These days we all are told to build stateless applications. Stateless apps are those that don’t store any state in the application process and fetch any state from a centralised datastore (it could be a global cache or a database). The sixth factor in 12 factor app also talk about the same principle.
Execute the app as one or more stateless processes
Twelve-factor processes are stateless and share-nothing. Any data that needs to persist must be stored in a stateful backing service, typically a database.
There are advantages in building stateless applications primary being ability to scale horizontally with ease. When we build stateless applications we push the scalability problem to the database. We expect our database to scale horizontally. This usually is solved by sticking a global cache (Redis or Memcached) in between. Scaling cache is relatively easy and solved problem. Keeping cache updated with updates is a hard problem. We will discuss it some other time.