git-annex/doc/sync.mdwn

The `git annex sync` command provides an easy way to keep several
repositories in sync. 

Often git is used in a centralized fashion with a central bare repository
which changes are pulled and pushed to using normal git commands.
That works fine, if you don't mind having a central repository.

But it can be harder to use git in a fully decentralized fashion, with no
central repository and still keep repositories in sync with one another.
You have to remember to pull from each remote, and merge the appropriate
branch after pulling. It's difficult to *push* to a remote, since git does
not allow pushes into the currently checked out branch.

`git annex sync` makes it easier using a scheme devised by Joachim
Breitner. The idea is to have a branch `synced/master` (actually,
`synced/$currentbranch`), that is never directly checked out, and serves
as a drop-point for other repositories to use to push changes.

When you run `git annex sync`, it merges the `synced/master` branch
into `master`, receiving anything that's been pushed to it. (If there is a
conflict in this merge, [[automatic_conflict_resolution]] is used to
resolve it). Then it fetches from each remote, and merges in any changes that
have been made to the remotes too. Finally, it updates `synced/master`
to reflect the new state of `master`, and pushes it out to each of the remotes.

This way, changes propagate around between repositories as `git annex sync`
is run on each of them. Every repository does not need to be able to talk
to every other repository; as long as the graph of repositories is
connected, and `git annex sync` is run from time to time on each, a given
change, made anywhere, will eventually reach every other repository.

The workflow for using `git annex sync` is simple:

* Make some changes to files in the repository, using `git-annex`,
  or anything else.
* Run `git annex sync` to save the changes.
* Next time you're working on a different clone of that repository,
  run `git annex sync` to update it.