Oh, sorry, I was imprecise: I meant "distributed database as a library". As in, you compile/bundle your application logic together with the persistence/replication/transaction logic and that is what you're running in a cluster.

Datomic is a distributed database written in Clojure, and is best described as "cqrs-as-a-service". It allows you to run a "peer" inside your local Java VM, which essentially does exactly this: it caches the parts your local vm needs, and if not it looks elsewhere.

https://docs.datomic.com/on-prem/peer-server.html

This is do-able with FoundationDB. You could distribute your application colocated with the database nodes and write a router to redirect requests to the machine which owns the data you're looking for. Then your application logic is computing against data on the same machine.

FoundationDB exposes the key-range to node mapping to you to make this router possible.

The BeamVM languages (Erlang, Elixir) seem to have this in the form of Mnesia, and libraries that are built on top of it, such as riak-core.

Ha! I was just about to ask about that.

‘I bet Elixir already has something like this’.

Riak uses Mnesia?

Not that I know, but it uses ETS to handle state, and the global process registry to store peer information, as far as I know.

Also, be aware that OP mentioned riak-core, which is a different thing from Riak. riak-core is a library to build dynamo-style distributed applications, which gives you virtual nodes, consistent hashing and data migration as building blocks.

Riak KV uses leveldb, but as @ergl mentioned, riak-core is essentially a hash-ring and replication (aka dynamo) layer.

EHCache has tons of capabilities and is bundled directly into your app('s war file)

Apache Geode, Hazelcast, how some people are using Kafka. I am just listing a few others, please don't consider this endorsement.