Trigger rules (all_success, one_success, none_failed) in Apache Airflow - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding all_success trigger rule

In Airflow, what does the all_success trigger rule mean for a task's execution?

A The task runs only if all upstream tasks have succeeded.

B The task runs if at least one upstream task has succeeded.

C The task runs only if no upstream tasks have run.

D The task runs even if some upstream tasks have failed.

💡 Hint

Think about when you want a task to run only after everything before it worked well.

The all_success trigger rule means the task will run only if every upstream task finished successfully. If any upstream task failed or was skipped, this task will not run.

🧠 Conceptual

intermediate

Meaning of one_success trigger rule

What does the one_success trigger rule do in Airflow?

A The task runs only if all upstream tasks have succeeded.

B The task runs if at least one upstream task has succeeded.

C The task runs only if no upstream tasks have succeeded.

D The task runs only if all upstream tasks have failed.

💡 Hint

Imagine you want to continue if any one of the previous tasks worked.

The one_success trigger rule allows the task to run if at least one upstream task succeeded, regardless of others failing or being skipped.

❓ query_result

advanced

Output of task execution with none_failed trigger rule

Given three upstream tasks with statuses: Task A - success, Task B - skipped, Task C - success. If a downstream task uses the none_failed trigger rule, will it run?

A No, because one task was skipped.

B Yes, because all upstream tasks succeeded.

C No, because at least one task succeeded.

D Yes, because no upstream task failed.

💡 Hint

Remember that none_failed means no failures allowed, but skips are okay.

The none_failed trigger rule allows the task to run if no upstream tasks failed. Skipped tasks do not count as failures, so the task will run.

🔧 Debug

advanced

Identify the trigger rule causing a task not to run

A downstream task is set with trigger rule all_success. Upstream tasks have statuses: success, failed, success. Why does the downstream task not run?

A Because <code>all_success</code> ignores failed tasks and runs anyway.

B Because <code>all_success</code> allows running if one task failed.

C Because <code>all_success</code> requires all upstream tasks to succeed, but one failed.

D Because <code>all_success</code> requires at least one task to be skipped.

💡 Hint

Check what all_success means about upstream failures.

The all_success trigger rule means the task runs only if every upstream task succeeded. Since one upstream task failed, the downstream task does not run.

🧠 Conceptual

expert

Choosing the correct trigger rule for conditional continuation

You want a task to run only if none of its upstream tasks failed, but it should run even if some were skipped. Which trigger rule should you use?

A none_failed

B one_success

C all_success

D all_done

💡 Hint

Think about allowing skips but no failures.

The none_failed trigger rule runs the task if no upstream tasks failed. It allows skipped tasks, unlike all_success which requires all to succeed.

Practice

1. In Airflow, which trigger rule ensures a task runs only if all upstream tasks have succeeded?

easy

A. all_success

B. one_success

C. none_failed

D. all_failed

Solution

1. Step 1: Understand the meaning of all_success

   This trigger rule requires every upstream task to complete successfully before the downstream task runs.

2. Step 2: Compare with other trigger rules

   one_success runs if any upstream succeeds, none_failed runs if no upstream failed (some may be skipped), and all_failed runs only if all upstream failed.

3. Final Answer:

   all_success -> Option A

4. Quick Check:

   All upstream success = all_success [OK]

Hint: All upstream must succeed means all_success [OK]

Common Mistakes:

• Confusing one_success with all_success
• Thinking none_failed means all succeeded
• Assuming all_failed runs on success

2. Which of the following is the correct way to set a task's trigger rule to run if at least one upstream task succeeds in Airflow DAG code?

easy

A. trigger_rule='all_success'

B. trigger_rule='all_done'

C. trigger_rule='none_failed'

D. trigger_rule='one_success'

Solution

1. Step 1: Recall the trigger rule for running on any upstream success

   The trigger rule one_success runs the task if at least one upstream task succeeded.

2. Step 2: Check syntax correctness

   Setting trigger_rule='one_success' is the correct syntax in Airflow task definition.

3. Final Answer:

   trigger_rule='one_success' -> Option D

4. Quick Check:

   At least one success = trigger_rule='one_success' [OK]

Hint: Use trigger_rule='one_success' for any upstream success [OK]

Common Mistakes:

• Using all_success instead of one_success
• Misspelling trigger_rule or using wrong quotes
• Confusing none_failed with one_success

3. Given a DAG with three upstream tasks: task1 (success), task2 (skipped), task3 (failed). If a downstream task uses trigger_rule='none_failed', will it run?

medium

A. Yes, because no upstream task failed

B. No, because one upstream task failed

C. Yes, because at least one upstream succeeded

D. No, because all upstream tasks must succeed

Solution

1. Step 1: Understand none_failed trigger rule

   This rule runs the task only if no upstream task has failed. If any upstream failed, it will not run.

2. Step 2: Analyze upstream task states

   task3 failed, so the condition of no failures is false. Therefore, the downstream task will not run.

3. Final Answer:

   No, because one upstream task failed -> Option B

4. Quick Check:

   Any failure blocks none_failed run [OK]

Hint: If any upstream failed, none_failed won't run [OK]

Common Mistakes:

• Ignoring failed upstream tasks when using none_failed
• Confusing skipped with failed states
• Assuming none_failed runs if some succeed

4. You have a task with trigger_rule='one_success' but it never runs even though some upstream tasks succeed. What is the likely cause?

medium

A. The task has no upstream dependencies defined

B. The DAG is paused

C. The trigger_rule is misspelled or not set correctly

D. All upstream tasks failed

Solution

1. Step 1: Check trigger_rule spelling and assignment

   If the trigger_rule is misspelled or not properly set, Airflow defaults to all_success, causing the task not to run if not all upstream succeed.

2. Step 2: Verify other conditions

   Since some upstream succeed, and the task has dependencies, the most common cause is incorrect trigger_rule syntax.

3. Final Answer:

   The trigger_rule is misspelled or not set correctly -> Option C

4. Quick Check:

   Incorrect trigger_rule causes unexpected task skipping [OK]

Hint: Check trigger_rule spelling if task doesn't run as expected [OK]

Common Mistakes:

• Assuming upstream failure when some succeeded
• Ignoring DAG paused state
• Not defining upstream dependencies

5. You want a downstream task to run if either all upstream tasks succeed or if none failed but some were skipped. Which trigger rule should you use?

hard

A. none_failed

B. one_success

C. all_success

D. all_done

Solution

1. Step 1: Understand the requirement

   The task should run if all upstream succeeded or if none failed but some were skipped. This means failures block the run, but skips do not.

2. Step 2: Match with trigger rules

   all_success requires all succeed (no skips allowed). none_failed allows skips but no failures. one_success runs if any succeed, which is too loose. all_done runs regardless of success or failure.

3. Step 3: Choose the correct trigger rule

   none_failed fits the requirement exactly.

4. Final Answer:

   none_failed -> Option A

5. Quick Check:

   No failures but allows skips = none_failed [OK]

Hint: Use none_failed to allow skips but no failures [OK]

Common Mistakes:

• Choosing all_success which disallows skips
• Using one_success which is too permissive
• Confusing all_done with failure conditions