Periodic tasks with Celery Beat in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a periodic task is scheduled with Celery Beat?

Consider a Flask app using Celery with Celery Beat to schedule a task every minute. What is the expected behavior when the scheduler triggers the task?

Flask

from celery import Celery
from celery.schedules import crontab

app = Celery('tasks', broker='redis://localhost:6379/0')

@app.task
def send_report():
    print('Report sent')

app.conf.beat_schedule = {
    'send-report-every-minute': {
        'task': 'tasks.send_report',
        'schedule': crontab(minute='*'),
    },
}

app.conf.timezone = 'UTC'

A The send_report task runs only if manually called in the Flask app code.

B The send_report task runs only once when the app starts.

C The send_report task runs every second due to the crontab schedule.

D The send_report task runs automatically every minute without manual triggering.

💡 Hint

Think about what Celery Beat does with the schedule configuration.

Celery Beat acts as a scheduler that triggers tasks at specified intervals automatically. Here, the task is set to run every minute, so it runs without manual calls.

📝 Syntax

intermediate

Identify the syntax error in this Celery Beat schedule configuration

Which option shows the correct way to define a periodic task schedule in Celery Beat?

Flask

app.conf.beat_schedule = {
    'task-name': {
        'task': 'tasks.example_task',
        'schedule': crontab(hour=0, minute=0)
    }
}

A app.conf.beat_schedule = {'task-name': {'task': 'tasks.example_task', 'schedule': crontab(hour=0, minute=0)}}

B app.conf.beat_schedule = {'task-name': {'task': 'tasks.example_task', 'schedule': crontab(hour=0, minute=0),}}

C app.conf.beat_schedule = {'task-name': {'task': 'tasks.example_task', 'schedule': crontab(hour=0 minute=0)}}

D app.conf.beat_schedule = {'task-name': {'task': 'tasks.example_task', 'schedule': crontab(hour=0; minute=0)}}

💡 Hint

Check for missing commas or invalid separators in the dictionary.

Option B correctly uses a trailing comma after the last item in the dictionary, which is valid in Python. Option B misses a comma between arguments, and D uses a semicolon which is invalid syntax.

🔧 Debug

advanced

Why does the periodic task not run even though Celery Beat is configured?

A Flask app has Celery and Celery Beat configured with a periodic task. The task is defined and scheduled correctly, but it never runs. What is the most likely cause?

Flask

from celery import Celery

app = Celery('tasks', broker='redis://localhost:6379/0')

@app.task
def cleanup():
    print('Cleanup done')

app.conf.beat_schedule = {
    'cleanup-task': {
        'task': 'tasks.cleanup',
        'schedule': 60.0,
    },
}

app.conf.timezone = 'UTC'

A The Celery Beat scheduler process is not running alongside the worker.

B The task name 'tasks.cleanup' is incorrect and should be 'cleanup'.

C The schedule value 60.0 is invalid and must be a crontab object.

D The broker URL is missing the password for Redis.

💡 Hint

Think about what components must run for periodic tasks to execute.

Celery Beat must be running as a separate scheduler process to send periodic task messages. If it is not running, tasks won't be triggered even if configured.

❓ state_output

advanced

What is the output of this periodic task with state update?

Given this Celery task that updates a state dictionary each time it runs, what will be the value of state['count'] after 3 executions triggered by Celery Beat?

Flask

state = {'count': 0}

@app.task
def increment():
    state['count'] += 1
    print(f"Count is {state['count']}")

app.conf.beat_schedule = {
    'increment-task': {
        'task': 'tasks.increment',
        'schedule': 10.0,
    },
}

app.conf.timezone = 'UTC'

A Count is 1 printed three times; state['count'] remains 1 due to task isolation.

B Count is 3 printed once; state['count'] resets after each run.

C Count is 0 printed three times; state['count'] never increments.

D Count is 3 printed three times; state['count'] is 3 after 3 runs.

💡 Hint

Consider how Celery tasks run in separate worker processes.

Each Celery task runs in a separate process, so the state dictionary is not shared or persisted between runs. The count resets to 0 each time, so it prints 1 each run.

🧠 Conceptual

expert

How to ensure a periodic task runs exactly once in a distributed Celery worker setup?

You have multiple Celery workers running in parallel with Celery Beat scheduling a periodic task. How do you guarantee the task runs only once per schedule, not multiple times?

A Run multiple Celery Beat schedulers to balance the load.

B Configure each worker with a unique queue and schedule the task on all queues.

C Use a distributed lock or singleton pattern to prevent multiple executions.

D Set the task's schedule to crontab with seconds precision.

💡 Hint

Think about how to avoid race conditions in distributed systems.

When multiple workers receive the same scheduled task, a distributed lock ensures only one executes it. Without locking, the task may run multiple times concurrently.