PDE Maintaining and Automating Data Workloads Practice Question
A data engineering team maintains several Cloud Composer DAGs that load files from Cloud Storage into BigQuery. They want the DAG to start only after a new file has landed in the source bucket, rather than polling on a fixed schedule and often finding nothing. Which Airflow construct should they use to trigger the DAG based on the arrival of the object?
⚠ Common exam trap
Candidates often confuse a time-based wait with an event-based wait, since both keep a task pending, but only a sensor that inspects Cloud Storage can confirm the object actually exists.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A GCSObjectExistenceSensor with a poke interval, followed by the load task.
The requirement is to begin processing only when a specific object appears, which is precisely what a Cloud Storage sensor provides. Sensors defer downstream tasks until their condition is met, so pairing a GCSObjectExistenceSensor with the load task converts the DAG from a blind schedule into a data-arrival trigger while keeping the existing load logic intact.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
A GCSObjectExistenceSensor with a poke interval, followed by the load task.
Why this is correct
The GCSObjectExistenceSensor waits until the specified object exists in Cloud Storage before allowing the DAG to proceed. This turns a time-driven schedule into an event-aware one: the load task runs only when the file is actually present, eliminating wasted polling runs that find nothing and reducing BigQuery load attempts against empty inputs.
- ✗
A TimeDeltaSensor set to the expected upload window before the load task.
Why it's wrong here
A TimeDeltaSensor simply pauses for a fixed duration and then succeeds regardless of whether any file exists. It cannot observe Cloud Storage, so if the upload is late the load task still runs against a missing object, and if the upload is early the DAG waits unnecessarily. It addresses timing, not object arrival.
- ✗
A BigQueryInsertJobOperator configured to load the file directly from the bucket path.
Why it's wrong here
The insert job operator submits a load job to BigQuery, which will fail if the referenced object does not yet exist. It performs the load but does not gate the DAG on file arrival, so the team would still need retries or scheduling luck. It is a loader, not a trigger condition, and does not satisfy the requirement.
- ✗
An ExternalTaskSensor pointed at a separate file-upload DAG.
Why it's wrong here
An ExternalTaskSensor waits for a task in another DAG to reach a given state. It has no knowledge of Cloud Storage objects; it only knows about task states. Unless a separate DAG already marks upload completion, this sensor cannot detect the new file, and introducing that extra DAG adds complexity without solving the arrival problem.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Google Cloud exam blueprint
This PDE practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PDE exam.