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.

Note that by default, git annex sync only synchronises the git repositories, but does not transfer the content of annexed files. If you want to fully synchronise two repositories content, you can use git annex sync --content. You can also configure preferred content settings to make only some content be synced.