https://github.com/cockroachdb/cockroach
Inspired by Google Spanner.
Or use posgres-xc, or mariadb 10 with galera and maxscale. Postgres has JSON storage. MariaDBB has dynamic and virtual columns, and the CONNECT engine which can now directly create new or edit existing JSON files using the SQL interface, instead of dedicated functions for JSON manipulation.
postgres-xc/xl has no HA
Maxscale looks like it doesn't yet offer sharding
Cassandra for example: you can't just put a secondary index on a table to enable sorting on it. There are secondary indexes, but they aren't useful for that (they don't perform well on high cardinality data).
Want a different sort? Make a new table. And duplicate the data, since there are no JOINs.
I find this really cumbersome.
Some other new database engines do it in different order - first adding as many RDBMS features as possible and then making some of them kinda scalable (in some circumstances, if the phase of the moon is right, etc.). Sure, that might feel less cumbersome at the beginning on a single laptop (feels like good old RDBMS), but once you scale it out, it is far from being all roses.
BTW, duplicating (denormalizing) data is the way to go if you really need scalability. In general case, joins or traditional indexes with references from the index leaves to the original data do not scale on distributed data sets.