Flask-Melodramatiq

Flask-Melodramatiq is a Flask extension that adds support for the Dramatiq task processing library.

Dramatiq is a great library, and Flask-Melodramatiq tries hard not to force you to change the way you interact with it. Flask-Melodramatiq defines thin wrappers around the broker types available in Dramatiq, so that all the power of Dramatiq's API remains available to you.

For example:

import dramatiq
from flask import Flask
from flask_melodramatiq import RabbitmqBroker

app = Flask(__name__)
broker = RabbitmqBroker(app)
dramatiq.set_broker(broker)

@dramatiq.actor
def task():
    print('Snakes appreciate good theatrical preformace.')

or, if you prefer the Flask application factory pattern:

import dramatiq
from flask import Flask
from flask_melodramatiq import RabbitmqBroker

broker = RabbitmqBroker()
dramatiq.set_broker(broker)

@dramatiq.actor
def task():
    print('Snakes appreciate good theatrical preformace.')

def create_app():
    app = Flask(__name__)
    broker.init_app(app)
    return app

In those examples, the broker instance that we created (we call it a "lazy broker") is a genuine Dramatiq broker, and can be used anywhere where a "native" broker can be used. (It has dramatiq.brokers.rabbitmq.RabbitmqBroker as a superclass!) Lazy brokers are thin wrappers which add several important features:

1. They honor the settings in the Flask application configuration.
2. init_app can be called on them before or after the actors have been defined.
3. The Flask application context is correctly set during the execution of the tasks.
4. They add few convenience methods. (The Broker.actor decorator for example.)

Configuration

You can change the configuration options for your broker by passing keyword arguments to the constructor, or by setting corresponding values for the DRAMATIQ_BROKER_* set of keys in the app config. For example, you can do either:

from flask_melodramatiq import RabbitmqBroker

broker = RabbitmqBroker(
    url='amqp://mybroker:5672', confirm_delivery=True)

or you can put this in your app config:

DRAMATIQ_BROKER_URL = 'amqp://mybroker:5672'
DRAMATIQ_BROKER_CONFIRM_DELIVERY = True

If the configuration values passed to the constructor are different from the ones set in the app config, the later take precedence. You can even set/override the type of the broker in the app config:

from flask_melodramatiq import Broker

broker = Broker()  # Broker's type is not specified here.

and instead, specify the type in the app config:

DRAMATIQ_BROKER_CLASS = 'StubBroker'

This feature can be quite useful when testing your code.

Starting workers

With Flask-Melodramatiq you have the whole power of Dramatiq's CLI on on your disposal. For example, to start worker processes for your broker instance you may run:

$ dramatiq wsgi:broker

and in wsgi.py you may have something like this:

from myapp import create_app, broker

app = create_app()

You may have as many broker instances as you want, but you need to start each one of them with a separate command.

Installation

You can install Flask-Melodramatiq with pip:

$ pip install Flask-Melodramatiq

You can read the docs here.