A company runs a web application on Amazon EC2 instances in an Auto Scaling group behind an Application Load Balancer that has sticky sessions enabled. The web server currently hosts the user session state. The company wants to ensure high availability and avoid user session state loss in the event of a web server outage. Which solution will meet these requirements?

The correct answer is to use Amazon ElastiCache for Redis. Redis is an in-memory data store that supports replication and persistence, making it ideal for highly available session state management. It offers fast read/write operations crucial for web applications and can be configured for high availability with master-replica setups. Using Amazon ElastiCache for Memcached is less suitable because Memcached is a simpler caching system that does not inherently offer persistence or replication for high availability of session state. If a Memcached node fails, session data could be lost. AWS Storage Gateway cached volumes are designed for hybrid cloud storage and file sharing, not for low-latency, high-throughput session state management for web applications. Amazon RDS is a relational database service. While it can store session data, it introduces higher latency and overhead compared to an in-memory solution like Redis, making it less optimal for performance-critical session management.