git-fat 0.1.0

Manage large binary files with git

git-fat
=======

Introduction
------------

Checking large binary files into a source repository (Git or otherwise) is a bad idea because repository size quickly
becomes unreasonable. Even if the instantaneous working tree stays manageable, preserving repository integrity
requires all binary files in the entire project history, which given the typically poor compression of binary diffs,
implies that the repository size will become impractically large. Some people recommend checking binaries into
different repositories or even not versioning them at all, but these are not satisfying solutions for most workflows.

Features of `git-fat`
---------------------

* clones of the source repository are small and fast because no binaries are transferred, yet fully functional (unlike `git clone --depth`)
* `git-fat` supports the same workflow for large binaries and traditionally versioned files, but internally manages the "fat" files separately
* `git-bisect` works properly even when versions of the binary files change over time
* selective control of which large files to pull into the local store
* local fat object stores can be shared between multiple clones, even by different users
* can easily support fat object stores distributed across multiple hosts
* depends only on stock Python and rsync

Installation and configuration
------------------------------

Place `git-fat` in your `PATH`.

Edit `.gitattributes` to regard any desired extensions as fat files.

$ cat >> .gitattributes
*.png filter=fat -crlf
*.jpg filter=fat -crlf
*.gz filter=fat -crlf
^D

Run `git fat init` to active the extension. Now add and commit as usual.
Matched files will be transparently stored externally, but will appear
complete in the working tree.

Set a remote store for the fat objects by editing `.gitfat`.

[rsync]
remote = your.remote-host.org:/share/fat-store

This file should typically be committed to the repository so that others
will automatically have their remote set. This remote address can use
any protocol supported by rsync. Most users will configure it to use
remote ssh in a directory with shared access.

You can also set an http url for a store in the `.gitfat` file. This url
serves as the root of the store and can only be cloned from, not pushed to.

[http]
remote = http://store.example.com/share/fat-store

This is useful for things like automated builds and public repositories
so that the anonymous users don't need to configure ssh.

A worked example
----------------

Before we start, let's turn on verbose reporting so we can see what's
happening. Without this environment variable, all the output lines
starting with `git-fat` will not be shown.

$ export GIT_FAT_VERBOSE=1

First, we create a repository and configure it for use with `git-fat`.

$ git init repo
Initialized empty Git repository in /tmp/repo/.git/
$ cd repo
$ git fat init
$ cat > .gitfat
[rsync]
remote = localhost:/tmp/fat-store
$ mkdir -p /tmp/fat-store # make sure the remote directory exists
$ echo '*.gz filter=fat -crlf' > .gitattributes
$ git add .gitfat .gitattributes
$ git commit -m'Initial repository'
[master (root-commit) eb7facb] Initial repository
2 files changed, 3 insertions(+)
create mode 100644 .gitattributes
create mode 100644 .gitfat

Now we add a binary file whose name matches the pattern we set in `.gitattributes`.

$ curl https://nodeload.github.com/jedbrown/git-fat/tar.gz/master -o master.tar.gz
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 6449 100 6449 0 0 7741 0 --:--:-- --:--:-- --:--:-- 9786
$ git add master.tar.gz
git-fat filter-clean: caching to /tmp/repo/.git/fat/objects/b3489819f81603b4c04e8ed134b80bace0810324
$ git commit -m'Added master.tar.gz'
[master b85a96f] Added master.tar.gz
git-fat filter-clean: caching to /tmp/repo/.git/fat/objects/b3489819f81603b4c04e8ed134b80bace0810324
1 file changed, 1 insertion(+)
create mode 100644 master.tar.gz

The patch itself is very simple and does not include the binary.

$ git show --pretty=oneline HEAD
918063043a6156172c2ad66478c6edd5c7df0217 Add master.tar.gz
diff --git a/master.tar.gz b/master.tar.gz
new file mode 100644
index 0000000..12f7d52
--- /dev/null
+++ b/master.tar.gz
@@ -0,0 +1 @@
+#$# git-fat 1f218834a137f7b185b498924e7a030008aee2ae

Pushing fat files
-----------------
Now let's push our fat files using the rsync configuration that we set up earlier.

$ git fat push
Pushing to localhost:/tmp/fat-store
building file list ...
1 file to consider

sent 61 bytes received 12 bytes 48.67 bytes/sec
total size is 6449 speedup is 88.34

We we might normally set a remote now and push the git repository.

Cloning and pulling
-------------------

Now let's look at what happens when we clone.

$ cd ..
$ git clone repo repo2
Cloning into 'repo2'...
done.
$ cd repo2
$ git fat init # don't forget
$ ls -l # file is just a placeholder
total 4
-rw-r--r-- 1 jed users 53 Nov 25 22:42 master.tar.gz
$ cat master.tar.gz # holds the SHA1 of the file
#$# git-fat 1f218834a137f7b185b498924e7a030008aee2ae

We can always get a summary of what fat objects are missing in our local cache.

Orphan objects:
1f218834a137f7b185b498924e7a030008aee2ae

Now get any objects referenced by our current `HEAD`. This command also
accepts the `--all` option to pull full history, or a revision to pull
selected history.

$ git fat pull
receiving file list ...
1 file to consider
1f218834a137f7b185b498924e7a030008aee2ae
6449 100% 6.15MB/s 0:00:00 (xfer#1, to-check=0/1)

sent 30 bytes received 6558 bytes 4392.00 bytes/sec
total size is 6449 speedup is 0.98
Restoring 1f218834a137f7b185b498924e7a030008aee2ae -> master.tar.gz
git-fat filter-smudge: restoring from /tmp/repo2/.git/fat/objects/1f218834a137f7b185b498924e7a030008aee2ae

Everything is in place

$ git status
git-fat filter-clean: caching to /tmp/repo2/.git/fat/objects/1f218834a137f7b185b498924e7a030008aee2ae
# On branch master
nothing to commit, working directory clean
$ ls -l # recovered the full file
total 8
-rw-r--r-- 1 jed users 6449 Nov 25 17:10 master.tar.gz

Summary
-------

* Set the "fat" file types in `.gitattributes`.
* Use normal git commands to interact with the repository without
thinking about what files are fat and non-fat. The fat files will be
treated specially.
* Synchronize fat files with `git fat push` and `git fat pull`.

Implementation notes
--------------------

The actual binary files are stored in `.git/fat/objects`, leaving `.git/objects` nice and small.

$ du -bs .git/objects
2212 .git/objects/
$ ls -l .git/fat/objects # This is where the file actually goes, but that's not important
total 8
-rw------- 1 jed users 6449 Nov 25 17:01 1f218834a137f7b185b498924e7a030008aee2ae

If you have multiple clones that access the same filesystem, you can make
`.git/fat/objects` a symlink to a common location, in which case all content
will be available in all repositories without extra copies. You still need to
`git fat push` to make it available to others.

For many commands, `git-fat` by default only checks the current HEAD for placeholder files to clone. This can
save on bandwidth for frequently changing large files and also saves on processing time for very large repositories.
To force commands to search the entire history for placeholders and pull all files, call `git-fat` with `-a`. e.g.

git fat -a pull

Related projects
----------------

* [git-annex](http://git-annex.branchable.com) is a far more comprehensive solution, but with less transparent workflow and with more dependencies.
* [git-media](https://github.com/schacon/git-media) adopts a similar approach to `git-fat`, but with a different synchronization philosophy and with many Ruby dependencies.

### Some refinements ###

* Allow pushing only select files
* Relate orphan objects to file system
* Put some more useful message in smudged (working tree) version of missing files.
* More friendly configuration for multiple fat remotes
* Make commands safer in presence of a dirty tree.
* Private setting of a different remote.
* Gracefully handle unmanaged files when the filter is called (either
legacy files or files matching the pattern that should some reason not
be treated as fat).