AZ-305 Design infrastructure solutions Practice Question
You are designing a containerized microservices application on Azure Kubernetes Service (AKS). The application must scale automatically based on HTTP traffic. You need to minimize cost by scaling down to zero pods when there is no traffic. Which scaling solution should you use?
⚠ Common exam trap
It's easy for candidates to assume the Horizontal Pod Autoscaler (HPA) can scale to zero pods because it is the default autoscaler for Kubernetes, but HPA has a hard-coded minimum of 1 replica and cannot scale down to zero, making KEDA the only correct option for cost minimization through zero-pod scaling.
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
✓
Kubernetes Event-driven Autoscaler (KEDA)
KEDA (Kubernetes Event-driven Autoscaler) is the correct choice because it can scale the number of pods in an AKS deployment down to zero when there is no HTTP traffic, and then scale up from zero when traffic resumes. Unlike the Horizontal Pod Autoscaler (HPA), which cannot scale below 1 replica by default, KEDA works with external event sources (like HTTP requests via an add-on) to achieve true zero-to-N scaling, minimizing cost during idle periods.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Horizontal Pod Autoscaler (HPA)
Why it's wrong here
Horizontal Pod Autoscaler (HPA) is the built-in Kubernetes controller that adjusts the number of pod replicas based on observed CPU, memory, or custom metrics, but it cannot scale a workload down to zero pods because the Deployment or StatefulSet must maintain a minimum replica count (typically minReplicas ≥ 1). HPA also relies on a continuous stream of metrics and lacks event-driven triggers, so during idle periods or while waiting for a burst of HTTP traffic it will keep at least one pod running, incurring unnecessary cost and resource usage. For a microservices application that may need true zero-idle scaling, HPA alone is insufficient.
- ✗
Cluster Autoscaler
Why it's wrong here
Cluster Autoscaler works at the infrastructure layer, not the application layer, by adding or removing worker nodes in the Kubernetes cluster based on pending pod scheduling conditions. It does not make decisions about individual pod replica counts; instead it reacts to pods that are unschedulable due to insufficient node capacity and scales the node pool accordingly. While Cluster Autoscaler can help reduce waste when combined with other autoscalers, it cannot itself reduce a microservice's pod count to zero based on HTTP traffic, so it does not satisfy the stated requirement.
- ✓
Kubernetes Event-driven Autoscaler (KEDA)
Why this is correct
Kubernetes Event-driven Autoscaler (KEDA) is the correct choice because it extends the Horizontal Pod Autoscaler (HPA) with event-driven triggers from HTTP requests, message queues, databases, or other external sources, allowing a microservice to scale from zero to N replicas and back down to zero when idle. KEDA installs a custom metrics API server that reports the current event stream length or request rate to the HPA, enabling fine-grained scaling that reacts to actual demand rather than only steady-state server metrics. This makes it ideal for containerized microservices that experience intermittent traffic and require cost-efficient running with no idle pods.
- ✗
Vertical Pod Autoscaler (VPA)
Why it's wrong here
Vertical Pod Autoscaler (VPA) adjusts CPU and memory requests/limits for existing pods, either restarting them or recommending new values, but it never changes the number of pod replicas. VPA is designed to ensure each pod is sized appropriately for its workload, not to handle fluctuations in request volume or to set replica counts to zero. Even if VPA frees resources on a node, it cannot provide the requested scale-to-zero behavior for HTTP-driven microservices, so it is fundamentally misaligned with the requirement.
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