With Prevayler, all data is kept in RAM, reachable from one root object, which Prevayler holds. All changes to the data model must be expressed as command objects. Each object is handed to Prevayler, which serializes it to a log and then executes it. Once in a while, you can snapshot the data out and start a new log. If there's a crash, you just load the latest snapshot and replay the log.

You got exactly as much data integrity as you wrote into your objects and your commands. Which without much work could be quite a bit, because you get a lot of data integrity by not writing things. E.g., if a kind of object should never be deleted, you just don't write any deletion code. If, say, account balances should never be changed directly, but only as part of properly structured credits and debits, then you just write your code like that.

Most databases, Redis I think included, are made for arbitrary operations on data. Developers add integrity and security later, hopefully. And that is often duplicative of the code base, so that one ends up having integrity checks both in the code and in the database.

I should say that made a lot of sense for the era databases came out of. Databases were a huge step forward in the 1970s and 1980s. My dad was a developer in that era and it was a big relief not to have to get a bunch of programmers to all follow the same conventions for exactly which record was stored in exactly which spot on their precious and expensive disk drives. Not having to know the minutia of the hardware let a lot of people just get in there and build business reports. But if we were starting fresh today, I don't think we would do anything like a SQL database. Redis was definitely a step away from that era, and I look forward to many more.