django-lightweight-queue 4.11.0
Lightweight & modular queue and cron system for Django
License: BSD-3-Clause
Requires Python: >=3.6.1,<4

# Django Lightweight Queue

DLQ is a lightweight & modular queue and cron system for Django. It powers
millions of production jobs every day at Thread.

## Installation

```shell
pip install django-lightweight-queue[redis]
```

Currently the only production-ready backends are redis-based, so the `redis`
extra is essentially required. Additional non-redis backed production-ready
backends are great candidates for community contributions.

## Basic Usage

Start by adding `django_lightweight_queue` to your `INSTALLED_APPS`:

```python
INSTALLED_APPS = [
    "django.contrib.admin",
    "django.contrib.auth",
    ...,
    "django_lightweight_queue",
]
```

After that, define your task in any file you want:

```python
import time
from django_lightweight_queue import task

# Define a task
@task()
def long_running_task(first_arg, second_arg):
    time.sleep(first_arg * second_arg)

# Request that the task be executed at some point
long_running_task(4, second_arg=9)
```

See the docstring on the [`task`](django_lightweight_queue/task.py) decorator
for more details.

## Configuration

All automatically picked up configuration options begin with `LIGHTWEIGHT_QUEUE_`
and can be found in `app_settings.py`. They should be placed in the usual Django
settings files, for example:

```python
LIGHTWEIGHT_QUEUE_BACKEND = 'django_lightweight_queue.backends.redis.RedisBackend'
```

#### Special Configuration

If desired, specific configuration overrides can be placed in a standalone
python file which passed on the command line. This is useful for applying
customisations for specific servers.

For example, given a `special.py` containing:

```python
LIGHTWEIGHT_QUEUE_REDIS_PORT = 12345
```

and then running:

```
$ python manage.py queue_runner --extra-settings=special.py
```

will result in the runner to use the settings from the specified configuration
file in preference to settings from the Django environment. Any settings not
present in the specified file are inherited from the global configuration.

## Backends

There are four built-in backends:

### Synchronous (Development backend)

`django_lightweight_queue.backends.synchronous.SynchronousBackend`

Executes the task inline, without any actual queuing.

### Redis (Production backend)

`django_lightweight_queue.backends.redis.RedisBackend`

Executes tasks at-most-once using [Redis][redis] for storage of the enqueued tasks.

### Reliable Redis (Production backend)

`django_lightweight_queue.backends.reliable_redis.ReliableRedisBackend`

Executes tasks at-least-once using [Redis][redis] for storage of the enqueued tasks (subject to Redis consistency). Does not guarantee the task _completes_.

### Debug Web (Debug backend)

`django_lightweight_queue.backends.debug_web.DebugWebBackend`

Instead of running jobs it prints the url to a view that can be used to run a task in a transaction which will be rolled back. This is useful for debugging and optimising tasks.

Use this to append the appropriate URLs to the bottom of your root `urls.py`:

```python
from django.conf import settings
from django.urls import path, include

urlpatterns = [
    ...
]

if settings.DEBUG:
    urlpatterns += [
        path(
            "",
            include(
                "django_lightweight_queue.urls", namespace="django-lightweight-queue"
            ),
        )
    ]
```

This backend may require an extra setting if your debug site is not on localhost:

```python
# defaults to http://localhost:8000
LIGHTWEIGHT_QUEUE_SITE_URL = "http://example.com:8000"
```

[redis]: https://redis.io/

## Running Workers

The queue runner is implemented as a Django management command:

```
$ python manage.py queue_runner
```

Workers can be distributed over multiple hosts by telling each runner that it is
part of a pool:

```
$ python manage.py queue_runner --machine 2 --of 4
```

Alternatively a runner can be told explicitly how to behave by having
extra settings loaded (any `LIGHTWEIGHT_QUEUE_*` constants found in the file
will replace equivalent django settings) and being configured to run exactly as
the settings describe:

```
$ python manage.py queue_runner --exact-configuration --extra-settings=special.py
```

When using `--exact-configuration` the number of workers is configured exactly,
rather than being treated as the configuration for a pool. Additionally,
exactly-configured runners will _not_ run any cron workers.

#### Example

Given a Django configuration containing:

```python
LIGHTWEIGHT_QUEUE_WORKERS = {
    'queue1': 3,
}
```

and a `special.py` containing:

```python
LIGHTWEIGHT_QUEUE_WORKERS = {
    'queue1': 2,
}
```

Running any of:

```
$ python manage.py queue_runner --machine 1 --of 3 # or,
$ python manage.py queue_runner --machine 2 --of 3 # or,
$ python manage.py queue_runner --machine 3 --of 3
```

will result in one worker for `queue1` on the current machine, while:

```
$ python manage.py queue_runner --exact-configuration --extra-settings=special.py
```

will result in two workers on the current machine.

## Cron Tasks

DLQ supports the use of a cron-like specification of Django management commands
to be run at certain times.

To specify that a management command should be run at a given time, place a
`cron.py` file in the root folder of the Django app which defines the command
and which contains a `CONFIG` variable:

```python
CONFIG = (
    {
        'command': 'my_cron_command',
        # Day values 1-7 to match datetime.datetime.utcnow().isoweekday()
        'days': '*',
        'hours': '*',
        'minutes': '*',
        # Equivalent behaviour to the kwarg to `task` of the same name
        'sigkill_on_stop': True,
    },
)
```

## Maintainers

This repository was created by [Chris Lamb](https://github.com/lamby) at
[Thread](https://www.thread.com/), and continues to be maintained by the [Thread
engineering team](https://github.com/thread).