What is "asynchronous"?

Asynchronous communication, transmission of data without the use of an external clock signal, where data can be transmitted intermittently rather than in a steady stream.

Wikipedia

In other words:

We don't want to wait and block our application awaiting the response.

You know JavaScript? ;)

What blocks our apps?

• I/O
  • Networking
  • Disk operations
• Computation
• Shared objects
• System calls

Possible solutions

(in Python)

Non-blocking solutions

• Processes
• Threads
• Low-level select-like
• Greenlets / Coroutines / Green Threads

Processes

are

expensive

Threads?

CPython threads are managed by OS

Threads are still expensive and not worth due to GIL

select?

So C-like solution!

Green threads

to the rescue!

Why user-space threads?

• Light weight and fast
• One code path at a time
• No condition races
• Similar API to threads
• High-level API
  • Event loop based
  • select-like for blocking I/O

But...

wrong code can block the process

Why for web servers?

• Fast
• Memory efficient
• Number of connections
  is virtually unlimited

Python implementations

• Asyncio
• Gevent
• Tornado
• Twisted
• Eventlet
• Greenlet
• ...

asyncio is

low-level API

Gevent is general purpose framework
(focused on networking
and synchronous API)

Tornado is a web framework

Twisted is
event-driven networking engine

Your choice should depend on a problem

Case study

Web application that shows tweets in realtime

Gevent vs Tornado

Tweepy library

for Twitter Streaming API

Tweepy uses

• uses blocking i/o
• threads

Gevent

• Greenlets
• Threads monkey patching
• Synchronous API
• One process

Tornado

• Event loop
• Bad idea to run with threads
• Asynchronous API
• Two processes

Let's dive into code

https://bitbucket.org/sunscrapers/twfeed

https://github.com/haxoza/tweefee

Thanks!

Questions?

@haxoza

Resources

• http://www.artima.com/weblogs/viewpost.jsp?thread=214235
• http://stackoverflow.com/questions/200469/what-is-the-difference-between-a-process-and-a-thread
• http://stackoverflow.com/questions/12758952/green-threads-and-thread-in-python
• http://vaidik.in/blog/understanding-non-blocking-io-with-python-part-1.html
• Gevent & Tornado docs