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How to use Flask with gevent (uWSGI and Gunicorn editions)

Python is booming and Flask is a pretty popular web-framework nowadays. Probably, quite some new projects are being started in Flask. But people should be aware, it's synchronous by design and ASGI is not a thing yet. So, if someday you realize that your project really needs asynchronous I/O but you already have a considerable codebase on top of Flask, this tutorial is for you. The charming gevent library will enable you to keep using Flask while start benefiting from all the I/O being asynchronous. In the tutorial we will see:

  • How to monkey patch a Flask app to make it asynchronous w/o changing its code.
  • How to run the patched application using gevent.pywsgi application server.
  • How to run the patched application using Gunicorn application server.
  • How to run the patched application using uWSGI application server.
  • How to configure Nginx proxy in front of the application server.

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From Callback Hell to async/await Heaven

In the previous article, we learned how to implement a simple but workable event loop. However, programs which are supposed to be run by the event loop are full of callbacks. This is the usual problem of event-loop-driven environments. When business logic becomes reasonably complicated, callbacks make program's code hardly readable and painfully maintainable. And the callback hell begins! There is plenty of ways to deal with the artificial complexity arose due to callbacks, but the most impressive one is to make the code great flat again. And by flat, I mean callback-less and synchronous-like. Usually, it's done by introducing async/await syntactic feature. But every high-level abstraction is built on top of some basic and fundamental ideas. Let's check the async/await sugar out and see what exactly happens under the hood.



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Explaining event loop in 100 lines of code

There is plenty of articles out there about the event loop. However, as a software engineer, I prefer to read code, not text. And there is no better way of learning a new concept than implementing it yourself. So, let's try to grasp the idea of the event loop by coding a new and shiny one.

NB: In the article, we will try to describe the idea of the event loop in general, not a specific implementation of the event loop in Node.js or Python, or some other language/library.

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Node.js Writable streams distilled

Writable streams are an abstraction for a destination to which data is written... And this time it's a concise abstraction! Compared to vague readable streams (multiple operation modes behind single abstraction) writable streams implement only one mode and hence expose only one essential method write(chunk). Nevertheless, the idea of writable streams is not trivial and it's due to one nice feature it provides - backpressure. Let's try to wrap our mind around writable stream abstraction.

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Node.js Readable streams distilled

Readable stream is an abstraction for some data source. Which could be hard to grasp and even harder to use...

Everybody knows that readable streams support two modes of operating (flowing and paused) and piping to writable streams. It's not that easy to understand the purposes of these mechanisms and behavioral differences though. Since one readable stream abstraction stands for multiple usage modes its public interface (i.e. the set of methods and events) is a bit inconsistent. Usage of readable streams might be totally confusing without the understanding of the underlying ideas. In this article, we will make an attempt to justify the abstraction of readable streams trying to implement our own file reader. Also, we will have a look at some nicer ways to consume readable streams.

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