the `turtle`
+branch" than "the changes are in commit bb69721404348e".
+Git often uses "ref" as shorthand for "reference".
+
+References can either refer to:
+
+1. An object ID, usually a <<commit,commit>> ID
+2. Another reference. This is called a "symbolic reference".
+
+References are stored in a hierarchy, and Git handles references
+differently based on where they are in the hierarchy.
+Most references are under `refs/`. Here are the main types:
+
+[[branch]]
+branches: `refs/heads/<name>`::
+    A branch refers to a commit ID.
+    That commit is the latest commit on the branch.
++
+To get the history of commits on a branch, Git will start at the commit
+ID the branch references, and then look at the commit's parent(s),
+the parent's parent, etc.
+
+[[tag]]
+tags: `refs/tags/<name>`::
+    A tag refers to a commit ID, tag object ID, or other object ID.
+    There are two types of tags:
+    1. "Annotated tags", which reference a <<tag-object,tag object>> ID
+       which contains a tag message
+    2. "Lightweight tags", which reference a commit, blob, or tree ID
+       directly
++
+Even though branches and tags both refer to a commit ID, Git
+treats them very differently.
+Branches are expected to change over time: when you make a commit, Git
+will update your <<HEAD,current branch>> to point to the new commit.
+Tags are usually not changed after they're created.
+
+[[HEAD]]
+HEAD: `HEAD`::
+    `HEAD` is where Git stores your current <<branch,branch>>,
+    if there is a current branch. `HEAD` can either be:
++
+1. A symbolic reference to your current branch, for example `ref:
+   refs/heads/main` if your current branch is `main`.
+2. A direct reference to a commit ID. In this case there is no current branch.
+   This is called "detached HEAD state", see the DETACHED HEAD section
+   of linkgit:git-checkout[1] for more.
+
+[[remote-tracking-branch]]
+remote-tracking branches: `refs/remotes/<remote>/<branch>`::
+    A remote-tracking branch refers to a commit ID.
+    It's how Git stores the last-known state of a branch in a remote
+    repository. `git fetch` updates remote-tracking branches. When
+    `git status` says "you're up to date with origin/main", it's looking at
+    this.
++
+`refs/remotes/<remote>/HEAD` is a symbolic reference to the remote's
+default branch. This is the branch that `git clone` checks out by default.
+
+[[other-refs]]
+Other references::
+    Git tools may create references anywhere under `refs/`.
+    For example, linkgit:git-stash[1], linkgit:git-bisect[1],
+    and linkgit:git-notes[1] all create their own references
+    in `refs/stash`, `refs/bisect`, etc.
+    Third-party Git tools may also create their own references.
++
+Git may also create references other than `HEAD` at the base of the
+hierarchy, like `ORIG_HEAD`.
+
+NOTE: Objects will only be deleted if they aren't "reachable" from any reference.
+An object is "reachable" if we can find it by following tags to whatever
+they tag, commits to their parents or trees, and trees to the trees or
+blobs that they contain.
+For example, if you amend a commit, with `git commit --amend`,
+the old commit will usually not be reachable, so it may be deleted eventually.
+
+[[index]]
+THE INDEX
+---------
+The index, also known as the "staging area", is a list of files and
+the contents of each file, stored as a <<blob,blob>>.
+You can add files to the index or update the contents of a file in the
+index with linkgit:git-add[1]. This is called "staging" the file for commit.
+
+Unlike a <<tree,tree>>, the index is a flat list of files.
+When you commit, Git converts the list of files in the index to a
+directory <<tree,tree>> and uses that tree in the new <<commit,commit>>.
+
+Each index entry has 4 fields:
+
+1. The *file mode*, which must be one of:
+  - `100644`: regular file (with <<object,object type>> `blob`)
+  - `100755`: executable file (with type `blob`)
+  - `120000`: symbolic link (with type `blob`)
+  - `160000`: gitlink, for use with submodules (with type `commit`)
+2. The *<<blob,blob>>* ID of the file,
+   or (rarely) the *<<commit,commit>>* ID of the submodule
+3. The *stage number*, either 0, 1, 2, or 3. This is normally 0, but if
+   there's a merge conflict there can be multiple versions of the same
+   filename in the index.
+4. The *file path*, for example `src/hello.py`
+
+It's extremely uncommon to look at the index directly: normally you'd
+run `git status` to see a list of changes between the index and <<HEAD,HEAD>>.
+But you can use `git ls-files --stage` to see the index.
+Here's the output of `git ls-files --stage` in a repository with 2 files:
+
+----
+100644 8728a858d9d21a8c78488c8b4e70e531b659141f 0 README.md
+100644 665c637a360874ce43bf74018768a96d2d4d219a 0 src/hello.py
+----
+
+[[reflogs]]
+REFLOGS
+-------
+
+Every time a branch, remote-tracking branch, or HEAD is updated, Git
+updates a log called a "reflog" for that <<references,reference>>.
+This means that if you make a mistake and "lose" a commit, you can
+generally recover the commit ID by running `git reflog <reference>`.
+
+A reflog is a list of log entries. Each entry has:
+
+1. The *commit ID*
+2. *Timestamp* when the change was made
+3. *Log message*, for example `pull: Fast-forward`
+
+Reflogs only log changes made in your local repository.
+They are not shared with remotes.
+
+You can view a reflog with `git reflog <reference>`.
+For example, here's the reflog for a `main` branch which has changed twice:
+
+----
+$ git reflog main --date=iso --no-decorate
+750b4ea main@{2025-09-29 15:17:05 -0400}: commit: Add README
+4ccb6d7 main@{2025-09-29 15:16:48 -0400}: commit (initial): Initial commit
+----
+
+GIT
+---
+Part of the linkgit:git[1] suite

Documentation/glossary-content.adoc

@@ -297,8 +297,8 @@ This commit is referred to as a "merge commit", or sometimes just a
 	identified by its <<def_object_name,object name>>. The objects usually
 	live in `$GIT_DIR/objects/`.
 
-[[def_object_identifier]]object identifier (oid)::
-	Synonym for <<def_object_name,object name>>.
+[[def_object_identifier]]object identifier, object ID, oid::
+	Synonyms for <<def_object_name,object name>>.
 
 [[def_object_name]]object name::
 	The unique identifier of an <<def_object,object>>.  The

Documentation/meson.build

@@ -192,6 +192,7 @@ manpages = {
   'gitcore-tutorial.adoc' : 7,
   'gitcredentials.adoc' : 7,
   'gitcvs-migration.adoc' : 7,
+  'gitdatamodel.adoc' : 7,
   'gitdiffcore.adoc' : 7,
   'giteveryday.adoc' : 7,
   'gitfaq.adoc' : 7,