PAS-C01 Operations and Maintenance Practice Question
A company runs a critical web application on a fleet of EC2 instances behind an Application Load Balancer (ALB) in an Auto Scaling group. The application stores session state in an ElastiCache Redis cluster (Cluster Mode disabled). The operations team receives alerts that the application is experiencing intermittent timeouts and high error rates. Upon investigation, they find that the Redis cluster's CPU utilization is consistently above 90%, and there are frequent 'MAXMEMORY' evictions. The application team confirms that they recently increased the number of concurrent users and added more session data per user. The current Redis cluster is a single cache.r5.large node with 13.37 GB of memory. Which of the following should the operations team do FIRST to address the performance issues?
⚠ Common exam trap
Watch out — candidates often choose vertical scaling (Option A) or eviction policy changes (Option C) as quick fixes, failing to recognize that the scenario's high CPU and memory evictions indicate a need for horizontal scaling via sharding, which is the only option that directly reduces per-node load and memory pressure.
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
✓
Enable Cluster Mode and add multiple shards to distribute the data across nodes.
The correct first step is to enable Cluster Mode and add multiple shards because the performance issue stems from CPU saturation and memory pressure on a single Redis node. Cluster Mode distributes data across multiple shards, reducing per-node CPU load and memory usage, which directly addresses the high CPU utilization and 'MAXMEMORY' evictions caused by increased concurrent users and session data.
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 instance type to cache.r5.2xlarge to double the memory and compute capacity.
Why it's wrong here
Scaling vertically provides only a temporary solution and does not scale horizontally for future growth.
- ✗
Create a read replica in a different Availability Zone and redirect read traffic to it.
Why it's wrong here
Read replicas do not help with write-heavy workloads or reduce CPU load on the primary node.
- ✗
Change the eviction policy to 'allkeys-lru' to maximize memory utilization.
Why it's wrong here
Changing the eviction policy may evict important session data, causing application errors.
- ✓
Enable Cluster Mode and add multiple shards to distribute the data across nodes.
Why this is correct
Enabling Cluster Mode and adding shards allows horizontal scaling, distributing CPU and memory load.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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