Cron expressions in Airflow - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a cron expression in Airflow?

A cron expression in Airflow is a string that defines the schedule for running a task. It uses five fields to specify minute, hour, day of month, month, and day of week.

beginner

What does the cron expression '0 12 * * *' mean in Airflow?

It means the task runs every day at 12:00 PM (noon). The fields are: minute=0, hour=12, day of month=any, month=any, day of week=any.

intermediate

How do you schedule a task to run every Monday at 8 AM using a cron expression in Airflow?

Use the cron expression '0 8 * * 1'. This means minute=0, hour=8, any day of month, any month, and day of week=1 (Monday).

beginner

What does the '*' symbol mean in a cron expression?

The '*' symbol means 'every' possible value for that field. For example, '*' in the month field means every month.

intermediate

How can you run a task every 15 minutes using a cron expression in Airflow?

Use '*/15 * * * *'. The '*/15' in the minute field means every 15 minutes, and the other fields are set to run every hour, day, month, and day of week.

What does the cron expression '30 6 * * *' schedule in Airflow?

A Runs at 6:30 AM every day

B Runs at 6:00 AM every 30 days

C Runs every 30 minutes starting at 6 AM

D Runs at 6:30 PM every day

The first field is minutes (30), second is hour (6), so it runs at 6:30 AM daily.

Which cron expression runs a task every Sunday at midnight in Airflow?

A 0 0 * * 0

B 0 0 * * 7

C 0 0 * * 1

D 0 0 7 * *

In cron, Sunday can be 0 or 7. '0 0 * * 0' and '0 0 * * 7' both mean midnight on Sunday.

What does the cron expression '15 14 1 * *' do in Airflow?

A Runs every day at 2:15 PM

B Runs at 1:14 PM every day

C Runs every 15 minutes starting at 2 PM

D Runs at 2:15 PM on the first day of every month

Minute=15, hour=14 (2 PM), day of month=1 means first day of each month at 2:15 PM.

How do you express 'every hour' in a cron expression for Airflow?

A * * * * *

B * 0 * * *

C 0 * * * *

D 0 0 * * *

"0 * * * *" means at minute 0 of every hour, every day.

Which part of the cron expression controls the day of the week in Airflow?

A Fifth field

B Second field

C First field

D Third field

The fifth field in a cron expression is for the day of the week (0-6 or SUN-SAT).

Explain how to read a cron expression in Airflow and give an example.

Think of each field as a time unit that controls when the task runs.

Describe how to schedule a task to run every weekday at 9 AM using a cron expression in Airflow.

Weekdays are Monday to Friday, which are days 1 to 5.

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