Periodic tasks with Celery Beat in Flask - Step-by-Step Execution

Concept Flow - Periodic tasks with Celery Beat

Define Celery app

↓

Configure Celery Beat schedule

↓

Start Celery worker and Beat

↓

Beat triggers task periodically

↓

Worker executes task

↓

Task completes and waits for next trigger

This flow shows how Celery Beat schedules tasks periodically and the worker executes them repeatedly.

Execution Sample

Flask

from celery import Celery
app = Celery('tasks', broker='redis://localhost')
app.conf.beat_schedule = {
  'say-hello-every-10-seconds': {
    'task': 'tasks.say_hello', 'schedule': 10.0
  }
}
@app.task
def say_hello():
  print('Hello!')

This code sets up a Celery app with a Beat schedule to run say_hello every 10 seconds.

Execution Table

Step	Action	Beat Schedule Check	Task Triggered	Worker Executes	Output
1	Start Celery Beat and Worker	Beat running, waiting 10s	No	No
2	10 seconds passed	Beat triggers say_hello	Yes	Yes	Hello!
3	Task completes	Beat resets timer	No	No
4	Another 10 seconds passed	Beat triggers say_hello	Yes	Yes	Hello!
5	Task completes	Beat resets timer	No	No

💡 This process repeats indefinitely until stopped.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
Beat Timer	0s	10s	0s	10s	0s	Repeats
Task Triggered	False	True	False	True	False	Repeats
Worker State	Idle	Running say_hello	Idle	Running say_hello	Idle	Repeats

Key Moments - 2 Insights

Why doesn't the task run immediately when Celery Beat starts?

Celery Beat waits for the first schedule interval (10 seconds) before triggering the task, as shown in execution_table step 1 where the timer counts up before triggering.

How does Celery Beat know when to trigger the task again?

After each task execution, Beat resets its timer and waits for the next interval, visible in execution_table steps 3 and 5 where the timer resets to 0s.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the worker doing?

A Executing the say_hello task

B Idle, waiting for task

C Starting Celery Beat

D Stopping the worker

💡 Hint

Check the 'Worker Executes' column at step 2 in execution_table

At which step does the Beat reset its timer after task completion?

A Step 1

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Beat Schedule Check' column for timer reset in execution_table

If the schedule was changed to 5 seconds, how would the 'Beat Timer' variable change in variable_tracker?

A It would count to 10 seconds before resetting

B It would count to 5 seconds before resetting

C It would never reset

D It would reset randomly

💡 Hint

Refer to the 'Beat Timer' row in variable_tracker and relate to schedule timing

Concept Snapshot

Periodic tasks with Celery Beat:
- Define Celery app and tasks
- Configure beat_schedule with task and interval
- Start Celery worker and Beat together
- Beat triggers tasks on schedule
- Worker executes tasks repeatedly
- Process runs continuously until stopped

Full Transcript

This visual execution shows how Celery Beat works with a Flask Celery app to run periodic tasks. First, the Celery app is defined with a Redis broker. Then, the beat_schedule config sets a task to run every 10 seconds. When Celery Beat and the worker start, Beat waits 10 seconds before triggering the task. The worker runs the task, printing 'Hello!'. After the task finishes, Beat resets its timer and waits for the next 10 seconds. This cycle repeats indefinitely. Variables like the Beat timer and worker state change accordingly. Key moments include understanding why the task doesn't run immediately and how Beat resets its timer. The quizzes test understanding of worker actions, timer resets, and schedule changes.