In the process of writing project, faced with the fact that many customers were required to forward applications and references and collect data on the activities of the branches. And to find a solution satisfying all the conditions failed. Came to the idea to write yourself.
What tools do you use, given these conditions (all or any combination of them)?
1. The server removed the long-distance (different cities)
2. Unstable/narrow channel of the Internet (e.g. communication with Kamchatka or Magadan)
3. Data synchronization in both directions (not just data collection or distribution directories)
Also interesting for some database servers used synchronization.
PS. The question was born out of a desire to write an article "how do we synchronize the deleted data".