The answer is a low max_active_runs_per_dag setting. This error occurs because Airflow’s scheduler, when attempting to create a new DagRun for a logical date, checks the DAG’s concurrency limits; if max_active_runs_per_dag is set too low—often the default of 1—any overlapping run for the same schedule interval is rejected with the “DagRun already exists” message. On the Google Professional Data Engineer exam, this scenario tests your understanding of Cloud Composer’s concurrency controls, especially for DAGs with long-running tasks and multi-day dependencies that naturally cause schedule overlaps. A common trap is confusing this with task-level parallelism or pool limits, but the key is that max_active_runs_per_dag governs entire DAG run concurrency, not individual tasks. Memory tip: think “one run per slot”—if the slot is full, new runs get the “already exists” boot.
PDE Practice Question: Building and operationalizing data processing systems
This PDE practice question tests your understanding of building and operationalizing data processing systems. 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 company uses Cloud Composer (Airflow) to orchestrate a data pipeline. One DAG has many tasks that run in parallel and dependencies that span multiple days. Recently, the DAG started failing with 'DagRun already exists' 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
✓
The DAG has max_active_runs_per_dag set to a low number, causing overlapping runs to be rejected.
The 'DagRun already exists' error occurs when Airflow attempts to create a new DAG run for a logical date that already has an active or completed run, and the DAG's concurrency settings prevent overlapping runs. Setting max_active_runs_per_dag to a low number (e.g., 1) restricts the number of concurrent runs, so if a previous run hasn't finished or been cleared, a new run for the same or overlapping schedule interval is rejected with this error. This is the most likely cause given the DAG has dependencies spanning multiple days, which can cause runs to overlap if not properly configured.
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 DAG has a large number of tasks, overwhelming the Airflow scheduler.
Why it's wrong here
Scheduler overload leads to queuing, not a DAG run duplicate error.
✓
The DAG has max_active_runs_per_dag set to a low number, causing overlapping runs to be rejected.
Why this is correct
If max_active_runs_per_dag is too low, a new DAG run cannot start while the previous one is active.
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 DAG's schedule interval is too short, causing task instances to be created with duplicate run IDs.
Why it's wrong here
Duplicate run IDs are not the error; DAG run IDs are unique per execution date.
✗
The DAG has a depends_on_past set to True, causing upstream failures to block new runs.
Why it's wrong here
depends_on_past would cause task failures, not a DAG run error.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the distinction between DAG-level concurrency settings (max_active_runs_per_dag) and task-level parallelism (e.g., pool, task concurrency), leading candidates to confuse the 'DagRun already exists' error with scheduler overload or task dependency issues.
Detailed technical explanation
How to think about this question
Under the hood, Airflow's DagRun creation is governed by the DAG's max_active_runs_per_dag parameter and the scheduler's logic to check for existing runs with the same run_id (typically the logical date). When a new run is triggered (e.g., by the schedule or manually), the scheduler queries the database for existing runs with that run_id; if one exists and the DAG's concurrency limit is reached, it raises the 'DagRun already exists' exception. In real-world scenarios, this often happens when a DAG is backfilled or manually triggered while a scheduled run for the same date is still active, especially if max_active_runs_per_dag is set to 1.
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.
Building and operationalizing data processing systems — This question tests Building and operationalizing data processing systems — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The DAG has max_active_runs_per_dag set to a low number, causing overlapping runs to be rejected. — The 'DagRun already exists' error occurs when Airflow attempts to create a new DAG run for a logical date that already has an active or completed run, and the DAG's concurrency settings prevent overlapping runs. Setting max_active_runs_per_dag to a low number (e.g., 1) restricts the number of concurrent runs, so if a previous run hasn't finished or been cleared, a new run for the same or overlapping schedule interval is rejected with this error. This is the most likely cause given the DAG has dependencies spanning multiple days, which can cause runs to overlap if not properly configured.
What should I do if I get this PDE 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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