Caché is more oriented towards vertical scaling, big monolithic servers, where FoundationDB is more towards horizontal scaling.

Also, the obvious difference is that Caché is closed source and prohibitively expensive.

FiS is only FOSS on selected platforms (Linux x86, OpenVMS Alpha), and proprietary on all other platforms.

Oh, Cache...

Worked a job once where that was the underlying data store.

I was only allowed to touch the SQL interface to it, which was....weird.

The SQL dialect was ancient, felt like something from about 1990 (and this was in.... 2012 or so, so not THAT long ago).

Query performance seemed invariant. A simple select * from foo where id=X and a monster programatically generated join across 15 tables would both take about 1.5 seconds to return results.

It seems that MUMPS doesn't have serializable isolation.

Yeah, when I read about that I thought it sounded neat—make sure the index updates with the data. On the other hand, I think of CAP theorem as an iron triangle. If you are gaining consistency, what’s the trade off?

This _is_ the usual trade off, but what makes FoundationDB so crazy is that it's a CP system that has a performance profile that AP systems would have a hard time matching.