The answer is a container port mismatch, where the application inside the container is not listening on the port Cloud Run expects. This occurs because Cloud Run routes all incoming requests to the port specified in the `containerPort` field of the service YAML, which defaults to 8080; if your application is bound to a different port, such as 3000 or 80, the request never reaches the application, causing health checks to fail and returning a 503 error. On the Google Professional Cloud Developer exam, this scenario tests your understanding of how Cloud Run’s networking layer maps external requests to container processes, and it often appears as a trap where the deployment succeeds but runtime fails. A common memory tip is to think of the container port as a door number—if your app is knocking on door 3000 but Cloud Run is only listening at door 8080, no one answers, so always verify your app’s listening port matches the `containerPort` value.
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. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Exhibit
Refer to the exhibit.
apiVersion: serving.knative.dev/v1
kind: Service
metadata:
name: my-service
spec:
template:
spec:
containers:
- image: gcr.io/my-project/my-image:latest
containerConcurrency: 80
A developer deployed the above Cloud Run service YAML. The service deploys successfully but any request fails with a 503 error. 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.
Refer to the exhibit.
apiVersion: serving.knative.dev/v1
kind: Service
metadata:
name: my-service
spec:
template:
spec:
containers:
- image: gcr.io/my-project/my-image:latest
containerConcurrency: 80
A
The container is not listening on the expected port.
Cloud Run requires the container to listen on the port specified by the PORT environment variable (default 8080). If the container listens on a different port, requests time out or fail.
B
The service has no ingress setting.
Why wrong: If not specified, the default ingress setting allows all traffic. Missing ingress setting does not cause failures.
C
The container image has a different entrypoint.
Why wrong: The entrypoint determines the command run, but even with a different entrypoint, as long as it listens on the correct port, it should work.
D
containerConcurrency is set too high.
Why wrong: containerConcurrency limits the number of concurrent requests per instance. A high value does not cause failures; it may affect performance but not result in 503 errors immediately.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The container is not listening on the expected port.
A 503 error from Cloud Run indicates that the service is failing to respond to health checks or requests. The most common cause is that the container is not listening on the port specified in the `containerPort` field of the YAML (default 8080). Cloud Run sends requests to that port, and if the application is bound to a different port (e.g., 3000 or 80), the request never reaches the application, resulting in a 503.
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 container is not listening on the expected port.
Why this is correct
Cloud Run requires the container to listen on the port specified by the PORT environment variable (default 8080). If the container listens on a different port, requests time out or fail.
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 service has no ingress setting.
Why it's wrong here
If not specified, the default ingress setting allows all traffic. Missing ingress setting does not cause failures.
✗
The container image has a different entrypoint.
Why it's wrong here
The entrypoint determines the command run, but even with a different entrypoint, as long as it listens on the correct port, it should work.
✗
containerConcurrency is set too high.
Why it's wrong here
containerConcurrency limits the number of concurrent requests per instance. A high value does not cause failures; it may affect performance but not result in 503 errors immediately.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between a container that fails to start (which would show a different error) and a container that runs but is unreachable on the expected port (which causes 503 errors).
Trap categories for this question
Command / output trap
The entrypoint determines the command run, but even with a different entrypoint, as long as it listens on the correct port, it should work.
Detailed technical explanation
How to think about this question
Cloud Run uses a built-in HTTP reverse proxy (Envoy) that forwards requests to the container's port. The health check (e.g., /healthz) is also sent to that port. If the container listens on a different port, the proxy cannot establish a TCP connection, leading to a 503. This is a common misconfiguration when developers use frameworks like Express (default 3000) or Flask (default 5000) without adjusting the port to match `containerPort`.
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
A cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
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 container is not listening on the expected port. — A 503 error from Cloud Run indicates that the service is failing to respond to health checks or requests. The most common cause is that the container is not listening on the port specified in the `containerPort` field of the YAML (default 8080). Cloud Run sends requests to that port, and if the application is bound to a different port (e.g., 3000 or 80), the request never reaches the application, resulting in a 503.
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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