The answer is to enable cluster mode on the ElastiCache Redis cluster and add more shards. This is the most cost-effective and scalable solution because horizontal scaling via sharding distributes write and read operations across multiple nodes, directly reducing CPU utilization on any single node during traffic spikes. Unlike vertical scaling, which upgrades to a larger node type and incurs higher fixed costs, adding shards allows granular, pay-as-you-grow capacity while cluster mode automatically handles data partitioning and rebalancing. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of horizontal versus vertical scaling for stateful caching layers, with a common trap being to choose larger instance types or read replicas—neither of which addresses high CPU from write-heavy session state. Remember the mnemonic: "Shards share the strain, vertical is vain."
DOP-C02 Resilient Cloud Solutions Practice Question
This DOP-C02 practice question tests your understanding of resilient cloud solutions. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company runs a stateless web application on a fleet of EC2 instances in an Auto Scaling group. The application stores session state in a shared ElastiCache Redis cluster. During traffic spikes, the application becomes slow. Monitoring shows that the Redis cluster has high CPU utilization. Which solution is MOST cost-effective and scalable?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable cluster mode on the ElastiCache Redis cluster and add more shards
Enabling cluster mode on ElastiCache Redis and adding more shards horizontally scales the cluster, distributing write and read operations across multiple nodes. This directly reduces CPU utilization on any single node and is more cost-effective than vertical scaling (upgrading to a larger node type) because it allows granular, pay-as-you-grow capacity. Cluster mode also supports automatic shard rebalancing and is ideal for stateless web applications with session state that can be partitioned by session key.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Upgrade the Redis instance to a larger node type to handle more operations
Why it's wrong here
Vertical scaling is limited and more expensive.
✓
Enable cluster mode on the ElastiCache Redis cluster and add more shards
Why this is correct
Cluster mode distributes data across shards, improving performance and scalability.
Related concept
Read the scenario before looking for a memorised answer.
✗
Add read replicas to offload read traffic from the primary node
Why it's wrong here
Read replicas require cluster mode and do not help with write-heavy workloads.
✗
Migrate session state to DynamoDB with DAX for caching
Why it's wrong here
DynamoDB is not optimized for session state latency like Redis.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse read replicas with horizontal scaling for write-heavy workloads, not realizing that replicas only help with read scaling and cannot reduce CPU from write operations, while cluster mode directly addresses both read and write scaling by splitting the data set.
Detailed technical explanation
How to think about this question
ElastiCache Redis cluster mode uses a sharding architecture where each shard is a primary node with up to 5 read replicas, and data is partitioned using hash slots (16384 total) based on the key's CRC16 checksum. The cluster automatically handles failover and rebalancing, and clients use the MOVED redirection or smart client libraries (e.g., Redis Cluster specification) to route requests to the correct shard. In a real-world scenario, a stateless web app with session stickiness can use a session key prefix (e.g., 'session:<user_id>') to ensure all requests for a given user hit the same shard, maximizing cache hit rates and minimizing cross-shard operations.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Resilient Cloud Solutions — This question tests Resilient Cloud Solutions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Enable cluster mode on the ElastiCache Redis cluster and add more shards — Enabling cluster mode on ElastiCache Redis and adding more shards horizontally scales the cluster, distributing write and read operations across multiple nodes. This directly reduces CPU utilization on any single node and is more cost-effective than vertical scaling (upgrading to a larger node type) because it allows granular, pay-as-you-grow capacity. Cluster mode also supports automatic shard rebalancing and is ideal for stateless web applications with session state that can be partitioned by session key.
What should I do if I get this DOP-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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