Can anyone elaborate as to how CockroachDB compares to Scylla? It seems to me like they're very similar on a surface level.
Cockroach DB is a full SQL compatible, strongly consistent key-value store. This means you can get the scalability and performance of a key-value store alongside the comfortable SQL query language. You, and all the other developers who work with you who already know SQL, can do pretty much all the things you know and love from SQL like joins, secondary indexes, etc with full ACID compliance. This means that when you read data from the database you always know with 100% certainty you're going to get up to date values (no stale reads).
ScyllaDB is similar in that data is stored in tables with a defined schema, but it uses a different query language, CQL[1], which is often similar to SQL. You can't to joins but you can have secondary indexes. You can store most of the same data types that you know and love from a standard SQL store. Interestingly enough, you get to CHOOSE the level of consistency you get, so you can make your ScyllaDB strongly consistent or choose from an array of eventually consistent options[2]. Most people however go with one of the eventually consistent options, which allow Scylla to be insanely performant and scalable. At the cost of strong consistency, you get an extremely high performance at an almost infinite scale. CockroachDB, while performant and scalable, can't match it here. It stands almost on a tier of it's own in terms of scalability and performance.
So really, the choice is yours based on what you're looking for. I'd choose CockroachDB for my purposes since I'm not storing Apple levels of data and consistency is important to my work, but your specifications and needs may be different.
[1] http://docs.scylladb.com/getting-started/ddl/ [2] http://docs.scylladb.com/architecture/architecture-fault-tol...