The answer is that some services lack the Istio sidecar proxy injection, which causes strict mTLS errors on GKE. When Istio’s default configuration enables STRICT mTLS mode, every service must have the Envoy sidecar injected to handle the required TLS handshakes; without it, services cannot authenticate or encrypt traffic, leading to failures like “upstream connect error” or “TLS handshake failure” in sidecar logs. On the Google Professional Cloud Developer exam, this tests your understanding that Istio’s default mTLS setting is strict, not permissive—a common trap is assuming sidecar injection is automatic for all pods, but it only applies to namespaces with the istio-injection label enabled. To avoid this, always verify that every service’s namespace has the label and that pods show two containers (app + sidecar) in kubectl describe. Memory tip: “Strict mTLS needs two containers per pod—no sidecar, no handshake.”
PCD Deploying applications Practice Question
This PCD practice question tests your understanding of deploying 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.
A team deploys a microservices architecture on GKE with Istio service mesh. They want to enforce mutual TLS (mTLS) between services. After enabling Istio with the default configuration, some services report connection errors. 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
✓
Some services do not have Istio sidecar injected, so strict mTLS fails.
Option C is correct because Istio's default configuration enables 'STRICT' mTLS mode, which requires all services to have an Envoy sidecar proxy injected to handle TLS handshakes. If any service lacks the sidecar, it cannot participate in mTLS, causing connection errors when other services attempt to communicate with it using TLS. The error typically manifests as 'upstream connect error' or 'TLS handshake failure' in the sidecar logs.
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 services need a ServiceEntry to communicate with each other.
Why it's wrong here
ServiceEntry is for external services, not intra-mesh.
✗
The namespace is not labeled with istio-injection=enabled.
Why it's wrong here
Missing label would prevent sidecar injection, but that is a deployment issue.
✓
Some services do not have Istio sidecar injected, so strict mTLS fails.
Why this is correct
Strict mTLS requires all services to have sidecars to handle TLS.
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 services are using a different service mesh protocol.
Why it's wrong here
Istio uses HTTP/1.1 or gRPC; no alternative protocol.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume the default Istio configuration uses PERMISSIVE mTLS (allowing both plaintext and TLS), but the actual default is STRICT, and they overlook the requirement that every service must have a sidecar for mTLS to work.
Detailed technical explanation
How to think about this question
Under the hood, Istio's mTLS is implemented via the Envoy sidecar proxy, which intercepts all inbound and outbound traffic using iptables rules. When STRICT mode is set, the sidecar rejects any plaintext (non-TLS) connections, causing the 'connection error' seen. A common real-world scenario is when a StatefulSet or Job is deployed without sidecar injection (e.g., due to missing annotation or label), breaking communication with other mesh services that expect mTLS.
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.
Deploying applications — This question tests Deploying applications — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Some services do not have Istio sidecar injected, so strict mTLS fails. — Option C is correct because Istio's default configuration enables 'STRICT' mTLS mode, which requires all services to have an Envoy sidecar proxy injected to handle TLS handshakes. If any service lacks the sidecar, it cannot participate in mTLS, causing connection errors when other services attempt to communicate with it using TLS. The error typically manifests as 'upstream connect error' or 'TLS handshake failure' in the sidecar logs.
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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