dulwich/docs/tutorial/0-introduction.txt

Introduction
============

Git repository format
---------------------

For a better understanding of Dulwich, we'll start by explaining most of the
Git secrets.

Open the ".git" folder of any Git-managed repository. You'll find folders
like "branches", "hooks"... We're only interested in "objects" here. Open it.

You'll mostly see 2 hex-digits folders. Git identifies content by its SHA-1
digest. The 2 hex-digits plus the 38 hex-digits of files inside these folders
form the 40 characters (or 20 bytes) id of Git objects you'll manage in
Dulwich.

We'll first study the three main objects:

- The Commit;

- The Tree;

- The Blob.

The Commit
----------

You're used to generate commits using Git. You have set up your name and
e-mail, and you know how to see the history using ``git log``.

A commit file looks like this::

  commit <content length><NUL>tree <tree sha>
  parent <parent sha>
  [parent <parent sha> if several parents from merges]
  author <author name> <author e-mail> <timestamp> <timezone>
  committer <author name> <author e-mail> <timestamp> <timezone>
 
  <commit message>

But where are the changes you commited? The commit contains a reference to a
tree.

The Tree
--------

A tree is a collection of file information, the state of your working copy at
a given point in time.

A tree file looks like this::

  tree <content length><NUL><file mode> <filename><NUL><blob sha>...

And repeats for every file in the tree.

Note that for a unknown reason, the SHA-1 digest is in binary form here.

The file mode is like the octal argument you could give to the ``chmod``
command.  Except it is in extended form to tell regular files from
directories and other types.

We now know how our files are referenced but we haven't found their actual
content yet. That's where the reference to a blob comes in.

The Blob
--------

A blob is simply the content of files you are versionning.

A blob file looks like this::

  blob <content length><NUL><content>

If you change a single line, another blob will be generated by Git at commit
time. This is how Git can fastly checkout any version in time.

On the opposite, several identical files with different filenames generate
only one blob. That's mostly how renames are so cheap and efficient in Git.

Dulwich Objects
---------------

Dulwich implements these three objects with an API to easily access the
information you need, while abstracting some more secrets Git is using to
accelerate operations and reduce space.

More About Git formats
----------------------

These three objects make 90 % of a Git repository. The rest is branch
information and optimizations.

For instance there is an index of the current state of the working copy.
There are also pack files to group several small objects in a single indexed
file.

For a more detailled explanation of object formats and SHA-1 digests, see:
http://www-cs-students.stanford.edu/~blynn/gitmagic/ch08.html

Just note that recent versions of Git compress object files using zlib.