m2wsgi 0.3.0

a mongrel2 => wsgi gateway

This module provides a WSGI gateway handler for the Mongrel2 webserver, allowing easy deployment of python apps on Mongrel2. It provides full support for chunked response encoding, streaming reads of large file uploads, and pluggable backends for evented I/O a la eventlet.

You might also find its supporting classes useful for developing non-WSGI handlers in python.

Command-line usage

The simplest way to use this package is as a command-line launcher:

m2wsgi dotted.app.name tcp://127.0.0.1:9999

This will connect to Mongrel2 on the specified request port and start handling requests by passing them through the specified WSGI app. By default you will get a single worker thread handling all requests, which is probably not what you want; increase the number of threads like so:

m2wsgi --num-threads=5 dotted.app.name tcp://127.0.0.1:9999

If threads aren’t your thing, you can just start several instances of the handler pointing at the same zmq socket and get the same effect. Better yet, you can use eventlet to shuffle the bits around like so:

m2wsgi --io=eventlet dotted.app.name tcp://127.0.0.1:9999

You can also use –io=gevent if that’s how you roll. Contributions for other async backends are most welcome.

Programmatic Usage

If you have more complicated needs, you can use m2wsgi from within your application. The main class is ‘WSGIHandler’ which provides a simple server interface. The equivalent of the above command-line usage is:

from m2wsgi.base import WSGIHandler
handler = WSGIHandler(my_wsgi_app,"tcp://127.0.0.1:9999")
handler.serve()

There are a lot of “sensible defaults” being filled in here. It assumes that the Mongrel2 recv socket is on the next port down from the send socket, and that it’s OK to connect the socket without a persistent identity.

For finer control over the connection between your handler and Mongrel2, create your own Connection object:

from m2wsgi.base import WSGIHandler, Connection
conn = Connection(send_spec="tcp://127.0.0.1:9999",
                  recv_spec="tcp://127.0.0.1:9992",
                  send_ident="9a5eee79-dbd5-4f33-8fd0-69b304c6035a")
handler = WSGIHandler(my_wsgi_app,conn)
handler.serve()

If you’re creating non-WSGI handlers for Mongrel2 you might find the following classes useful:

Connection:

represents the connection from your handler to Mongrel2, through which you can read requests and rend responses.

Request:

represents a client request to which you can asynchronously send response data at any time.

Don’t we already have one of these?

Yes, there are several existing WSGI gateways for Mongrel2:

• https://github.com/berry/Mongrel2-WSGI-Handler

• https://bitbucket.org/dholth/mongrel2_wsgi

None of them fully met my needs. In particular, this package has transparent support for:

• chunked response encoding

• streaming reads of large “async upload” requests

• pluggable IO backends (e.g. eventlet, gevent)

It’s also designed from the ground up specifically for Mongrel2. This means it gets a lot of functionality for free, and the code is simpler and lighter as a result.

For example, there is no explicit management of a threadpool and request queue as you might find in e.g. the CherryPy server. Instead, you just start up as many threads as you need, have them all connect to the same handler socket, and mongrel2 (via zmq) will automatically load-balance the requests to them.

Similarly, there’s no explicit support for reloading when the code changes. Just kill the old handler and start up a new one. If you’re using fixed handler UUIDs then zmq will ensure that the handover happens gracefully.

Current bugs, limitations and things to do

It’s not all perfect just yet, although it does seem to mostly work:

• Needs tests something fierce! I just have to find the patience to write the necessary setup and teardown cruft.

• When running multiple threads, ctrl-C doesn’t cleanly exit the process. Seems like the background threads get stuck in a blocking recv(). I really don’t want to emulate interrupts using zmq_poll…

• The zmq load-balancing algorithm is greedy round-robin, which isn’t ideal. For example, it can schedule several fast requests to the same thread as a slow one, making them wait even if other threads become available. I’m working on a zmq adapter that can do something better (see the pull2xreq device in this distribution).

• It would be great to grab connection details straight from the mongrel2 config database. Perhaps a Connection.from_config method with keywords to select the connection by handler id, host, route etc.