The answer is a taint and toleration mismatch. This is correct because even when the cluster autoscaler successfully provisions new nodes, the Kubernetes scheduler will refuse to place pods onto those nodes if the pods lack the required tolerations for the node’s taints. In GKE, node auto-upgrade can introduce new taints on replacement nodes, and if your pod specifications do not include matching tolerations, the scheduler sees the nodes as incompatible and leaves the pods in a Pending state. On the Google Professional Cloud Developer exam, this scenario tests your understanding of how scheduling constraints override capacity—a common trap is assuming autoscaling alone guarantees pod placement, when in reality taints and tolerations act as a hard filter. A useful memory tip: think of taints as “reject” labels on nodes and tolerations as “admit” passes on pods; without the correct pass, the pod is denied entry regardless of available seats.
PCD Practice Question: Designing highly scalable, available, and reliable cloud-native applications
This PCD practice question tests your understanding of designing highly scalable, available, and reliable cloud-native applications. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company uses GKE with cluster autoscaling and node auto-upgrade. During a traffic spike, new pods are unschedulable even though the cluster autoscaler adds nodes. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The pods have tolerations that don't match node taints
Option D is correct because if pods have tolerations that do not match the taints on the nodes, the scheduler will not place them on those nodes, even if the cluster autoscaler has added new nodes. This mismatch prevents scheduling, leading to unschedulable pods despite sufficient node capacity.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 pods have resource requests that exceed available node capacity
Why it's wrong here
New nodes are added, so capacity should be available; if pods still unschedulable, another issue.
✗
The cluster autoscaler is disabled
Why it's wrong here
The scenario states the autoscaler is adding nodes.
✗
The node pool has reached its maximum size limit
Why it's wrong here
If max size were reached, the autoscaler would not add new nodes.
✓
The pods have tolerations that don't match node taints
Why this is correct
Newly added nodes may have taints (e.g., from node auto-upgrade) that the pods do not tolerate, preventing scheduling.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The nodes are in unhealthy status
Why it's wrong here
If nodes were unhealthy, they might not be added; but the autoscaler adds healthy nodes.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between resource-based scheduling failures (like insufficient capacity) and policy-based scheduling failures (like taint/toleration mismatches), where candidates mistakenly assume that adding nodes always solves unschedulable pods.
Trap categories for this question
Scenario analysis trap
The scenario states the autoscaler is adding nodes.
Detailed technical explanation
How to think about this question
In Kubernetes, taints and tolerations work together to control pod placement: nodes with taints repel pods that do not have matching tolerations. The cluster autoscaler adds nodes based on pending pods, but if those pods lack the required tolerations for the new nodes' taints, they remain unschedulable. This is a common misconfiguration when using node pools with different taints for workload isolation.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Designing highly scalable, available, and reliable cloud-native applications — This question tests Designing highly scalable, available, and reliable cloud-native applications — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The pods have tolerations that don't match node taints — Option D is correct because if pods have tolerations that do not match the taints on the nodes, the scheduler will not place them on those nodes, even if the cluster autoscaler has added new nodes. This mismatch prevents scheduling, leading to unschedulable pods despite sufficient node capacity.
What should I do if I get this PCD question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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