Periodic tasks with Celery Beat in Flask - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Celery Beat in the context of Celery?

Celery Beat is a scheduler that kicks off tasks at regular intervals, which are then executed by Celery workers. It helps automate periodic tasks like sending emails or cleaning databases.

intermediate

How do you define a periodic task using Celery Beat in Flask?

You define periodic tasks by adding a schedule to Celery Beat's configuration, usually in the 'beat_schedule' dictionary, specifying the task name, the function to call, and the interval or crontab timing.

beginner

What is the role of the 'beat_schedule' setting in Celery Beat?

'beat_schedule' is a dictionary where each key is a task name and the value defines when and how often the task runs, using intervals or crontab expressions.

beginner

Why do you need to run both Celery worker and Celery Beat for periodic tasks?

Celery Beat only schedules tasks but does not execute them. The Celery worker listens for tasks and runs them. Both must run simultaneously for periodic tasks to work.

beginner

How can you test if your periodic task with Celery Beat is working correctly?

You can check the Celery worker logs to see if the task runs at the scheduled times, or add print/log statements inside the task function to confirm execution.

What does Celery Beat do in a Flask app?

A Schedules tasks to run periodically

B Executes tasks immediately

C Stores task results

D Manages database connections

Celery Beat schedules tasks to run at set intervals but does not execute them.

Which configuration dictionary holds periodic task schedules in Celery Beat?

A worker_concurrency

B task_routes

C beat_schedule

D broker_url

'beat_schedule' defines when periodic tasks run.

To run periodic tasks, you must start:

A Both Celery Beat and Celery worker

B Only Celery worker

C Only Celery Beat

D Neither, tasks run automatically

Celery Beat schedules tasks; Celery worker executes them.

Which of these can you use to specify task timing in Celery Beat?

A CSS selectors

B SQL query

C HTML tags

D crontab

Crontab expressions define schedules like 'every day at 5pm'.

How can you confirm a periodic task ran successfully?

A Restart the Flask app

B Check Celery worker logs

C Delete the task

D Change the broker URL

Logs show when tasks are executed by the worker.

Explain how Celery Beat and Celery worker work together to run periodic tasks in a Flask app.

Think about who plans the task and who actually does it.

Describe how to set up a simple periodic task using Celery Beat in Flask.

Start from writing the task to running the scheduler and worker.

Practice

1. What is the main purpose of Celery Beat in a Flask application?

easy

A. To schedule tasks to run automatically at specific times

B. To handle HTTP requests asynchronously

C. To manage database migrations

D. To serve static files efficiently

Solution

1. Step 1: Understand Celery Beat's role

   Celery Beat is a scheduler that triggers tasks periodically without manual intervention.

2. Step 2: Compare with other options

   Handling HTTP requests, managing migrations, and serving static files are unrelated to scheduling tasks.

3. Final Answer:

   To schedule tasks to run automatically at specific times -> Option A

4. Quick Check:

   Celery Beat = Task scheduler [OK]

Hint: Celery Beat is all about timing tasks automatically [OK]

Common Mistakes:

• Confusing Celery Beat with request handling
• Thinking it manages database or static files
• Assuming it runs tasks only once

2. Which of the following is the correct way to define a periodic task in Celery Beat using Flask?

easy

A. @app.route('/task')\ndef my_task():\n print('Hello')

B. @celery.task\ndef my_task():\n print('Hello')

C. def my_task():\n print('Hello')

D. @celery.task\nasync def my_task():\n print('Hello')

Solution

1. Step 1: Identify task decorator

   Periodic tasks in Celery are defined with the @celery.task decorator.

2. Step 2: Check syntax correctness

   @celery.task\ndef my_task():\n print('Hello') correctly uses @celery.task and a normal function. @app.route('/task')\ndef my_task():\n print('Hello') uses Flask route decorator, which is unrelated. def my_task():\n print('Hello') lacks any decorator. @celery.task\nasync def my_task():\n print('Hello') uses async which is not standard for Celery tasks.

3. Final Answer:

   @celery.task\ndef my_task():\n print('Hello') -> Option B

4. Quick Check:

   Use @celery.task to define Celery tasks [OK]

Hint: Use @celery.task decorator for Celery tasks [OK]

Common Mistakes:

• Using Flask route decorator instead of @celery.task
• Defining tasks without any decorator
• Using async def which Celery does not support by default

3. Given this Celery Beat schedule snippet, what will happen?

beat_schedule = {
    'print-every-10-seconds': {
        'task': 'app.print_message',
        'schedule': 10,
    },
}

medium

A. The task runs once after 10 seconds

B. The task runs every 10 minutes

C. The task 'print_message' runs every 10 seconds

D. This code causes a syntax error

Solution

1. Step 1: Understand the schedule value

   The 'schedule' key with value 10 means the task runs every 10 seconds.

2. Step 2: Confirm task repetition

   Celery Beat schedules tasks repeatedly at the interval given, so it runs every 10 seconds, not once or every 10 minutes.

3. Final Answer:

   The task 'print_message' runs every 10 seconds -> Option C

4. Quick Check:

   Schedule=10 means every 10 seconds [OK]

Hint: Schedule number means seconds interval by default [OK]

Common Mistakes:

• Thinking schedule is minutes instead of seconds
• Assuming task runs only once
• Confusing syntax with crontab format

4. Identify the error in this Celery Beat configuration:

beat_schedule = {
    'task1': {
        'task': 'app.task1',
        'schedule': crontab(minute='*/5')
    },
    'task2': {
        'task': 'app.task2',
        'schedule': '10',
    }
}

medium

A. The dictionary keys 'task1' and 'task2' must be integers

B. 'task' names must be functions, not strings

C. Missing import for crontab causes runtime error

D. 'schedule' for 'task2' should be an integer or crontab, not a string

Solution

1. Step 1: Check 'schedule' types

   For 'task1', schedule uses crontab correctly. For 'task2', schedule is a string '10', which should be an integer 10 or a crontab object.

2. Step 2: Validate other parts

   Task names as strings are correct references. Import errors are not shown here. Dictionary keys can be strings.

3. Final Answer:

   'schedule' for 'task2' should be an integer or crontab, not a string -> Option D

4. Quick Check:

   Schedule must be int or crontab, not string [OK]

Hint: Schedule values must be int or crontab, not quoted strings [OK]

Common Mistakes:

• Using string instead of int for schedule interval
• Confusing task names with function objects
• Assuming dictionary keys must be integers

5. You want to run a Flask Celery task every Monday at 8:30 AM using Celery Beat. Which beat_schedule entry is correct?

hard

A. {'task': 'app.weekly_task', 'schedule': crontab(hour=8, minute=30, day_of_week='mon')}

B. {'task': 'app.weekly_task', 'schedule': crontab(hour=8, minute=30, day_of_week=2)}

C. {'task': 'app.weekly_task', 'schedule': crontab(hour=8, minute=30, day_of_week='tue')}

D. {'task': 'app.weekly_task', 'schedule': crontab(hour=8, minute=30, day_of_week='Monday')}

Solution

1. Step 1: Understand crontab day_of_week format

   Celery's crontab accepts day_of_week as string names like 'mon', 'tue', or integers 0-6 (0=Sunday).

2. Step 2: Evaluate options

   {'task': 'app.weekly_task', 'schedule': crontab(hour=8, minute=30, day_of_week='mon')} uses 'mon' string which is correct. {'task': 'app.weekly_task', 'schedule': crontab(hour=8, minute=30, day_of_week=2)} uses integer 2 which is Tuesday (0=Sunday). {'task': 'app.weekly_task', 'schedule': crontab(hour=8, minute=30, day_of_week='tue')} uses 'tue' string which is Tuesday. {'task': 'app.weekly_task', 'schedule': crontab(hour=8, minute=30, day_of_week='Monday')} uses 'Monday' full name which is not supported.

3. Final Answer:

   {'task': 'app.weekly_task', 'schedule': crontab(hour=8, minute=30, day_of_week='mon')} -> Option A

4. Quick Check:

   Use 'mon' string for Monday in crontab [OK]

Hint: Use three-letter lowercase day names like 'mon' for crontab [OK]

Common Mistakes:

• Using full day names like 'Monday' instead of 'mon'
• Using string numbers like '1' instead of integers or names
• Confusing day_of_week numbering starting at Sunday