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Motor: An Asynchronous MongoDB Driver (Python / Tornado)

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Tornado is a popular asynchronous Python web server, and MongoDB a widely used non-relational database. Alas, to connect to MongoDB from a Tornado app requires a tradeoff: You can either use PyMongo and give up the advantages of an async web server, or use AsyncMongo, which is non-blocking but lacks key features.

I decided to fill the gap by writing a new async driver called Motor (for "MOngo + TORnado"), and it's reached the public alpha stage. Please try it out and tell me what you think. I'll maintain a homepage for it here, including basic documentation.


Motor is alpha. It is certainly buggy. Its implementation and possibly its API will change in the coming months. I hope you'll help me by reporting bugs, requesting features, and pointing out how it could be better.


Two good projects, AsyncMongo and APyMongo, took the straightforward approach to implementing an async MongoDB driver: they forked PyMongo and rewrote it to use callbacks. But this approach creates a maintenance headache: now every improvement to PyMongo must be manually ported over. Motor sidesteps the problem. It uses a Gevent-like technique to wrap PyMongo and run it asynchronously, while presenting a classic callback interface to Tornado applications. This wrapping means Motor reuses all of PyMongo's code and, aside from GridFS support, Motor is already feature-complete. Motor can easily keep up with PyMongo development in the future.


Motor depends on greenlet and, of course, Tornado. It's been tested only with Python 2.7. You can get the code from my fork of the PyMongo repo, on the motor branch:

pip install tornado greenlet    
pip install git+https://github.com/ajdavis/mongo-python-driver.git@motor

To keep up with development, watch my repo and do

pip install -U git+https://github.com/ajdavis/mongo-python-driver.git@motor

when you want to upgrade.


Here's an example of an application that can create and display short messages:

import tornado.web, tornado.ioloop
import motor

class NewMessageHandler(tornado.web.RequestHandler):
    def get(self):
        """Show a 'compose message' form"""
        <form method="post">
            <input type="text" name="msg">
            <input type="submit">

    # Method exits before the HTTP request completes, thus "asynchronous"
    def post(self):
        """Insert a message
        msg = self.get_argument('msg')

        # Async insert; callback is executed when insert completes
            {'msg': msg},

    def _on_response(self, result, error):
        if error:
            raise tornado.web.HTTPError(500, error)

class MessagesHandler(tornado.web.RequestHandler):
    def get(self):
        """Display all messages
        self.write('<a href="/compose">Compose a message</a><br>')
        db = self.settings['db']
        db.messages.find().sort([('_id', -1)]).each(self._got_message)

    def _got_message(self, message, error):
        if error:
            raise tornado.web.HTTPError(500, error)
        elif message:
            self.write('<li>%s</li>' % message['msg'])
            # Iteration complete

db = motor.MotorConnection().open_sync().test

application = tornado.web.Application([
        (r'/compose', NewMessageHandler),
        (r'/', MessagesHandler)
    ], db=db

print 'Listening on http://localhost:8888'

Other examples are Chirp, a Twitter-like demo app, and Motor-Blog, which runs this site.


For now, you can ask for help in the comments, or email me directly at jesse@10gen.com if you have any questions or feedback. I'm going on Zencation July 25th through August 13; aside from that time I'll do my best to respond immediately.


In the next few months I'll implement the PyMongo feature I'm missing, GridFS, and make Motor work with all the Python versions Tornado does: Python 2.5, 2.6, 2.7, 3.2, and PyPy. (Only Python 2.7 is tested now.) Once the public alpha and beta stages have shaken out the bugs and revealed missing features, I hope Motor will be included as a module in the official PyMongo distribution.






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Published at DZone with permission of A. Jesse Jiryu Davis , DZone MVB .

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