A developer is creating an application that will store personal health information (PHI). The PHI needs to be encrypted at all times. An encrypted…

The optimal solution is Amazon ElastiCache for Redis. It is a high-performance, in-memory caching service that meets all specified requirements. For handling sensitive Personal Health Information (PHI), ElastiCache for Redis supports both encryption in transit via Transport Layer Security (TLS) and encryption at rest. This ensures the data is protected at all times. Critically, Redis supports advanced data structures, including "Sorted Sets," which are specifically designed for maintaining ordered collections of data. This directly fulfills the requirement to efficiently sort and rank the cached datasets, a feature not available in other caching options like Memcached.

B: Amazon ElastiCache for Memcached does not support encryption at rest, which is a critical requirement for storing PHI and ensuring it is "encrypted at all times." C: An Amazon RDS read replica is a database instance, not a dedicated in-memory cache. It offloads read traffic but does not provide the low-latency performance of ElastiCache. D: This solution introduces unnecessary complexity by adding a new database system (DynamoDB) and its specific cache (DAX) to cache data from an existing RDS for MySQL database.

1. AWS ElastiCache User Guide, "Comparing Memcached and Redis": This document explicitly states that Redis supports advanced data types like "sorted sets," which are ideal for ranking. It also confirms that Memcached does not support encryption at rest. (See section: "Redis" and "Memcached" feature comparison table).

2. AWS ElastiCache User Guide, "At-Rest Encryption in ElastiCache for Redis": "ElastiCache for Redis at-rest encryption is an optional feature to increase data security by encrypting on-disk data... This is especially important when you have personally identifiable information (PII) or other confidential data." (See section: "At-Rest Encryption").

3. AWS ElastiCache User Guide, "In-Transit Encryption (TLS) in ElastiCache for Redis": "To help keep your data secure, Amazon ElastiCache for Redis and Amazon EC2 work together to protect your data in transit... You can enable in-transit encryption for your ElastiCache for Redis clusters." (See section: "In-Transit Encryption (TLS)").

4. AWS RDS User Guide, "Working with Amazon RDS read replicas": This document describes read replicas as a feature for scaling read-heavy database workloads, distinguishing their purpose from that of an in-memory cache designed for sub-millisecond latency. (See section: "Read replica use cases").