The answer is to use Cloud Build with a step that checks `git diff` against the previous commit and runs tests only on affected modules using a test runner that supports file-based filtering. This approach is correct because it directly implements selective test execution, allowing the CI pipeline to skip unchanged code and dramatically reduce build times by running only changed code tests in CI with Cloud Build. On the Google Professional Cloud Developer exam, this scenario tests your understanding of optimizing CI/CD pipelines for microservices, where full test suites are too slow; the common trap is assuming parallel execution (Option A) is sufficient, but it still runs all tests, wasting resources. Remember the memory tip: "diff to decide"—use git diff to determine what changed, then test only that scope for maximum efficiency.
PCD Building and testing applications Practice Question
This PCD practice question tests your understanding of building and testing 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 needs to build a CI/CD pipeline for a microservices architecture. They want to run unit tests quickly by only testing code that has changed. Which approach should they use?
Use Cloud Build with a step that caches test results based on file hashes.
Why wrong: Caching can reduce runtime but still runs tests on all files initially.
B
Use Cloud Build with a step that runs all tests in parallel.
Why wrong: Running all tests in parallel improves speed but does not selectively test only changed code.
C
Use Cloud Build with a step that uses `git log` to find changed files and run tests.
Why wrong: Using `git log` is less precise than `git diff` and may miss uncommitted changes.
D
Use Cloud Build with a step that checks `git diff` against the previous commit and runs tests only on affected modules using a test runner that supports file-based filtering.
This approach directly targets changed files, minimizing test execution time.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use Cloud Build with a step that checks `git diff` against the previous commit and runs tests only on affected modules using a test runner that supports file-based filtering.
Option D is correct because using a custom builder to check the diff and run only relevant tests is a best practice for fast CI. Option A is suboptimal because running all tests in parallel is not selective. Option B is not a built-in feature and can be unreliable. Option C is not as efficient as a custom diff-based approach.
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.
✗
Use Cloud Build with a step that caches test results based on file hashes.
Why it's wrong here
Caching can reduce runtime but still runs tests on all files initially.
✗
Use Cloud Build with a step that runs all tests in parallel.
Why it's wrong here
Running all tests in parallel improves speed but does not selectively test only changed code.
✗
Use Cloud Build with a step that uses `git log` to find changed files and run tests.
Why it's wrong here
Using `git log` is less precise than `git diff` and may miss uncommitted changes.
✓
Use Cloud Build with a step that checks `git diff` against the previous commit and runs tests only on affected modules using a test runner that supports file-based filtering.
Why this is correct
This approach directly targets changed files, minimizing test execution time.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 PCD exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Building and testing applications — This question tests Building and testing applications — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use Cloud Build with a step that checks `git diff` against the previous commit and runs tests only on affected modules using a test runner that supports file-based filtering. — Option D is correct because using a custom builder to check the diff and run only relevant tests is a best practice for fast CI. Option A is suboptimal because running all tests in parallel is not selective. Option B is not a built-in feature and can be unreliable. Option C is not as efficient as a custom diff-based approach.
What should I do if I get this PCD question wrong?
Identify which PCD exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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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