You’ve been tasked with moving an e-commerce web application from a customer’s data center into a VPC. The application must be fault-tolerant and well as highly scalable. Moreover, the customer is adamant that service interruptions do not affect the user experience. As you near launch, you discover that the application currently uses multicast to share session state between web servers. In order to handle session state within the VPC, you choose the following option.

Answer options:

A.Enable session stickiness via Elastic Load Balancing.
B.Store session state in Amazon ElastiCache for Redis.
C.Create a mesh VPN between instances and allow multicast on it.
D.Store session state in Amazon Relational Database Service.

Answer – B
Option A is incorrect because ELB sticky sessions only cache user data locally for better performance. If the EC2 instance fails, the session data will still be lost.
Option B is CORRECT because Redis is a fast, open-source, in-memory data store and caching service. It is highly available, reliable, and with high performance suitable for the most demanding applications such as this one.
Option C is incorrect because Mesh VPN is not fault-tolerant or highly scalable - the client`s real priorities. Its failure would impact users. The supernode that handles the registration is a single point of failure. In case of failure, new VPN nodes would not be able to register. Also, the nodes wouldn`t register across multiple AZs. Even if it is possible, it is very cumbersome.
Option D is incorrect because storing the session state in RDS is not a good option.
For more information on Elastic Cache, please visit the below URL:
https://aws.amazon.com/elasticache/
Note:
Our main requirement is to provide fault tolerance and high scalability. 
Redis data resides in-memory, in contrast to databases that store data on disk or SSDs. By eliminating the need to access disks, in-memory data stores such as Redis avoid seeking time delays and access microseconds data. Redis is a popular choice for caching, session management, real-time analytics, geospatial, chat/messaging, media streaming, and gaming leaderboards.
ElastiCache Redis can provide high scalability and is fault-tolerant.