# Using Redis as a Broker for Celery

Redis is a popular choice for a Celery message broker because it is fast, easy to set up, and lightweight. While RabbitMQ is often the recommended broker for complex enterprise setups due to its reliability guarantees, Redis is excellent for many use cases.

## 1. Installation

You need to install Celery with the Redis bundle.

```bash
pip install "celery[redis]"
```

## 2. Configuration

To use Redis, you simply need to configure the `broker_url` in your Celery app.

### Basic Setup

```python
from celery import Celery

# Format: redis://:password@hostname:port/db_number
app = Celery('tasks', broker='redis://localhost:6379/0')

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

### Using Redis as Result Backend

Redis is also commonly used to store the *results* of tasks (the return values).

```python
app = Celery('tasks',
             broker='redis://localhost:6379/0',
             backend='redis://localhost:6379/1')
```

*Note: It is good practice to use different Redis databases (e.g., `/0` and `/1`) for the broker and the backend to avoid key collisions, although Celery keys are usually namespaced.*

## 3. Running Redis (via Docker)

The easiest way to get Redis running locally is using Docker.

```bash
docker run -d -p 6379:6379 --name my-redis redis
```

## 4. Important Considerations

### Visibility Timeout

One specific behavior of Redis as a broker is the **Visibility Timeout**.

If a worker picks up a task but crashes (or takes too long) before acknowledging it, Redis needs to know when to give that task to another worker.

*   **Default:** 1 hour (3600 seconds).
*   **Issue:** If a task is lost, it won't be retried for an hour.
*   **Configuration:** Set `broker_transport_options`.

```python
app.conf.broker_transport_options = {
    'visibility_timeout': 3600, # 1 hour
}
```

If you have very long-running tasks, you must increase this value, otherwise, the task might be redelivered to another worker while the first one is still processing it, causing duplication.

[[programming/python/celery]]