Redis has optional durability

https://redis.io/docs/manual/persistence/

More like RocksDb, LevelDb or Sled - which are all just just libraries instead of services with a remote API.

Why write anything to disk if you can just store it in an array?

Just one example: sharing data between processes

If only there was a standardized, robust, widely available cross-platform Message Passing Interface that could do this.

I don't grok why people outside of HPC seem to be shunning MPI. The shared-nothing memory model and asynchronous nature of MPI makes it very similar in spirit to a lot of the current web dev tech, AFAICT.

If you’re not going to store the data on a fixed medium, you can use mmap for that.

And redis/memcached are like 1% of use cases for database.