SAP-C02 Design for New Solutions Practice Question
A company is designing a new application that will run on Amazon EC2 instances in an Auto Scaling group behind an Application Load Balancer. The application requires that users' session data be stored durably and with low latency. The session data is accessed frequently but is rarely updated. Which solution is MOST cost-effective?
⚠ Common exam trap
Test-takers frequently choose DynamoDB for its durability and scalability, overlooking that its on-demand pricing is significantly more expensive than a provisioned Redis cluster for frequent reads with rare updates, or they may choose Memcached for its simplicity, forgetting that it lacks persistence and replication, which are required for durable session storage.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use ElastiCache for Redis with replication.
ElastiCache for Redis with replication is the most cost-effective solution because it provides durable, low-latency session storage with built-in replication for high availability. Redis supports persistence (e.g., snapshots or AOF logs) to ensure session data survives node failures, and its in-memory nature delivers sub-millisecond latency for frequent reads with rare updates, avoiding the higher costs of DynamoDB on-demand or the lack of durability in Memcached.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Use ElastiCache for Redis with replication.
Why this is correct
Redis provides low latency and durability via replication, suitable for session storage.
- ✗
Store session data in Amazon S3 with reduced redundancy.
Why it's wrong here
S3 has higher latency and is not designed for frequent access.
- ✗
Use DynamoDB with on-demand capacity.
Why it's wrong here
DynamoDB is more expensive than ElastiCache for this access pattern.
- ✗
Use ElastiCache for Memcached with multiple nodes.
Why it's wrong here
Memcached is not durable; data is lost on node failure.
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