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

Concept Flow - Periodic tasks with Celery Beat

Define Celery Task

↓

Configure Celery Beat Schedule

↓

Start Celery Worker

↓

Start Celery Beat Scheduler

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Scheduler triggers task at intervals

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Worker executes task

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Task completes and waits for next trigger

This flow shows how a periodic task is defined, scheduled, and executed repeatedly by Celery Beat and the Celery worker.

Execution Sample

Django

from celery import shared_task

@shared_task
def say_hello():
    print('Hello from Celery Beat!')

Defines a simple Celery task that prints a message when run.

Execution Table

Step	Action	Scheduler Time	Task Triggered	Worker Output
1	Celery Beat scheduler starts	00:00	No	No output
2	Scheduler checks schedule	00:05	No	No output
3	Scheduler time matches task interval	00:10	Yes	Task queued
4	Worker picks up task	00:10	Yes	Prints: Hello from Celery Beat!
5	Task completes	00:10	Yes	Idle until next trigger
6	Scheduler waits for next interval	00:15	No	No output
7	Scheduler time matches task interval again	00:20	Yes	Task queued
8	Worker executes task again	00:20	Yes	Prints: Hello from Celery Beat!
9	Task completes	00:20	Yes	Idle until next trigger

💡 The scheduler runs continuously; this trace shows two task executions at 10 and 20 seconds.

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 7	After Step 8	Final
Scheduler Time	00:00	00:10	00:10	00:20	00:20	00:20
Task Triggered	No	Yes	Yes	Yes	Yes	Yes
Worker Output	No output	Task queued	Prints: Hello from Celery Beat!	Task queued	Prints: Hello from Celery Beat!	Idle until next trigger

Key Moments - 3 Insights

Why doesn't the task run immediately when the scheduler starts?

The scheduler waits until the first scheduled interval matches the current time before triggering the task, as shown in steps 1 and 2 where no task is triggered.

How does the worker know when to execute the task?

The scheduler queues the task at the scheduled time (step 3), and the worker continuously checks the queue to pick up and execute tasks (step 4).

Does the task run only once or repeatedly?

The task runs repeatedly at each scheduled interval, demonstrated by the repeated triggers and executions at steps 3-5 and 7-9.

Visual Quiz - 3 Questions

Look at the execution table, at which step does the worker first print the message?

A Step 4

B Step 3

C Step 2

D Step 1

💡 Hint

Check the 'Worker Output' column to see when the message is printed.

At which scheduler time does the task get triggered again after the first execution?

A 00:15

B 00:20

C 00:10

D 00:05

💡 Hint

Look at the 'Scheduler Time' and 'Task Triggered' columns for repeated triggers.

If the task interval was changed to 15 seconds, which step would change in the execution table?

A Step 3

B Step 7

C Step 6

D Step 9

💡 Hint

Changing the interval affects when the scheduler waits and triggers tasks, check the 'Scheduler waits' step.

Concept Snapshot

Periodic tasks with Celery Beat:
- Define tasks with @shared_task decorator.
- Configure schedule in Celery Beat settings.
- Run Celery worker and Beat scheduler.
- Beat triggers tasks at intervals.
- Worker executes tasks repeatedly.
- Useful for regular background jobs.

Full Transcript

This visual trace shows how periodic tasks work with Celery Beat in Django. First, you define a task using the @shared_task decorator. Then, you configure the schedule so Celery Beat knows when to run it. When you start the Celery worker and the Beat scheduler, the scheduler waits until the scheduled time to trigger the task. The worker picks up the task from the queue and runs it, printing the message. This process repeats at each interval, showing how Celery Beat automates periodic background jobs.