Is there a CAHCE / LRU Cache solution built for Appian ?

so that process model are not instantiated every time

Instantiated as in when you open some given process model in the Process Modeler?

Basically have a Singleton pattern for the process models

Could you please describe a bit more what you want to do. I implemented a solution that uses DB for a lock-like pattern to prevent simultaneous modification of a record with related actions.

Simple use case for the cache will be a cache the values from a database table and not hit the DB everytime