The idea is provide more than one layer support as fallback.

Similar to cache L1 and L2 approach.
https://people.csail.mit.edu/sanchez/papers/2016.model.hpca.pdf

Just an example aproximation using redis and memory implementation.

get operations:

• first try to get the value from redis instance.
• it it fails, try to find the value in the memory implementation.
• under memory hit, move the key to redis.

set operations:

• if redis have a free slot, use it.
• if redis doesn't have a free slot but the slot it's available on memory, use it.
• otherwise, remove the oldest reference entry at memory implementation.