git-annex/doc/how_it_works.mdwn
Joey Hess 7189dfd77d git-annex (5.20131127) unstable; urgency=low
* webapp: Detect when upgrades are available, and upgrade if the user
    desires.
    (Only when git-annex is installed using the prebuilt binaries
    from git-annex upstream, not from eg Debian.)
  * assistant: Detect when the git-annex binary is modified or replaced,
    and either prompt the user to restart the program, or automatically
    restart it.
  * annex.autoupgrade configures both the above upgrade behaviors.
  * Added support for quvi 0.9. Slightly suboptimal due to limitations in its
    interface compared with the old version.
  * Bug fix: annex.version did not get set on automatic upgrade to v5 direct
    mode repo, so the upgrade was performed repeatedly, slowing commands down.
  * webapp: Fix bug that broke switching between local repositories
    that use the new guarded direct mode.
  * Android: Fix stripping of the git-annex binary.
  * Android: Make terminal app show git-annex version number.
  * Android: Re-enable XMPP support.
  * reinject: Allow to be used in direct mode.
  * Futher improvements to git repo repair. Has now been tested in tens
    of thousands of intentionally damaged repos, and successfully
    repaired them all.
  * Allow use of --unused in bare repository.

# imported from the archive
2013-11-27 18:41:44 -04:00

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Markdown

This page gives a high-level view of git-annex. 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 instructions.
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 large 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.
---
Next: [[install]] or [[walkthrough]]