Python Concurrency with asyncio

Matthew Fowler

• Publisher: Manning
• Year: 2022
• ISBN:
  9781638357087
• Topic: Informatica
• Subtopic: Programmazione in Python

Learn how to speed up slow Python code with concurrent programming and the cutting-edge asyncio library. Use coroutines and tasks alongside async/await syntax to run code concurrently
Build web APIs and make concurrency web requests with aiohttp
Run thousands of SQL queries concurrently
Create a map-reduce job that can process gigabytes of data concurrently
Use threading with asyncio to mix blocking code with asyncio code Python is flexible, versatile, and easy to learn. It can also be very slow compared to lower-level languages. Python Concurrency with asyncio teaches you how to boost Python's performance by applying a variety of concurrency techniques. You'll learn how the complex-but-powerful asyncio library can achieve concurrency with just a single thread and use asyncio's APIs to run multiple web requests and database queries simultaneously. The book covers using asyncio with the entire Python concurrency landscape, including multiprocessing and multithreading. About the technology
It's easy to overload standard Python and watch your programs slow to a crawl. Th e asyncio library was built to solve these problems by making it easy to divide and schedule tasks. It seamlessly handles multiple operations concurrently, leading to apps that are lightning fast and scalable. About the book
Python Concurrency with asyncio introduces asynchronous, parallel, and concurrent programming through hands-on Python examples. Hard-to-grok concurrency topics are broken down into simple flowcharts that make it easy to see how your tasks are running. You'll learn how to overcome the limitations of Python using asyncio to speed up slow web servers and microservices. You'll even combine asyncio with traditional multiprocessing techniques for huge improvements to performance. What's inside Build web APIs and make concurrency web requests with aiohttp
Run thousands of SQL queries concurrently
Create a map-reduce job that can process gigabytes of data concurrently
Use threading with asyncio to mix blocking code with asyncio codeAbout the reader
For intermediate Python programmers. No previous experience of concurrency required. About the author
Matthew Fowler has over 15 years of software engineering experience in roles from architect to engineering director.Table of Contents
1 Getting to know asyncio
2 asyncio basics
3 A first asyncio application
4 Concurrent web requests
5 Non-blocking database drivers
6 Handling CPU-bound work
7 Handling blocking work with threads
8 Streams
9 Web applications
10 Microservices
11 Synchronization
12 Asynchronous queues
13 Managing subprocesses
14 Advanced asyncio

1 Getting to know asyncio

This chapter covers

• What asyncio is and the benefits it provides
• Concurrency, parallelism, threads, and processes
• The global interpreter lock and the challenges it poses to concurrency
• How non-blocking sockets can achieve concurrency with only one thread
• The basics of how event-loop-based concurrency works

Many applications, especially in today’s world of web applications, rely heavily on I/O (input/output) operations. These types of operations include downloading the contents of a web page from the internet, communicating over a network with a group of microservices, or running several queries together against a database such as MySQL or Postgres. A web request or communication with a microservice may take hundreds of milliseconds, or even seconds if the network is slow. A database query could be time intensive, especially if that database is under high load or the query is complex. A web server may need to handle hundreds or thousands of requests at the same time.

Making many of these I/O requests at once can lead to substantial performance issues. If we run these requests one after another as we would in a sequentially run application, we’ll see a compounding performance impact. As an example, if we’re writing an application that needs to download 100 web pages or run 100 queries, each of which takes 1 second to execute, our application will take at least 100 seconds to run. However, if we were to exploit concurrency and start the downloads and wait simultaneously, in theory, we could complete these operations in as little as 1 second.

asyncio was first introduced in Python 3.4 as an additional way to handle these highly concurrent workloads outside of multithreading and multiprocessing. Properly utilizing this library can lead to drastic performance and resource utilization improvements for applications that use I/O operations, as it allows us to start many of these long-running tasks together.

In this chapter, we’ll introduce the basics of concurrency to better understand how we can achieve it with Python and the asyncio library. We’ll explore the differences between CPU-bound work and I/O-bound work to know which concurrency model best suits our specific needs. We’ll also learn about the basics of processes and threads and the unique challenges to concurrency in Python caused by its global interpreter lock (GIL). Finally, we’ll get an understanding of how we can utilize a concept called non-blocking I/O with an event loop to achieve concurrency using only one Python process and thread. This is the primary concurrency model of asyncio.

1.1 What is asyncio?

In a synchronous application, code runs sequentially. The next line of code runs as soon as the previous one has finished, and only one thing is happening at once. This model works fine for many, if not most, applications. However, what if one line of code is especially slow? In that case, all other code after our slow line will be stuck waiting for that line to complete. These potentially slow lines can block the application from running any other code. Many of us have seen this before in buggy user interfaces, where we happily click around until the application freezes, leaving us with a spinner or an unresponsive user interface. This is an example of an application being blocked leading to a poor user experience.

While any operation can block an application if it takes long enough, many applications will block waiting on I/O. I/O refers to a computer’s input and output devices such as a keyboard, hard drive, and, most commonly, a network card. These operations wait for user input or retrieve the contents from a web-based API. In a synchronous application, we’ll be stuck waiting for those operations to complete before we can run anything else. This can cause performance and responsiveness issues, as we can only have one long operation running at any given time, and that operation will stop our application from doing anything else.

One solution to this issue is to introduce concurrency. In the simplest terms, concurrency means allowing more than one task being handled at the same time. In the case of concurrent I/O, examples include allowing multiple web requests to be made at the same time or allowing simultaneous connections to a web server.

There are several ways to achieve this concurrency in Python. One of the most recent additions to the Python ecosystem is the asyncio library. asyncio is short for asynchronous I/O. It is a Python library that allows us to run code using an asynchronous programming model. This lets us handle multiple I/O operations at once, while still allowing our application to remain responsive.

So what is asynchronous programming? It means that a particular long-running task can be run in the background separate from the main application. Instead of blocking all other application code waiting for that long-running task to be completed, the system is free to do other work that is not dependent on that task. Then, once the long-running task is completed, we’ll be notified that it is done so we can process the result.

In Python version 3.4, asyncio was first introduced with decorators alongside generator yield from syntax to define coroutines. A coroutine is a method that can be paused when we have a potentially long-running task and then resumed when that task is finished. In Python version 3.5, the language implemented first-class support for coroutines and asynchronous programming when the keywords async and await were explicitly added to the language. This syntax, common in other programming languages such as C# and JavaScript, allows us to make asynchronous code look like it is run synchronously. This makes asynchronous code easy to read and understand, as it looks like the sequential flow most software engineers are familiar with. asyncio is a library to execute these coroutines in an asynchronous fashion using a concurrency model known as a single-threaded event loop.

While the name of asyncio may make us think that this library is only good for I/O operations, it has functionality to handle other types of operations as well by interoperating with multithreading and multiprocessing. With this interoperability, we can use async and await syntax with threads and processes making these workflows easier to understand. This means this library not only is good for I/O based concurrency but can also be used with code that is CPU intensive. To better understand what type of workloads asyncio can help us with and which concurrency model is best for each type of concurrency, let’s explore the differences between I/O and CPU-bound operations.

1.2 What is I/O-bound and what is CPU-bound?

When we refer to an operation as I/O-bound or CPU-bound we are referring to the limiting factor that prevents that operation from running faster. This means that if we increased the performance of what the operation was bound on, that operation would complete in less time.

In the case of a CPU-bound operation, it would complete faster if our CPU was more powerful, for instance by increasing its clock speed from 2 GHz to 3 GHz. In the case of an I/O-bound operation, it would get faster if our I/O devices could handle more data in less time. This could be achieved by increasing our network bandwidth through our ISP or upgrading to a faster network card.

CPU-bound operations are typically computations and processing code in the Python world. An example of this is computing the digits of pi or looping over the contents of a dictionary, applying business logic. In an I/O-bound operation we spend most of our time waiting on a network or other I/O device. An example of an I/O-bound operation would be making a request to a web server or reading a file from our machine’s hard drive.

Listing 1.1 I/O-bound and CPU-bound operations

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