Now that I have a good candidate, I find myself wondering whether the performances would be good enough to ditch traditional DBs (I know: I should define "workload" before thinking about "performances"...)
I guess is the curse of tradeoffs: nothing in life is free.
At least, TiDB, CRDB, RethinkDB are open source :)
So is CockroachDB too?
For any other issues you run into when using it, you may want to see the discussion about postgrex compatibility at https://github.com/cockroachdb/cockroach/issues/5582. If you run into different problems please do file the issue :).
I'm also an Elixir junkie, so I would also love to see ORM compatibility here! I definitely want to dedicated time to it if I can. (Disclaimer: I'm currently interning at Cockroach Labs.)
I know little about CockroachDB, but I vaguely remember that it has consistent replication. If so, then writes would require some round trips before consensus is reached, killing any latency benefits from having the master follow the load (unless you move all your replicas around!).
Reads go from 1 RTT to 0 RTTs, while writes go from 2 RTTs to 1 RTT. For example, consider a cluster in which the nodes are 200ms RTT away from each other.
* A read will take 200ms if the leaseholder isn't in the local datacenter vs single-digit milliseconds if it is local.
* A write will take 400ms if the leaseholder isn't in the local datacenter vs 200 milliseconds if it is local.
The write takes 400ms because it takes 100ms to get from the local datacenter to leaseholder, 200ms to reach consensus (the leaseholder needs to send a request to and get a response from one of the followers), and another 100ms for the response from the leaseholder to get back to the local datacenter where the request originated.
Is there a bit more detailed technical doc on how this is actually achieved ideally with some examples? When is it decided that the lease holder moves from one node to another? Is it based on some sort of stats?
Edit: found the doc follow-the-workload here: https://www.cockroachlabs.com/docs/stable/demo-follow-the-wo...
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...
CockroachDB also uses a key/value store but puts a postgres-compatible SQL layer on top, derived from the Google Spanner approach, so you can get (almost) all the querying abilities and data modeling of a relational database. They're slated to have JSON datatypes soon that will make it very compelling as a general purpose, highly reliable datastore for all of your core data in multiple regions.
For a more practical summary, compare the architecture overviews of Cassandra[2] and Cockroach[3].
0: http://www.allthingsdistributed.com/files/amazon-dynamo-sosp...
1: https://static.googleusercontent.com/media/research.google.c...
2: https://docs.datastax.com/en/cassandra/3.0/cassandra/archite...
3: https://www.cockroachlabs.com/docs/stable/architecture/overv...
The doc you found is the best resource. If you want even more detail, you can check out the original RFC: https://github.com/cockroachdb/cockroach/blob/master/docs/RF...
For the thing that does matter, which is ACID within the same partition/entity group, I would argue that RethinkDB already does have that.
Mike Stonebraker says that there are no fast distributed transactions [1]. I'm inclined to believe him, and that's not just an appeal to authority.
Suppose you write value a to leader A and then value b to leader A. Node B got both updates, node C got only the a value because of network timeouts.
Now you do a read from B of both values, you get the current a and b. Then you do a read from C, you receive the current a and the old b. Inconsistent.
Basically a violation of serializability and linearizability across the cluster in respect to the client.
The parent poster was referring to CockroachDB as CRDB.