Robust scalable and fault-tolerant architectures are designed using stateless servers: servers which do not handle any user state like shopping carts, state between pages and authentication tokens. Services like DynamoDB and ElastiCache are great options to offload temporary session data to an external service, so when applications fail they do it gracefully.

Stateful applications, on the other hand, do manage conversational state with the end user and are not fault tolerant, for these legacy applications you can enable sticky sessions which are a good way to provide session affinity with the persistent server; once this attribute is enabled the load balancer will send a session cookie named AWSELB to route all future requests to the same server.