# Python Celery for Distributed Task Queues

Celery is a simple, flexible, and reliable distributed system to process vast amounts of messages, while providing operations with the tools required to maintain such a system. It is a task queue with focus on real-time processing, while also supporting task scheduling.

## Installation

Celery requires a message broker to send and receive messages. RabbitMQ and Redis are the most common choices.

```bash
pip install celery
# For Redis support
pip install "celery[redis]"
```

## Basic Application

Create a file named `tasks.py`.

```python
from celery import Celery

# Initialize Celery
# Broker URL format: transport://userid:password@hostname:port/virtual_host
app = Celery('tasks', broker='redis://localhost:6379/0', backend='redis://localhost:6379/0')

@app.task
def add(x, y):
    return x + y
```

## Running the Worker

You can start the worker by executing the program with the `worker` argument.

```bash
celery -A tasks worker --loglevel=INFO
```

*   `-A tasks`: The name of the module (file) containing the app instance.
*   `worker`: Starts the worker process.
*   `--loglevel=INFO`: Sets the logging level.

## Calling Tasks

To call a task, use the `delay()` method. This is a shortcut for `apply_async()`.

```python
from tasks import add

# This sends the task to the broker and returns an AsyncResult instance
result = add.delay(4, 4)

# Check if the task is ready
print(f"Ready: {result.ready()}")

# Get the result (blocks until ready)
try:
    print(f"Result: {result.get(timeout=1)}")
except Exception as e:
    print(f"Task failed: {e}")
```

## Configuration

Celery can be configured via a configuration object or a separate file.

```python
app.conf.update(
    task_serializer='json',
    accept_content=['json'],  # Ignore other content
    result_serializer='json',
    timezone='UTC',
    enable_utc=True,
)
```

## Periodic Tasks (Celery Beat)

Celery Beat is a scheduler that kicks off tasks at regular intervals.

```bash
celery -A tasks beat
```

[[programming/python/python]]