Merge branch 'je/doc-data-model'

Add a new manual that describes the data model.

* je/doc-data-model:
  doc: add an explanation of Git's data model

Author: gitster

Documentation/Makefile

@@ -53,6 +53,7 @@ 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 +1,307 @@
+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 these required fields
+    (though there are other optional fields):
++
+1. The full directory structure of all the files in that version of the
+   repository and each file's contents, stored as the *<<tree,tree>>* ID
+   of the commit's top-level 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 type*, which must be one of these five types:
+  - *regular file*
+  - *executable file*
+  - *symbolic link*
+  - *directory*
+  - *gitlink* (for use with submodules)
+3. The <<object-id,*object ID*>> with the contents of the file, directory,
+   or gitlink.
++
+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
+----
+
+NOTE: In the output above, Git displays the file type of each tree entry
+using a format that's loosely modelled on Unix file modes (`100644` is
+"regular file", `100755` is "executable file", `120000` is "symbolic
+link", `040000` is "directory", and `160000` is "gitlink"). It also
+displays the object's type: `blob` for files and symlinks, `tree` for
+directories, and `commit` for gitlinks.
+
+[[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 *ID* of the object it references
+2. The *type* of the object it references
+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: Git may delete objects that aren't "reachable" from any reference
+or <<reflogs,reflog>>.
+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`,
+there will no longer be a branch that points at the old commit.
+The old commit is recorded in the current branch's <<reflogs,reflog>>,
+so it is still "reachable", but when the reflog entry expires it may
+become unreachable and get deleted.
+
+the old commit will usually not be reachable, so it may be deleted eventually.
+Reachable objects will never be deleted.
+
+[[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 type*, which must be one of:
+  - *regular file*
+  - *executable file*
+  - *symbolic link*
+  - *gitlink* (for use with submodules)
+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

@@ -193,6 +193,7 @@ manpages = {
   'gitcore-tutorial.adoc' : 7,
   'gitcredentials.adoc' : 7,
   'gitcvs-migration.adoc' : 7,
+  'gitdatamodel.adoc' : 7,
   'gitdiffcore.adoc' : 7,
   'giteveryday.adoc' : 7,
   'gitfaq.adoc' : 7,