Cron expressions in Airflow - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the next execution time for this cron expression?

Given the cron expression 0 6 * * 1-5 in Airflow, what is the next scheduled run time if the current time is Monday 2024-06-03 05:00 UTC?

A Monday 2024-06-10 06:00 UTC

B Tuesday 2024-06-04 06:00 UTC

C Monday 2024-06-03 06:00 UTC

D Sunday 2024-06-02 06:00 UTC

💡 Hint

Remember that the cron runs at 6 AM on weekdays (Monday to Friday).

The cron expression means run at 6:00 AM on Monday through Friday. Since current time is Monday 5 AM, the next run is the same day at 6 AM.

🧠 Conceptual

intermediate

Understanding the meaning of a cron expression

What does the cron expression 30 23 15 * * mean in Airflow scheduling?

A Run at 11:30 AM on the 15th day of every month

B Run at 11:30 PM on the 15th day of every month

C Run every 15 minutes starting at 11:30 PM

D Run at 11:30 PM every day

💡 Hint

The first two numbers represent minutes and hours.

The cron fields are: minute=30, hour=23, day=15, month=any, weekday=any. So it runs once a month at 23:30 on day 15.

❓ Troubleshoot

advanced

Why does this Airflow DAG not run as expected?

An Airflow DAG has the cron schedule 0 0 31 2 *. It never runs. Why?

A February never has 31 days, so the schedule never matches

B The timezone is not set, so the DAG runs at a different time

C Airflow does not support scheduling on specific days of the month

D The cron expression syntax is invalid and causes an error

💡 Hint

Check if the date specified actually exists in the calendar.

February has at most 29 days, so day 31 never occurs. The DAG never triggers.

✅ Best Practice

advanced

Choosing a cron expression for daily midnight runs

Which cron expression is the best choice to schedule an Airflow DAG to run daily at midnight UTC?

A 0 0 * * *

B 0 24 * * *

C 0 0 1 * *

D 0 0 * * 0

💡 Hint

Midnight is hour 0, minute 0 every day.

The expression '0 0 * * *' means every day at 00:00. Option A is invalid (hour 24), C runs only on the first day of the month, D runs only on Sundays.

🔀 Workflow

expert

Order the steps to update an Airflow DAG's schedule using a cron expression

Put these steps in the correct order to update an Airflow DAG's schedule with a new cron expression.

A 3,1,2,4

B 2,1,3,4

C 1,3,2,4

D 1,2,3,4

💡 Hint

Think about editing first, then testing, then deploying, then verifying.

First edit the schedule in code, then test it, then deploy to Airflow, finally check the UI.

Practice

1. What does the cron expression 0 12 * * * mean in Airflow scheduling?

easy

A. Run the task every day at 12:00 PM (noon)

B. Run the task every hour at minute 12

C. Run the task every 12 minutes

D. Run the task every day at midnight

Solution

1. Step 1: Understand cron fields

   The expression has five parts: minute, hour, day of month, month, day of week. Here, 0 12 * * * means minute 0, hour 12, any day, any month, any weekday.

2. Step 2: Interpret the time

   Minute 0 and hour 12 means the task runs once daily at 12:00 PM (noon).

3. Final Answer:

   Run the task every day at 12:00 PM (noon) -> Option A

4. Quick Check:

   Minute=0, Hour=12 means noon daily [OK]

Hint: Minute and hour fields show exact run time [OK]

Common Mistakes:

• Confusing hour and minute positions
• Thinking * means a specific time
• Assuming 12 means midnight

2. Which of the following is the correct cron expression to run a task every 15 minutes in Airflow?

easy

A. */15 * * * *

B. 15 * * * *

C. 0 15 * * *

D. * 15 * * *

Solution

1. Step 1: Understand the meaning of '*/15' in cron

   '*/15' in the minute field means every 15 minutes starting at 0.

2. Step 2: Check other options

   15 * * * * runs at minute 15 every hour, 0 15 * * * runs at 3 PM daily, * 15 * * * is invalid because '*' in minute field with 15 in hour is not correct for every 15 minutes.

3. Final Answer:

   */15 * * * * -> Option A

4. Quick Check:

   */15 in minutes means every 15 minutes [OK]

Hint: Use '*/number' in minutes for periodic runs [OK]

Common Mistakes:

• Using fixed minute instead of interval
• Confusing hour and minute fields
• Misplacing the slash '/' syntax

3. Given the cron expression 30 6 * * 1-5, when will the Airflow task run?

medium

A. At 6:30 AM every day

B. At 6:00 AM every weekday

C. At 6:30 PM Monday through Friday

D. At 6:30 AM Monday through Friday

Solution

1. Step 1: Decode each cron field

   Minute=30, Hour=6, Day of month=any, Month=any, Day of week=1-5 (Monday to Friday).

2. Step 2: Interpret the schedule

   The task runs at 6:30 AM on weekdays (Monday to Friday).

3. Final Answer:

   At 6:30 AM Monday through Friday -> Option D

4. Quick Check:

   Day of week 1-5 means weekdays [OK]

Hint: Day of week 1-5 means Monday to Friday [OK]

Common Mistakes:

• Mixing AM and PM hours
• Ignoring day of week range
• Assuming day of month restricts schedule

4. You wrote the cron expression 60 24 * * * in Airflow to run a task daily. What is the problem?

medium

A. It runs every 60 minutes and 24 hours

B. It will run correctly at midnight

C. Minute 60 and hour 24 are invalid values

D. It runs only on the 60th day of the month

Solution

1. Step 1: Check valid ranges for cron fields

   Minute must be 0-59, hour must be 0-23. Here, 60 and 24 are out of range.

2. Step 2: Understand impact of invalid values

   Using invalid values causes syntax errors or the schedule will not run as expected.

3. Final Answer:

   Minute 60 and hour 24 are invalid values -> Option C

4. Quick Check:

   Minute max=59, Hour max=23 [OK]

Hint: Minutes max 59, hours max 23 in cron [OK]

Common Mistakes:

• Using 60 for minutes instead of 0
• Using 24 for hour instead of 0
• Assuming cron accepts out-of-range values

5. You want to schedule an Airflow DAG to run at 11:45 PM on the last day of every month. Which cron expression should you use?

hard

A. 45 23 31 * *

B. 45 23 L * *

C. 45 23 * * 31

D. 45 23 * * L

Solution

1. Step 1: Understand 'L' in cron expressions

   'L' means the last day of the month in the day-of-month field.

2. Step 2: Check each option

   45 23 31 * * runs only on 31st (not all months have 31 days). 45 23 L * * uses 'L' in day-of-month field correctly. 45 23 * * 31 uses 31 in day-of-week field (invalid). 45 23 * * L uses 'L' in day-of-week field (invalid).

3. Final Answer:

   45 23 L * * -> Option B

4. Quick Check:

   'L' in day-of-month means last day [OK]

Hint: Use 'L' in day-of-month for last day of month [OK]

Common Mistakes:

• Using 31 for all months (fails in shorter months)
• Placing 'L' in wrong cron field
• Confusing day-of-week and day-of-month fields