PAS-C01 Operations and Maintenance Practice Question
A company uses AWS Lambda functions with reserved concurrency to process messages from an SQS queue. The operations team notices that the Lambda function sometimes throttles, causing messages to remain in the queue. What is the MOST likely cause and solution?
⚠ Common exam trap
It's easy for candidates to confuse throttling with message processing failures and incorrectly focus on DLQ or visibility timeout settings, rather than recognizing that reserved concurrency directly controls the maximum number of concurrent Lambda executions and is the primary cause of throttling when set too low.
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 reserved concurrency is set too low; increase the reserved concurrency for the function.
Lambda throttling occurs when the reserved concurrency limit is reached, preventing the function from processing new invocations. Since the function uses reserved concurrency, setting it too low restricts the number of concurrent executions, causing SQS messages to remain in the queue until concurrency becomes available. Increasing the reserved concurrency allows more parallel invocations, reducing throttling and improving message processing throughput.
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 SQS visibility timeout is too short; increase it.
Why it's wrong here
Visibility timeout affects retries, not throttling.
- ✗
The Lambda function's dead-letter queue (DLQ) is not configured; set up a DLQ.
Why it's wrong here
DLQ handles failed messages, not throttling.
- ✗
The SQS queue's redrive policy is too aggressive; reduce the maxReceiveCount.
Why it's wrong here
Not related to throttling.
- ✓
The reserved concurrency is set too low; increase the reserved concurrency for the function.
Why this is correct
Reserved concurrency limits concurrent executions, causing throttling when exceeded.
Quick reference
Cloud Service Model Comparison
| Model | You Manage | Provider Manages | Examples |
|---|---|---|---|
| IaaS | OS, runtime, apps, data | Hardware, hypervisor, networking | EC2, Azure VMs, GCP Compute Engine |
| PaaS | Apps and data | OS, runtime, middleware, hardware | Elastic Beanstalk, Azure App Service |
| SaaS | Data and settings only | Everything else | Microsoft 365, Salesforce, Workday |
| FaaS / Serverless | Function code only | Infra, scaling, runtime | Lambda, Azure Functions, Cloud Run |
| CaaS | Containers and apps | Kubernetes, OS, hardware | EKS, AKS, GKE |
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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