15.10Stateless design becomes important

If you have multiple replicas:

request 1 → replica A
request 2 → replica B

don't assume:

local in-memory variable from request 1

exists on replica B.

This applies to:

caches
dedupe state
watermarks
locks
session-like state

Durable/shared state needs an appropriate external mechanism such as:

Object Store
database
Redis
queue

depending on architecture.