async-stagger 0.1.2

Happy eyeballs and underlying scheduling algorithm in asyncio

Quick, what’s the situation?

To get all the benefits of Happy Eyeballs connection establishment algorithm, simply use async_stagger.open_connection like you would use asyncio.open_connection:

reader, writer = await async_stagger.open_connection('www.example.com', 80)

Now your connections are more dual-stack friendly and will complete faster! A replacement for loop.create_connection is also provided.

The long version

What is Happy Eyeballs, and why should I use it?

Happy Eyeballs is an algorithm for establishing TCP connections to destinations specified by host names. It is described in RFC 6555 and RFC 8305. The primary benefit is that when host name resolution returns multiple addresses, and some of the address are unreachable, Happy Eyeballs will establish the connection much faster than conventional algorithms. For more information, check the Wikipedia article on Happy Eyeballs.

Python’s standard library provides several high-level methods of establishing TCP connections towards a host name: The socket module has socket.create_connection, and asyncio has loop.create_connection and asyncio.open_connection. These methods have the same behavior when a host name resolves to several IP addresses: they try to connect to the first address in the list, and only after the attempt fails (which may take tens of seconds) will the second one be tried, and so on. In contrast, the Happy Eyeballs algorithm will start an attempt with the second IP address in parallel to the first one hasn’t completed after some time, typically around 300 milliseconds. As a result several attempts may be in flight at the same time, and whenever one of the attempts succeed, all other connections are cancelled, and the winning connection is used. This means a much shorter wait before one of the IP addresses connect successfully.

Happy Eyeballs is particularly important for dual-stack clients, when some hosts may have resolvable IPv6 addresses that are somehow unreachable.

What does async_stagger has to offer?

async_stagger provides open_connection and create_connection with Happy Eyeballs support. They are mostly drop-in replacements for their asyncio counterparts (Well, not exactly: create_connection takes a loop argument instead of being a method on an event loop). There are two additional optional arguments, delay and interleave, that can be left at default for most purposes.

Another public coroutine create_connected_sock returns a connected socket.socket object, for when you want lower level access.

Happy Eyeballs sounds great! I want to use similar logic somewhere else!

You’re in luck! async_stagger actually exposes the underlying scheduling logic as a reusable block: staggered_race. It can be use when:

• There are several ways to achieve one goal. Some of the ways may fail, but you have to try it to find out.

• Making attempts strictly in sequence is too slow.

• You want to parallelize, but also don’t want to start the attempts all at the same time. Maybe you want to give preference to some of the attempts, so they should be started earlier and given more time to complete. Maybe you want to avoid straining the system with simultaneous attempts.

• An attempt done half-way can be rolled back safely.

Where can I get it?

(At the time I’m writing this README, none of the links below are live yet. Hopefully I remember to check them.)

async_stagger requires Python 3.6 or later. (It doesn’t work on Python 3.5 for a somewhat trivial reason: trailing commas in function definition argument lists. If those pesky commas are removed, the code should probably run well enough.) It does not have any external dependencies. Install it from PyPI the usual way:

pip install async-stagger

The documentation can be found here: http://async-stagger.readthedocs.io/en/latest/

This project is licensed under the MIT license.

Acknowledgments

The Happy Eyeballs scheduling algorithm implementation is inspired by the implementation in trio.