Task dependencies (>> and << operators) in Apache Airflow - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the >> operator do in Airflow task dependencies?

The >> operator sets a downstream dependency, meaning the task on the left must run before the task on the right.

beginner

How does the << operator work in Airflow task dependencies?

The << operator sets an upstream dependency, meaning the task on the right must run before the task on the left.

beginner

How would you express that task A runs before tasks B and C using >> operator?

You write: A >> [B, C]. This means task A runs first, then tasks B and C run after.

beginner

Can you chain multiple tasks using >> operator? Example?

Yes. For example, A >> B >> C means A runs first, then B, then C in order.

intermediate

What happens if you mix >> and << operators in the same dependency chain?

They represent the same dependencies but from different directions. For example, A >> B is the same as B << A.

In Airflow, what does the expression 'task1 >> task2' mean?

A task1 runs before task2

B task2 runs before task1

C task1 and task2 run at the same time

D task1 depends on task2 to finish

The >> operator means the left task runs before the right task.

Which operator sets an upstream dependency in Airflow?

A ==

B >>

C <<

D !=

The << operator sets an upstream dependency, meaning the right task runs before the left.

How do you specify that task A runs before tasks B and C?

A A << [B, C]

B [B, C] >> A

C [B, C] << A

D A >> [B, C]

Using A >> [B, C] means A runs first, then B and C run after.

What does 'A >> B >> C' mean in Airflow?

A C runs, then B runs, then A runs

B A runs, then B runs, then C runs

C A, B, and C run simultaneously

D B depends on A and C

The chain means tasks run in sequence: A, then B, then C.

Is 'A >> B' equivalent to 'B << A' in Airflow?

A Yes, both mean A runs before B

B No, they mean opposite things

C Only if tasks are in the same DAG

D Only if tasks are different types

Both expressions set the same dependency: A runs before B.

Explain how to use the >> and << operators to set task dependencies in Airflow.

Think about which task runs first and which runs after.

Describe how to express multiple downstream tasks from a single upstream task using Airflow operators.

Use square brackets to group tasks.

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 >>