Task dependencies (>> and << operators) in Apache Airflow - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the effect of >> operator in Airflow DAGs

Given the following Airflow task dependencies:

task1 >> task2 >> task3

Which statement best describes the execution order of these tasks?

A task1 runs first, then task2, then task3

B task3 runs first, then task2, then task1

C All tasks run in parallel without order

D task2 runs first, then task1, then task3

💡 Hint

The >> operator sets the order from left to right.

The >> operator in Airflow sets a downstream dependency. So task1 runs before task2, and task2 runs before task3.

💻 Command Output

intermediate

Output of task dependencies using << operator

Consider the following Airflow task dependencies:

task3 << task2 << task1

What is the correct execution order of the tasks?

A task2 runs first, then task3, then task1

B task3 runs first, then task2, then task1

C task1 runs first, then task2, then task3

D All tasks run simultaneously

💡 Hint

The << operator sets upstream dependencies.

The << operator means the task on the right runs before the task on the left. So task1 runs before task2, and task2 runs before task3.

🔀 Workflow

advanced

Creating a diamond-shaped task dependency graph

You want to create a DAG where task1 runs first, then task2 and task3 run in parallel, and finally task4 runs after both task2 and task3 complete.

Which code snippet correctly sets these dependencies using >> and/or << operators?

A [task2, task3] >> task1 >> task4

B task1 << [task2, task3] << task4

C task4 >> [task2, task3] >> task1

D task1 >> [task2, task3] >> task4

💡 Hint

Use >> to set downstream dependencies and list to set parallel tasks.

Using task1 >> [task2, task3] >> task4 means task1 runs first, then task2 and task3 run in parallel, then task4 runs after both finish.

❓ Troubleshoot

advanced

Identifying incorrect task dependency causing cyclic error

Given the following dependencies:

task1 >> task2
 task2 >> task3
 task3 >> task1

What error will Airflow raise when parsing this DAG?

A AirflowDagCycleException due to cyclic dependency

B TaskNotFound error because task3 is missing

C No error, DAG runs successfully

D SyntaxError due to invalid operator usage

💡 Hint

Check if the dependencies form a loop.

The dependencies form a cycle: task1 -> task2 -> task3 -> task1, which Airflow does not allow and raises a cycle exception.

✅ Best Practice

expert

Choosing the best way to set multiple dependencies for clarity

You have tasks start, middle1, middle2, and end. You want start to run first, then middle1 and middle2 in parallel, and finally end after both middles finish.

Which of the following is the clearest and most maintainable way to set these dependencies using >> and/or << operators?

A start >> [middle1, middle2] >> end

B

start >> middle1
start >> middle2
middle1 >> end
middle2 >> end

C [start, middle1] >> middle2 >> end

D end << middle1 << middle2 << start

💡 Hint

Consider readability and explicitness for future maintenance.

Option B explicitly sets each dependency on its own line, making it clear and easy to maintain. Option B is concise but less explicit. Option B and D do not represent the correct order clearly.

Practice

1. In Airflow, what does the operator task1 >> task2 mean?

easy

A. task1 runs before task2

B. task1 runs after task2

C. task1 and task2 run at the same time

D. task1 is unrelated to task2

Solution

1. Step 1: Understand the meaning of >> operator

   The >> operator in Airflow means "runs before" or "upstream to downstream" dependency.

2. Step 2: Apply to given tasks

   Given task1 >> task2, task1 must finish before task2 starts.

3. Final Answer:

   task1 runs before task2 -> Option A

4. Quick Check:

   >> means runs before [OK]

Hint: Remember: >> means runs before, << means runs after [OK]

Common Mistakes:

• Confusing >> with <<
• Thinking >> means runs after
• Assuming tasks run in parallel

2. Which of the following is the correct syntax to set task2 to run after task1 using Airflow operators?

easy

A. task1 >> task2

B. task2 << task1

C. task2 >> task1

D. task1 << task2

Solution

1. Step 1: Identify the operator meaning

   >> means "runs before"; so task1 >> task2 means task1 runs before task2.

2. Step 2: Match the requirement

   To make task2 run after task1, task1 must run before task2, so task1 >> task2 is correct.

3. Final Answer:

   task1 >> task2 -> Option A

4. Quick Check:

   task2 after task1 = task1 >> task2 [OK]

Hint: Use >> to say runs before, so task1 >> task2 means task2 after task1 [OK]

Common Mistakes:

• Using << backwards
• Swapping task order
• Confusing which task is upstream

3. Given the code snippet:

task1 >> task2 << task3

Which statement is true about the task execution order?

medium

A. task2 runs before task1 and task3

B. task1 runs after task2, and task3 runs after task2

C. task1 runs before task2, and task3 runs before task2

D. task1, task2, and task3 run in parallel

Solution

1. Step 1: Break down the dependencies

   task1 >> task2 means task1 runs before task2; task3 << task2 means task3 runs before task2 (because << means runs after, so task2 runs after task3).

2. Step 2: Combine the relations

   Both task1 and task3 must finish before task2 starts.

3. Final Answer:

   task1 runs before task2, and task3 runs before task2 -> Option C

4. Quick Check:

   task1 and task3 before task2 = task1 runs before task2, and task3 runs before task2 [OK]

Hint: >> means runs before; << means runs after (reverse order) [OK]

Common Mistakes:

• Misreading << as runs before
• Assuming task2 runs first
• Thinking tasks run in parallel

4. Identify the error in this Airflow task dependency code:

task1 >> >> task2

medium

A. task2 runs before task1

B. Syntax error due to double >> operators

C. No error, tasks run sequentially

D. task1 and task2 run in parallel

Solution

1. Step 1: Analyze the operator usage

   The code has two consecutive >> operators without a task in between, which is invalid syntax in Python and Airflow.

2. Step 2: Understand Airflow dependency syntax

   Only one >> operator is allowed between two tasks to set dependency; double >> causes syntax error.

3. Final Answer:

   Syntax error due to double >> operators -> Option B

4. Quick Check:

   Double >> is invalid syntax [OK]

Hint: Only one >> or << between tasks, no doubles [OK]

Common Mistakes:

• Writing multiple >> operators in a row
• Ignoring syntax errors
• Assuming double >> means stronger dependency

5. You want to create a DAG where task1 and task3 run first in parallel, then task2 runs after both finish. Which code correctly sets this dependency?

hard

A. task2 >> [task1, task3]

B. task1 << task2 >> task3

C. task1 >> task2 >> task3

D. [task1, task3] >> task2

Solution

1. Step 1: Understand the requirement

   task1 and task3 run in parallel first, then task2 runs after both finish.

2. Step 2: Translate to Airflow syntax

   Using list syntax [task1, task3] >> task2 means both task1 and task3 run before task2.

3. Step 3: Check other options

   task1 >> task2 >> task3 runs tasks sequentially; B and D have wrong directions.

4. Final Answer:

   [task1, task3] >> task2 -> Option D

5. Quick Check:

   List >> task means all list tasks before next [OK]

Hint: Use list syntax [task1, task3] >> task2 for parallel upstream tasks [OK]

Common Mistakes:

• Using >> in wrong order
• Not using list for parallel tasks
• Confusing << with >>