The answer is that the Airflow worker service account lacks the proper OAuth scopes to authenticate with Vertex AI. This authentication error occurs because Cloud Composer executes DAG tasks using a dedicated worker service account, and without the `https://www.googleapis.com/auth/cloud-platform` scope explicitly granted, the worker cannot obtain a valid token to call Vertex AI APIs, resulting in a 403 or 401 failure. On the Google Professional Machine Learning Engineer exam, this scenario tests your understanding of how Cloud Composer’s service account scopes differ from IAM roles—a common trap is confusing IAM permissions with OAuth scopes, but scopes gate the initial token request. Remember that even with the correct IAM role (like `aiplatform.user`), missing the cloud-platform scope will still block authentication. Memory tip: “Scopes first, roles second—without the scope, the token won’t work.”
PMLE Automating and orchestrating ML pipelines Practice Question
This PMLE practice question tests your understanding of automating and orchestrating ml 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.
The exhibit shows a Cloud Composer environment variable configuration. An ML pipeline DAG fails with an authentication error when trying to access Vertex AI. 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 Airflow worker does not have the proper scopes to access Vertex AI
The authentication error when accessing Vertex AI from Cloud Composer most likely occurs because the Airflow worker's service account lacks the necessary OAuth scopes or IAM permissions. Cloud Composer uses a worker service account to execute tasks; if this account does not have the `https://www.googleapis.com/auth/cloud-platform` scope or the `aiplatform.user` role, the Airflow worker cannot authenticate to Vertex AI APIs, resulting in a 403 or 401 error.
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 Airflow worker does not have the proper scopes to access Vertex AI
Why this is correct
The environment variable 'GOOGLE_APPLICATION_CREDENTIALS' is set to a service account key path, but the worker VM may not have the necessary scopes.
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 account key in the environment variable is expired
Why it's wrong here
The error is about authentication; a key could be expired, but the exhibit shows a key path, not the key itself.
✗
The DAG file is missing a required Python library
Why it's wrong here
Authentication error is not about missing libraries.
✗
The Cloud Composer environment is in a different project than Vertex AI
Why it's wrong here
Cross-project access is possible with proper permissions; the error is authentication, not permission.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the distinction between authentication (scopes/identity) and authorization (IAM roles), so candidates mistakenly blame cross-project configuration or missing libraries when the root cause is the worker's service account lacking the required OAuth scopes.
Trap categories for this question
Command / output trap
The error is about authentication; a key could be expired, but the exhibit shows a key path, not the key itself.
Detailed technical explanation
How to think about this question
Cloud Composer's Airflow workers authenticate to Google APIs using the Compute Engine default service account or a custom service account specified at environment creation. This account must have the `cloud-platform` OAuth scope (or specific scopes) and the appropriate IAM roles (e.g., `roles/aiplatform.user`) on the Vertex AI project. Under the hood, the Airflow worker uses Application Default Credentials (ADC) to obtain an access token; if the scopes are missing, the token lacks the `aud` claim for Vertex AI, causing the API to reject the request.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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.
Automating and orchestrating ML pipelines — This question tests Automating and orchestrating ML pipelines — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The Airflow worker does not have the proper scopes to access Vertex AI — The authentication error when accessing Vertex AI from Cloud Composer most likely occurs because the Airflow worker's service account lacks the necessary OAuth scopes or IAM permissions. Cloud Composer uses a worker service account to execute tasks; if this account does not have the `https://www.googleapis.com/auth/cloud-platform` scope or the `aiplatform.user` role, the Airflow worker cannot authenticate to Vertex AI APIs, resulting in a 403 or 401 error.
What should I do if I get this PMLE 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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