SAP-C02 Continuous Improvement for Existing Solutions Practice Question
A company runs a critical web application on Amazon EC2 instances behind an Application Load Balancer. The application stores session state in a self-managed Redis cluster on a single EC2 instance. During a recent load test, the Redis instance became a bottleneck, causing session timeouts and degraded performance. The company wants to improve the scalability and availability of the session store while minimizing application changes. Which solution meets these requirements?
⚠ Common exam trap
The trap here is assuming that sticky sessions or a second Redis instance can solve the scalability and availability issues without application changes.
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
✓
Migrate the session store to Amazon ElastiCache for Redis with cluster mode enabled and multiple shards.
ElastiCache for Redis with cluster mode enabled offers a managed, scalable, and highly available session store that integrates with existing Redis clients. It removes the single-instance bottleneck by distributing data across shards and provides automatic failover. Other options either require extensive application changes or do not address scalability and availability.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Deploy a second Redis instance on another EC2 instance and configure the application to use both instances with client-side sharding.
Why it's wrong here
Client-side sharding requires significant application changes and manual management of shard distribution. It does not provide automatic failover or high availability. While it adds capacity, it increases operational overhead and does not meet the requirement of minimizing application changes.
- ✗
Replace the Redis cluster with an Amazon RDS for MySQL Multi-AZ database and store sessions in a table.
Why it's wrong here
RDS for MySQL Multi-AZ provides high availability but is not optimized for high-throughput session storage. It introduces higher latency and requires significant application changes to use SQL instead of Redis commands. It does not scale horizontally as easily as ElastiCache and may become a bottleneck under heavy load.
- ✗
Enable sticky sessions on the Application Load Balancer and continue using the single Redis instance.
Why it's wrong here
Sticky sessions only ensure that a user's requests go to the same EC2 instance, but they do not reduce the load on the Redis instance. The Redis instance remains a single point of failure and a bottleneck. This approach does not improve scalability or availability of the session store.
- ✓
Migrate the session store to Amazon ElastiCache for Redis with cluster mode enabled and multiple shards.
Why this is correct
ElastiCache for Redis with cluster mode enabled provides horizontal scaling and high availability across multiple shards, eliminating the single-instance bottleneck. The application can use the Redis cluster endpoint with minimal changes, as the Redis protocol remains the same. This directly addresses scalability and availability requirements.
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Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Amazon Web Services exam blueprint
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