There Is No Now – Problems with Simultaneity in Distributed Systems
Keeping an always consistent state in a large distributed you are fighting against the law of physics.Like it is mentioned, Google did it with their F1/Spanner SQL database. But that also mean GPS receivers with antennas on the roofs of data centers.Which is yet another thing that can fail and it, either by itself or in a cascade of other failures will lead to unspecified and possibly undesirable behavior.Recently I see a lot of advocates of dropping NoSQL databases and moving back to Postgress or other SQL databases.The problem is SQL and schemas is not the only reason NoSQL databases became popular, they became popular because they also started to have a default and more defined behavior in respect to replication and distribution.Most solutions don't need that and sticking with a solid single database works very well. But those that need distributed operation have a pretty hard task ahead of them.One heuristic you can look at is if and how is distribution implemented. Is it something that is bolted on top, like you download some proxy or addon, or added as an afterthought, be very careful. Those things should be baked into the core. For example, does it support CRTDs?  Does it have master to master replication and so on. If it claims to have consistency instead availability figure out how it is implemented, Paxos, Raft, or something else.So far I think Riak is probably the database that thought the hardest and did the most reasonable job. The other simpler database is CouchDB, they have well specified conflict resolution behavior and master to master replication. But you have to usually write your own cluster topology. There are probably others but those are the two I know of first hand.