DOP-C02 Incident and Event Response Practice Question
A company's production environment consists of EC2 instances in an Auto Scaling group behind an Application Load Balancer (ALB). The instances run a web application that stores session data in an ElastiCache Redis cluster. The company has enabled detailed CloudWatch metrics and set up a dashboard. The operations team notices that the average CPU utilization across the Auto Scaling group spikes to 95% every 15 minutes, coinciding with a high number of Redis connections. What is the MOST likely cause?
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
✓
The ALB has session stickiness enabled, causing traffic to be routed to the same instances.
ALB session stickiness (sticky sessions) causes the load balancer to route requests from the same client to the same EC2 instance. When a large number of clients connect simultaneously (e.g., due to a periodic batch job), they may all be routed to the same few instances, causing CPU spikes on those instances. The high number of Redis connections corresponds to the sessions being stored in Redis. This explains the 15-minute periodic spikes. Option A is incorrect because Memcached is not used; the question states ElastiCache Redis. Option B is incorrect because the scaling policy is based on CPU utilization, not memory. Option D is incorrect because the cluster size doesn't cause periodic spikes; it would cause consistent high utilization.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The application is using Memcached instead of Redis, causing increased load.
Why it's wrong here
Memcached is not part of the described architecture; the environment uses ElastiCache with Redis, so the cache engine type is irrelevant to the observed CPU spikes. Even if Memcached were substituted, it would primarily increase cache miss rates and database latency rather than directly cause EC2 CPU utilization spikes. The actual issue is uneven traffic distribution from ALB session stickiness, not the cache engine selection.
- ✗
The Auto Scaling group's scaling policy is based on memory utilization instead of CPU.
Why it's wrong here
An Auto Scaling policy that targets memory utilization does not create CPU spikes; it simply defines a different alarm threshold for scaling actions. Under such a policy, the group would fail to scale out in response to CPU pressure, which could leave the fleet with fewer instances, but it would not explain why individual instance CPU usage is spiking. The spikes stem from stickiness routing disproportionately to a subset of instances, not from the metric chosen for the scaling policy.
- ✓
The ALB has session stickiness enabled, causing traffic to be routed to the same instances.
Why this is correct
With session stickiness enabled on the ALB target group, the load balancer consistently routes a given client's requests to the same EC2 instance for the stickiness duration. When a few power users or long-lived connections generate heavy traffic, those specific instances absorb a disproportionate share of load, causing CPU utilization to spike even if the aggregate fleet average remains moderate. This creates a hot-spot pattern where some targets are saturated while others idle, and the Auto Scaling group, if scaling on average CPU, may not react in time.
- ✗
The ElastiCache cluster is not large enough to handle the number of requests.
Why it's wrong here
An undersized ElastiCache cluster would increase cache miss rates and add latency to responses, but it would not directly cause CPU spikes on the EC2 application servers. The cache is a separate service; serving from a smaller cache might increase database read load, yet the application CPU spikes in question are occurring on the instances themselves. The described behavior matches sticky sessions forcing disproportionate traffic to certain instances, not a cache capacity problem.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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