What are the best software libraries to synchronize changes to JSON data stored on a server?

There are many problems with this topic.

1. There is no such thing as JSON-ish data. It is either JSON, or not.
2. Syncronization targets are not given. Is it server-to-server or client-to-server?
3. Syncronization directions are not given. Is it one-way (to the server) or two-way (to and from the server, and/or client)?
4. No library can sync data on its own. It needs to be running on/in some application. If by library you mean "package X for language Y", please clarify.

This question was emailed on 2017-08-27; if possible, somebody should take this down / archive it / delete it.