Cron expressions in Airflow - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to schedule a DAG to run every day at midnight using a cron expression.

Apache Airflow

schedule_interval = '[1]'

Drag options to blanks, or click blank then click option'

A0 0 * * *

B0 12 * * *

C* * * * *

D0 0 * * 0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 12 instead of 0 for hour to represent midnight.

Using '*' for hour which means every hour.

The cron expression 0 0 * * * means the task runs daily at midnight (00:00).

2fill in blank

medium

Complete the code to schedule a DAG to run every Monday at 6 AM.

Apache Airflow

schedule_interval = '[1]'

Drag options to blanks, or click blank then click option'

A0 18 * * 1

B0 6 * * 0

C0 6 * * 1

D0 6 1 * *

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 for Monday instead of 1.

Using 18 for hour which means 6 PM instead of 6 AM.

The cron expression 0 6 * * 1 means the task runs at 6:00 AM every Monday (day 1 of the week).

3fill in blank

hard

Fix the error in the cron expression to run a DAG every 15 minutes.

Apache Airflow

schedule_interval = '[1]'

Drag options to blanks, or click blank then click option'

A*/15 * * * *

B15 * * * *

C0/15 * * * *

D0 * * * */15

Attempts:

3 left

💡 Hint

Common Mistakes

Using '15 * * * *' which runs only at minute 15 every hour.

Using '0/15' which is not standard cron syntax.

The correct cron expression for every 15 minutes is */15 * * * *. It means every 15 minutes at any hour, day, month, and weekday.

4fill in blank

hard

Fill both blanks to schedule a DAG to run at 3:30 PM on the 1st day of every month.

Apache Airflow

schedule_interval = '[1] [2] 1 * *'

Drag options to blanks, or click blank then click option'

A30

B15

C3

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping hour and minute fields.

Using 3 instead of 15 for hour which is 3 AM, not 3 PM.

The minute field should be 30 and the hour field 15 (3 PM in 24-hour format). The rest means every 1st day of the month.

5fill in blank

hard

Fill all three blanks to schedule a DAG to run every weekday at 8:00 AM.

Apache Airflow

schedule_interval = '[1] [2] * * [3]'

Drag options to blanks, or click blank then click option'

A0

B8

C1-5

D0-6

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0-6 for day of week which includes weekends.

Swapping hour and minute fields.

The minute is 0, hour is 8 (8 AM), and day of week is 1-5 which means Monday to Friday (weekdays).

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