DOP-C02 Monitoring and Logging Practice Question
A DevOps engineer is troubleshooting a slow-running Lambda function. The function processes messages from an SQS queue. Which CloudWatch metric should be examined first to determine if the function is experiencing throttling?
⚠ Common exam trap
Many candidates confuse high ConcurrentExecutions with throttling, but throttling is a separate metric that directly counts rejected invocations, not the number of concurrent runs.
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
✓
Throttles
The Throttles metric directly indicates when Lambda is rejecting invocation requests due to concurrency limits being reached. Since the question asks specifically about throttling, this is the first metric to examine to confirm whether the function is being rate-limited by AWS.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Invocations
Why it's wrong here
The Invocations metric records the number of requests that actually began executing your function, including both successful runs and those that ended in a function error. Throttled requests are never counted as invocations because Lambda rejects them with a 429 before the code starts, so a sustained burst of throttling can leave Invocations looking normal or even low. Consequently, while it is a useful indicator of overall demand, it cannot reveal how many calls were blocked due to concurrency limits.
- ✗
ConcurrentExecutions
Why it's wrong here
ConcurrentExecutions measures the instantaneous count of function instances that are actively processing events, such as three invocations running at the same moment. It only reflects capacity in use; if the value is pinned at the reserved concurrency limit, throttling is likely happening, but the metric never reports the actual number of rejected requests. You would still need the Throttles metric to quantify lost invocations and distinguish between a function that is exactly at capacity and one that is heavily throttling event delivery.
- ✗
Duration
Why it's wrong here
Duration tracks the wall-clock time Lambda spends executing your function's handler code, from the moment the runtime starts running it until it returns a response or fails. A slow-running function will typically show elevated Duration, but high execution time has no direct relationship to throttling, which occurs before execution begins. Even if Duration is low, the function may be throttled repeatedly due to concurrency constraints, so this metric cannot be used to diagnose throttled requests.
- ✓
Throttles
Why this is correct
The Throttles metric is the definitive CloudWatch counter for how many invocation requests Lambda rejected because the function had no available concurrency capacity. Each time Lambda receives a request while the function or account is over its concurrency limit, it increments Throttles and returns a 429 TooManyRequestsException, prompting SDK retries or event-source retry logic. For a slow-running function, elevated Throttles explains intermittent delays caused by client-side retries, making this the correct metric to inspect when troubleshooting such issues.
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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