Once files are added (or removed or moved), need to send those changes to
all the other git clones, at both the git level and the key/value level.

## git syncing

1. At regular intervals, just run `git annex sync`, which already handles
   bidirectional syncing.
2. Use a git merge driver that adds both conflicting files,
   so conflicts never break a sync.
3. Investigate the XMPP approach like dvcs-autosync does, or other ways of
   signaling a change out of band.
4. Add a hook, so when there's a change to sync, a program can be run.

## data syncing

There are two parts to data syncing. First, map the network and second,
decide what to sync when.

Mapping the network can reuse code in `git annex map`. Once the map is
built, we want to find paths through the network that reach all nodes
eventually, with the least cost. This is a minimum spanning tree problem,
except with a directed graph, so really a Arborescence problem.

With the map, we can determine which nodes to push new content to. Then we
need to control those data transfers, sending to the cheapest nodes first,
and with appropriate rate limiting and control facilities.

This probably will need lots of refinements to get working well.

## other considerations

This assumes the network is connected. It's often not, so the
cloud needs to be used to bridge between LANs.

It would be nice if, when a USB drive is connected,
syncing starts automatically.