The Object Database
-------------------
-The object database is literally just a content-addressable collection
-of objects. All objects are named by their content, which is
-approximated by the SHA1 hash of the object itself. Objects may refer
-to other objects (by referencing their SHA1 hash), and so you can
-build up a hierarchy of objects.
-
-All objects have a statically determined "type" which is
-determined at object creation time, and which identifies the format of
-the object (i.e. how it is used, and how it can refer to other
-objects). There are currently four different object types: "blob",
-"tree", "commit", and "tag".
-A <<def_blob_object,"blob" object>> cannot refer to any other object,
-and is, as the name implies, a pure storage object containing some
-user data. It is used to actually store the file data, i.e. a blob
-object is associated with some particular version of some file.
-
-A <<def_tree_object,"tree" object>> is an object that ties one or more
-"blob" objects into a directory structure. In addition, a tree object
-can refer to other tree objects, thus creating a directory hierarchy.
-
-A <<def_commit_object,"commit" object>> ties such directory hierarchies
-together into a <<def_DAG,directed acyclic graph>> of revisions - each
-"commit" is associated with exactly one tree (the directory hierarchy at
-the time of the commit). In addition, a "commit" refers to one or more
-"parent" commit objects that describe the history of how we arrived at
-that directory hierarchy.
-
-As a special case, a commit object with no parents is called the "root"
-commit, and is the point of an initial project commit. Each project
-must have at least one root, and while you can tie several different
-root objects together into one project by creating a commit object which
-has two or more separate roots as its ultimate parents, that's probably
-just going to confuse people. So aim for the notion of "one root object
-per project", even if git itself does not enforce that.
-
-A <<def_tag_object,"tag" object>> symbolically identifies and can be
-used to sign other objects. It contains the identifier and type of
-another object, a symbolic name (of course!) and, optionally, a
-signature.
+We already saw in <<understanding-commits>> that all commits are stored
+under a 40-digit "object name". In fact, all the information needed to
+represent the history of a project is stored in objects with such names.
+In each case the name is calculated by taking the SHA1 hash of the
+contents of the object. The SHA1 hash is a cryptographic hash function.
+What that means to us is that it is impossible to find two different
+objects with the same name. This has a number of advantages; among
+others:
+
+- Git can quickly determine whether two objects are identical or not,
+ just by comparing names.
+- Since object names are computed the same way in ever repository, the
+ same content stored in two repositories will always be stored under
+ the same name.
+- Git can detect errors when it reads an object, by checking that the
+ object's name is still the SHA1 hash of its contents.
+
+(See <<object-details>> for the details of the object formatting and
+SHA1 calculation.)
+
+There are four different types of objects: "blob", "tree", "commit", and
+"tag".
+
+- A <<def_blob_object,"blob" object>> is used to store file data.
+- A <<def_tree_object,"tree" object>> is an object that ties one or more
+ "blob" objects into a directory structure. In addition, a tree object
+ can refer to other tree objects, thus creating a directory hierarchy.
+- A <<def_commit_object,"commit" object>> ties such directory hierarchies
+ together into a <<def_DAG,directed acyclic graph>> of revisions - each
+ commit contains the object name of exactly one tree designating the
+ directory hierarchy at the time of the commit. In addition, a commit
+ refers to "parent" commit objects that describe the history of how we
+ arrived at that directory hierarchy.
+- A <<def_tag_object,"tag" object>> symbolically identifies and can be
+ used to sign other objects. It contains the object name and type of
+ another object, a symbolic name (of course!) and, optionally, a
+ signature.
The object types in some more detail:
Commit Object
~~~~~~~~~~~~~
-The "commit" object is an object that introduces the notion of
-history into the picture. In contrast to the other objects, it
-doesn't just describe the physical state of a tree, it describes how
-we got there, and why.
-
-A "commit" is defined by the tree-object that it results in, the
-parent commits (zero, one or more) that led up to that point, and a
-comment on what happened. Again, a commit is not trusted per se:
-the contents are well-defined and "safe" due to the cryptographically
-strong signatures at all levels, but there is no reason to believe
-that the tree is "good" or that the merge information makes sense.
-The parents do not have to actually have any relationship with the
-result, for example.
-
-Note on commits: unlike some SCM's, commits do not contain
-rename information or file mode change information. All of that is
-implicit in the trees involved (the result tree, and the result trees
-of the parents), and describing that makes no sense in this idiotic
-file manager.
-
-A commit is created with gitlink:git-commit-tree[1] and
-its data can be accessed by gitlink:git-cat-file[1].
+The "commit" object links a physical state of a tree with a description
+of how we got there and why. Use the --pretty=raw option to
+gitlink:git-show[1] or gitlink:git-log[1] to examine your favorite
+commit:
+
+------------------------------------------------
+$ git show -s --pretty=raw 2be7fcb476
+commit 2be7fcb4764f2dbcee52635b91fedb1b3dcf7ab4
+tree fb3a8bdd0ceddd019615af4d57a53f43d8cee2bf
+parent 257a84d9d02e90447b149af58b271c19405edb6a
+author Dave Watson <dwatson@mimvista.com> 1187576872 -0400
+committer Junio C Hamano <gitster@pobox.com> 1187591163 -0700
+
+ Fix misspelling of 'suppress' in docs
+
+ Signed-off-by: Junio C Hamano <gitster@pobox.com>
+------------------------------------------------
+
+As you can see, a commit is defined by:
+
+- a tree: The SHA1 name of a tree object (as defined below), representing
+ the contents of a directory at a certain point in time.
+- parent(s): The SHA1 name of some number of commits which represent the
+ immediately prevoius step(s) in the history of the project. The
+ example above has one parent; merge commits may have more than
+ one. A commit with no parents is called a "root" commit, and
+ represents the initial revision of a project. Each project must have
+ at least one root. A project can also have multiple roots, though
+ that isn't common (or necessarily a good idea).
+- an author: The name of the person responsible for this change, together
+ with its date.
+- a committer: The name of the person who actually created the commit,
+ with the date it was done. This may be different from the author, for
+ example, if the author was someone who wrote a patch and emailed it
+ to the person who used it to create the commit.
+- a comment describing this commit.
+
+Note that a commit does not itself contain any information about what
+actually changed; all changes are calculated by comparing the contents
+of the tree referred to by this commit with the trees associated with
+its parents. In particular, git does not attempt to record file renames
+explicitly, though it can identify cases where the existence of the same
+file data at changing paths suggests a rename. (See, for example, the
+-M option to gitlink:git-diff[1]).
+
+A commit is usually created by gitlink:git-commit[1], which creates a
+commit whose parent is normally the current HEAD, and whose tree is
+taken from the content currently stored in the index.
[[tree-object]]
Tree Object
~~~~~~~~~~~
-The next hierarchical object type is the "tree" object. A tree object
-is a list of mode/name/blob data, sorted by name. Alternatively, the
-mode data may specify a directory mode, in which case instead of
-naming a blob, that name is associated with another TREE object.
-
-Like the "blob" object, a tree object is uniquely determined by the
-set contents, and so two separate but identical trees will always
-share the exact same object. This is true at all levels, i.e. it's
-true for a "leaf" tree (which does not refer to any other trees, only
-blobs) as well as for a whole subdirectory.
-
-For that reason a "tree" object is just a pure data abstraction: it
-has no history, no signatures, no verification of validity, except
-that since the contents are again protected by the hash itself, we can
-trust that the tree is immutable and its contents never change.
-
-So you can trust the contents of a tree to be valid, the same way you
-can trust the contents of a blob, but you don't know where those
-contents 'came' from.
-
-Side note on trees: since a "tree" object is a sorted list of
-"filename+content", you can create a diff between two trees without
-actually having to unpack two trees. Just ignore all common parts,
-and your diff will look right. In other words, you can effectively
-(and efficiently) tell the difference between any two random trees by
-O(n) where "n" is the size of the difference, rather than the size of
-the tree.
-
-Side note 2 on trees: since the name of a "blob" depends entirely and
-exclusively on its contents (i.e. there are no names or permissions
-involved), you can see trivial renames or permission changes by
-noticing that the blob stayed the same. However, renames with data
-changes need a smarter "diff" implementation.
-
-A tree is created with gitlink:git-write-tree[1] and
-its data can be accessed by gitlink:git-ls-tree[1].
-Two trees can be compared with gitlink:git-diff-tree[1].
+The ever-versatile gitlink:git-show[1] command can also be used to
+examine tree objects, but gitlink:git-ls-tree[1] will give you more
+details:
+
+------------------------------------------------
+$ git ls-tree fb3a8bdd0ce
+100644 blob 63c918c667fa005ff12ad89437f2fdc80926e21c .gitignore
+100644 blob 5529b198e8d14decbe4ad99db3f7fb632de0439d .mailmap
+100644 blob 6ff87c4664981e4397625791c8ea3bbb5f2279a3 COPYING
+040000 tree 2fb783e477100ce076f6bf57e4a6f026013dc745 Documentation
+100755 blob 3c0032cec592a765692234f1cba47dfdcc3a9200 GIT-VERSION-GEN
+100644 blob 289b046a443c0647624607d471289b2c7dcd470b INSTALL
+100644 blob 4eb463797adc693dc168b926b6932ff53f17d0b1 Makefile
+100644 blob 548142c327a6790ff8821d67c2ee1eff7a656b52 README
+...
+------------------------------------------------
+
+As you can see, a tree object contains a list of entries, each with a
+mode, object type, SHA1 name, and name, sorted by name. It represents
+the contents of a single directory tree.
+
+The object type may be a blob, representing the contents of a file, or
+another tree, representing the contents of a subdirectory. Since trees
+and blobs, like all other objects, are named by the SHA1 hash of their
+contents, two trees have the same SHA1 name if and only if their
+contents (including, recursively, the contents of all subdirectories)
+are identical. This allows git to quickly determine the differences
+between two related tree objects, since it can ignore any entries with
+identical object names.
+
+(Note: in the presence of submodules, trees may also have commits as
+entries. See gitlink:git-submodule[1] and gitlink:gitmodules.txt[1]
+for partial documentation.)
+
+Note that the files all have mode 644 or 755: git actually only pays
+attention to the executable bit.
[[blob-object]]
Blob Object
~~~~~~~~~~~
-A "blob" object is nothing but a binary blob of data, and doesn't
-refer to anything else. There is no signature or any other
-verification of the data, so while the object is consistent (it 'is'
-indexed by its sha1 hash, so the data itself is certainly correct), it
-has absolutely no other attributes. No name associations, no
-permissions. It is purely a blob of data (i.e. normally "file
-contents").
+You can use gitlink:git-show[1] to examine the contents of a blob; take,
+for example, the blob in the entry for "COPYING" from the tree above:
+
+------------------------------------------------
+$ git show 6ff87c4664
+
+ Note that the only valid version of the GPL as far as this project
+ is concerned is _this_ particular version of the license (ie v2, not
+ v2.2 or v3.x or whatever), unless explicitly otherwise stated.
+...
+------------------------------------------------
-In particular, since the blob is entirely defined by its data, if two
-files in a directory tree (or in multiple different versions of the
-repository) have the same contents, they will share the same blob
-object. The object is totally independent of its location in the
-directory tree, and renaming a file does not change the object that
-file is associated with in any way.
+A "blob" object is nothing but a binary blob of data. It doesn't refer
+to anything else or have attributes of any kind.
-A blob is typically created when gitlink:git-update-index[1]
-is run, and its data can be accessed by gitlink:git-cat-file[1].
+Since the blob is entirely defined by its data, if two files in a
+directory tree (or in multiple different versions of the repository)
+have the same contents, they will share the same blob object. The object
+is totally independent of its location in the directory tree, and
+renaming a file does not change the object that file is associated with.
+
+Note that any tree or blob object can be examined using
+gitlink:git-show[1] with the <revision>:<path> syntax. This can
+sometimes be useful for browsing the contents of a tree that is not
+currently checked out.
[[trust]]
Trust
~~~~~
-An aside on the notion of "trust". Trust is really outside the scope
-of "git", but it's worth noting a few things. First off, since
-everything is hashed with SHA1, you 'can' trust that an object is
-intact and has not been messed with by external sources. So the name
-of an object uniquely identifies a known state - just not a state that
-you may want to trust.
+If you receive the SHA1 name of a blob from one source, and its contents
+from another (possibly untrusted) source, you can still trust that those
+contents are correct as long as the SHA1 name agrees. This is because
+the SHA1 is designed so that it is infeasible to find different contents
+that produce the same hash.
-Furthermore, since the SHA1 signature of a commit refers to the
-SHA1 signatures of the tree it is associated with and the signatures
-of the parent, a single named commit specifies uniquely a whole set
-of history, with full contents. You can't later fake any step of the
-way once you have the name of a commit.
+Similarly, you need only trust the SHA1 name of a top-level tree object
+to trust the contents of the entire directory that it refers to, and if
+you receive the SHA1 name of a commit from a trusted source, then you
+can easily verify the entire history of commits reachable through
+parents of that commit, and all of those contents of the trees referred
+to by those commits.
So to introduce some real trust in the system, the only thing you need
to do is to digitally sign just 'one' special note, which includes the
Tag Object
~~~~~~~~~~
-Git provides the "tag" object to simplify creating, managing and
-exchanging symbolic and signed tokens. The "tag" object at its
-simplest simply symbolically identifies another object by containing
-the sha1, type and symbolic name.
-
-However it can optionally contain additional signature information
-(which git doesn't care about as long as there's less than 8k of
-it). This can then be verified externally to git.
+A tag object contains an object, object type, tag name, the name of the
+person ("tagger") who created the tag, and a message, which may contain
+a signature, as can be seen using the gitlink:git-cat-file[1]:
-Note that despite the tag features, "git" itself only handles content
-integrity; the trust framework (and signature provision and
-verification) has to come from outside.
+------------------------------------------------
+$ git cat-file tag v1.5.0
+object 437b1b20df4b356c9342dac8d38849f24ef44f27
+type commit
+tag v1.5.0
+tagger Junio C Hamano <junkio@cox.net> 1171411200 +0000
+
+GIT 1.5.0
+-----BEGIN PGP SIGNATURE-----
+Version: GnuPG v1.4.6 (GNU/Linux)
+
+iD8DBQBF0lGqwMbZpPMRm5oRAuRiAJ9ohBLd7s2kqjkKlq1qqC57SbnmzQCdG4ui
+nLE/L9aUXdWeTFPron96DLA=
+=2E+0
+-----END PGP SIGNATURE-----
+------------------------------------------------
-A tag is created with gitlink:git-mktag[1],
-its data can be accessed by gitlink:git-cat-file[1],
-and the signature can be verified by
-gitlink:git-verify-tag[1].
+See the gitlink:git-tag[1] command to learn how to create and verify tag
+objects. (Note that gitlink:git-tag[1] can also be used to create
+"lightweight tags", which are not tag objects at all, but just simple
+references in .git/refs/tags/).
[[the-index]]
be useful when doing unusual things with git, or just as a way to
understand its inner workings.
+[[object-manipulation]]
+Object access and manipulation
+------------------------------
+
+The gitlink:git-cat-file[1] command can show the contents of any object,
+though the higher-level gitlink:git-show[1] is usually more useful.
+
+The gitlink:git-commit-tree[1] command allows constructing commits with
+arbitrary parents and trees.
+
+A tree can be created with gitlink:git-write-tree[1] and its data can be
+accessed by gitlink:git-ls-tree[1]. Two trees can be compared with
+gitlink:git-diff-tree[1].
+
+A tag is created with gitlink:git-mktag[1], and the signature can be
+verified by gitlink:git-verify-tag[1], though it is normally simpler to
+use gitlink:git-tag[1] for both.
+
[[the-workflow]]
The Workflow
------------
projects / git.git / commitdiff