Data Sync

https://code.google.com/p/nsync/wiki/SyncAlgorithm
http://stackoverflow.com/questions/1928482/whats-the-standard-algorithm-for-syncing-two-lists-of-related-objects
http://programmers.stackexchange.com/questions/135423/best-practice-patterns-for-two-way-data-synchronisation
https://neil.fraser.name/writing/sync/