What Color is Your Python async Library?

Since I've began to program, I've heard everywhere that synchronous, sequential programming is a way of the past. In a manycore future, we will need to write programs that execute in parallel. But aside from embarassingly parallel programs, this has always been considered to be too difficult to be worth it.

One of the first issues to deal with is that we often confuse parallelism, concurrency and multithreading. While concurrency enables parallelism, multithreading is only one way to attain concurrency. And programming with threads is very hard because of all the things that we must consider when using them.

What alternative models of concurrency do we have? While threads are general, there are other models that make different trade-offs. Indeed, most problems that need concurrency are IO-bound, not CPU-bound. When you're reading a file or making an HTTP request, your CPU stays idle most of the time. How can we take advantage of this?

The answer is asynchronous I/O. During that HTTP request, your program can continue doing something else. While the idea is old, it gained popularity recently. node.js, released in 2009, made this idea very popular. But it also made it easy to enter callback hell. In 2012, C# 5.0 introduced async/await to the world. It was a great step forward that was adopted in Python and JavaScript. async/await, in some sense, is just a way to let the compiler write the callbacks itself. (Read about continuation-passing style if you would like to learn more about this.) But it does makes writing asynchronous I/O much more approachable, and I would not consider writing asynchronous code without that feature.

Asynchronous I/O has attracted a lot of praise and criticism. One obvious "problem" is that we do not actually use more CPU cores, we're just using one more efficiently. Anyway, the best criticism came from Bob Nystrom who wrote a very influential post named What Color is Your Function? in which he explains that asynchronous programming forces us to divide the world in two. For every synchronous library that does I/O you now need an asynchronous library too. This happens in the node.js standard library, but also in the Python world where the community reimplements many libraries using asyncio, for example as part of the aio-libs project.

If the cost is so great, why continue to write asynchronous code? Throughput is not the (only) answer. After all, carefully written threaded code can also handle more than 10K concurrent connections, despite what some proponents of asynchronous I/O say. However, asynchronous programming makes it much easier to reason about concurrency for large programs with lots of users while still writing fast code. And this is a huge deal.

The many worlds

However, in Python, the situation is even worse than the theoretical two worlds. We have way more than two worlds.

First, there's synchronous code. The requests library was written with only synchronous support in mind, and there's a lot of work needed before async can happen. Among other things, Python 2 support has to be dropped, and it won't happen before a few years. It is possible to use a threadpool to simulate synchronous code in a asynchronous way, but it's only a measure of last resort.

Second, there's callback-based asynchronous programming. This style was made popular with node.js before promises started to gain traction. In Python, that's mostly Twisted, but also the callback-based part of asyncio, the standard library module. The two are mostly compatible, and in fact a project named txaio allows to write code that's compatibly between the two. Twisted is not going anywhere, mostly because it includes a lot of stuff that does not exist elsewhere yet. The main issue with the callback approach is that while it's an improvement over using threads, the code can very easily end up as spaghetti code that is very hard to follow.

Third, we have the async/await part of asyncio in the standard library. This syntax allows the code to be way more natural to read because it's close to sequential code and is much easier to reason about. It attracted a lot of mindshare because it was the standard approach, and this is what the ambitious aio-libs project supports. That includes aiohttp, but also aiomysql, aiopg, aioredis, and so on. Unfortunately, while using it is simple, asyncio itself is a complex beast with many concepts to understand. Starting with Python 3.6, asyncio is no longer included on a provisional basis and is now guaranteed to be stable. It will continue to be the first place newcomers come to. Not going away either.

Fourth, we have curio. It reuses the syntax introduced with asyncio. After all, in Python async/await is a protocol and asyncio is only a library, and nothing prevents building other libraries to continue to innovate in the space. Unlike asyncio, it does not try to support callbacks. Surprisingly, this allows curio users to write less code which is more correct. Unfortunately, curio is still small and does not have all the libraries offered by the options above.

And there are others. eventlet, gevent, Tornado, trio. The list never ends. And they're not compatible: writing code that works between all of them is close to impossible.

How do we cope?

We could lament at the specific Python situation, but it actually enabled a lot of innovation. Each new library appears to be better than the previous one. But still, how do we cope? One way would be to keep the async functions in our code to a minimum. In The Function Colour Myth, Cory Benfield argues that with care, no more than 10% of your codebase needs to be async functions. But this is only true in new codebases, and still difficult to do: as long as you have an async function, all callers must be async.

In many existing codebases, we don't try to isolate I/O code. When we do try, we only isolate I/O in a specific class or function, but it still gets called all over the place. requests is the well-designed library in Python, but it stills YOLOs bytes into and out of sockets like nobody's business according to one of its maintainers (Cory Benfield, again).

The best way to isolate I/O is to perform no I/O at all. This radical idea from... Cory Benfield allowed to write hyper-h2 and h11. They're implementations of HTTP/2 and HTTP/1.1 that don't perform any I/O. How is this possible? Well, they introduce a toolkit which can be used in any Python code, be it synchronous, using async/await, curio, callbacks, and so on. It allows the Python ecosystem to start sharing code again. And since there's no I/O in those implementations, they are easy to test, too. h11, for example, has 100% coverage for both statements and branches.

The idea is generic, and a library named python-effect tries to help with it. The names comes from programming language research, but what it basically does is isolating I/O and state-manipulation code in objects. Those objects capture the I/O computation, which allows to perform I/O in a really small part of the code. Time will tell how successful this library becomes.

The concept itself, however, is already succesful, and is named sans I/O. If you write Python asynchronous libraries, I encourage you to think about staying compatible. Please don't get stuck in a single world, the other ones have many things to offer!

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