git-annex/doc/how_it_works.mdwn

This page gives a high-level view of how git-annex works. For a detailed
low-level view, see [[the_man_page|git-annex]] and [[internals]].

You do not need to read this page to get started with using git-annex. The
[[walkthrough]] provides step-by-step examples, and [[workflow]] discusses
different ways you can use git-annex.

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Still reading? Ok. Git's man page calls it "a stupid content
tracker". With git-annex, git is instead "a stupid filename and metadata
tracker". The contents of annexed files are not stored in git, only the
names of the files and some other metadata remain there.

The contents of the files are kept by git-annex in a distributed key/value
store consisting of every clone of a given git repository. That's a fancy
way to say that git-annex stores the actual file content somewhere under
`.git/annex/`. (See [[internals]] for details.)

That was the values; what about the keys? Well, a key is calculated for a
given file when it's first added into git-annex. Normally this uses a hash
of its contents, but various [[backends]] can produce different sorts of
keys. The file that gets checked into git is just a symlink to the key
under `.git/annex/`. If the content of a file is modified, that produces
a different key (and the symlink is changed).

A file's content can be [[transferred|transferring_data]] from one
repository to another by git-annex. Which repositories contain a given
value is tracked by git-annex (see [[location_tracking]]). It stores this
tracking information in a separate branch, named "git-annex". All you ever
do with the "git-annex" branch is push/pull it around between repositories,
to [[sync]] up git-annex's view of the world.

That's really all there is to it. Oh, there are [[special_remotes]] that
let values be stored other places than git repositories (anything from
Amazon S3 to a USB key), and there's a pile of commands listed in
[[the_man_page|git-annex]] to handle moving the values around and managing
them. But if you grok the description above, you can see through all that.
It's really just symlinks, keys, values, and a git-annex branch to store
additional metadata.

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Next: [[install]] or [[walkthrough]]