Task queue concept in Flask - Step-by-Step Execution

Concept Flow - Task queue concept

Client sends request

↓

Add task to queue

↓

Worker picks task

↓

Worker processes task

↓

Worker updates status/result

↓

Client checks result/status

This flow shows how a task is added to a queue, processed by a worker, and the result is updated for the client.

Execution Sample

Flask

from flask import Flask
from rq import Queue
from redis import Redis

app = Flask(__name__)
redis_conn = Redis()
queue = Queue(connection=redis_conn)

def background_task(n):
    return n * 2

@app.route('/start/<int:n>')
def start_task(n):
    job = queue.enqueue(background_task, n)
    return f'Task {job.id} started'

This Flask app adds a background task to a Redis queue and returns the task ID.

Execution Table

Step	Action	Queue State	Worker State	Output/Result
1	Client calls /start/5	Empty	Idle	Task added to queue with ID 1
2	Task 1 enqueued	Task 1 (background_task(5))	Idle	Waiting for worker
3	Worker picks Task 1	Empty	Processing Task 1	Task 1 started
4	Worker runs background_task(5)	Empty	Processing Task 1	Calculating 5 * 2
5	Worker finishes Task 1	Empty	Idle	Result 10 stored
6	Client can check result	Empty	Idle	Result 10 available

💡 Task processed and result stored; queue is empty again.

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 5	Final
queue	Empty	Task 1 added	Task 1 removed	Empty	Empty
worker_state	Idle	Idle	Processing Task 1	Idle	Idle
task_result	None	None	None	10	10

Key Moments - 2 Insights

Why does the client get a task ID immediately before the task finishes?

Because the task is added to the queue instantly (see Step 2), the client receives the ID right away while the worker processes it later (Step 3-5).

What happens if the worker is busy when a new task is added?

The new task stays in the queue (Step 2) until the worker finishes current tasks and picks it up (Step 3).

Visual Quiz - 3 Questions

Look at the execution table, what is the worker state at Step 4?

A Idle

B Processing Task 1

C Waiting for task

D Task completed

💡 Hint

Check the 'Worker State' column at Step 4 in the execution table.

At which step does the queue become empty again?

A Step 2

B Step 3

C Step 5

D Step 6

💡 Hint

Look at the 'Queue State' column to see when tasks are removed.

If the client sends two tasks quickly, how would the queue state change at Step 2?

A Queue has two tasks waiting

B Queue is empty

C Worker processes both tasks immediately

D Queue has one task only

💡 Hint

Refer to the queue behavior in the variable_tracker and execution_table for multiple tasks.

Concept Snapshot

Task queue lets Flask handle long jobs outside request time.
Client adds task to queue, gets task ID immediately.
Worker runs tasks one by one from queue.
Results stored separately for client to check later.
This keeps app responsive and scalable.

Full Transcript

In Flask, a task queue helps run long jobs in the background. When a client sends a request, the app adds the job to a queue and returns a task ID right away. A worker process picks tasks from the queue and runs them separately. This way, the app stays fast and doesn't wait for the job to finish. The worker stores the result, which the client can check later. The execution table shows each step: adding the task, worker picking it, processing, and finishing. Variables like queue state and worker state change as tasks move through the system. This method is common to keep web apps responsive when doing heavy work.