AZ-400 Practice Question: Design and implement build and release pipelines
This AZ-400 practice question tests your understanding of design and implement build and release pipelines. 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 uses Azure Pipelines to build a .NET Core application. The build pipeline runs successfully, but the release pipeline fails when deploying to Azure App Service with the error: 'ERROR_FILE_IN_USE'. 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 'Take App Offline' setting is not enabled in the deployment task.
The 'ERROR_FILE_IN_USE' error occurs when the deployment process tries to overwrite files that are currently locked by the running application. Enabling the 'Take App Offline' setting in the Azure App Service deploy task places an app_offline.htm file in the wwwroot directory, which gracefully shuts down the application and releases all file locks before the new binaries are copied. Without this setting, the running process holds locks on the DLLs, causing the deployment to fail.
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 deployment slot is not configured correctly.
Why it's wrong here
Slot configuration does not directly cause file lock errors.
✓
The 'Take App Offline' setting is not enabled in the deployment task.
Why this is correct
Taking the app offline releases file locks.
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 Azure App Service plan is not scaled appropriately.
Why it's wrong here
Scaling does not cause file lock errors.
✗
The build configuration is set to Release instead of Debug.
Why it's wrong here
Build configuration does not affect deployment file locks.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse 'ERROR_FILE_IN_USE' with a slot configuration or scaling issue, but the root cause is always the running process holding file locks, which is directly resolved by the 'Take App Offline' setting in the deployment task.
Detailed technical explanation
How to think about this question
Under the hood, the 'Take App Offline' setting leverages the ASP.NET Core module's detection of the app_offline.htm file. When present, the module stops the application pool and waits for all in-flight requests to complete, ensuring no process holds a handle on the binaries. In a real-world scenario, this is critical for zero-downtime deployments when using Web Deploy (MSDeploy) with the 'AllowUntrustedCertificate' flag, as the file lock can persist even after the app pool recycles if the 'autoHealEnabled' setting is misconfigured.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this AZ-400 question in full detail.
Design and implement build and release pipelines — This question tests Design and implement build and release pipelines — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The 'Take App Offline' setting is not enabled in the deployment task. — The 'ERROR_FILE_IN_USE' error occurs when the deployment process tries to overwrite files that are currently locked by the running application. Enabling the 'Take App Offline' setting in the Azure App Service deploy task places an app_offline.htm file in the wwwroot directory, which gracefully shuts down the application and releases all file locks before the new binaries are copied. Without this setting, the running process holds locks on the DLLs, causing the deployment to fail.
What should I do if I get this AZ-400 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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