Git File 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 committed? The commit contains a reference to a tree.

The Tree

A tree is a collection of file information, the state of a single directory at a given point in time.

A tree file looks like this:

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

And repeats for every file in the tree.

Note that 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 versioning.

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 up most of the contents of a Git repository and are used for the history. They can either appear as simple files on disk (one file per object) or in a pack file, which is a container for a number of these objects.

The is also an index of the current state of the working copy in the repository as well as files to track the existing branches and tags.

For a more detailed 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.