Hacker News Reader

Use Redis. It's an in-memory data structure server, first created to have the same data structures usable and accessible across processes and languages.

Or plenty of other in-memory database products (Apache Ignite) or fast key/value (ScyllaDB) or just use RDBMS (Postgres or SQLite).

I have been using SQLite DBs to that end with some amount of success. If I remember well a lot of software of yore used BDB to the same effect.

Apache Arrow?

As far as I understand it, it's more for cross-process in-memory fast access (e.g. numbers crunching), but it's mutable.

As far as I can tell, Apache Arrow is not very mutable. It basically has to reallocate the universe.

I don't need large data between processes. I need short-lived medium data ... large enough and short-lived enough to make serialization painful but mutable enough to make "stores" kind of annoying.