SQLite provides a similar benefit with tremendous results using its in-process database engine, although the benefit there is more muted by default because of the very small default cache size. We do have one app where we do this. There's no database server, the app server uses SQLite to talk directly to S3 and the app server itself caches its working set in memory. I can definitely see the benefit of some situations, but for us it was a pretty unusual situation that we might not ever encounter again.
All that said... can't Datomic also do traditional query execution on the server? I thought it had support for scale-out compute for that. AIUI, you have the option to run as either a full peer or just an RPC client on a case-by-case basis? I thought you wouldn't need to resort to writing your own API intermediary, you could just connect to Datomic via RPC, right?
AIUI, the full peer is Datomic; the RPC server is just a full peer that exposes the API over http and is mainly intended to be used with clients that don't run on the JVM (and so can't run Datomic itself in-process).
All that said... can't Datomic also do traditional query execution on the server? I thought it had support for scale-out compute for that. AIUI, you have the option to run as either a full peer or just an RPC client on a case-by-case basis? I thought you wouldn't need to resort to writing your own API intermediary, you could just connect to Datomic via RPC, right?