Python Tutor

Python Tutor, created by Philip Guo, helps people overcome a fundamental barrier to learning programming: understanding what happens as the computer executes each line of a program’s source code.

Using this tool, you can write Python, Java, JavaScript, TypeScript, Ruby, C, and C++ programs in your Web browser and visualize what the computer is doing step-by-step as it executes those programs. So far, over 1.5 million people in over 180 countries have used Python Tutor to visualize over 15 million pieces of code, often as a supplement to textbooks, lecture notes, and online programming tutorials.


How we ended up with microservices

“When I was at SoundCloud, I was responsible for the migration from a monolithic Ruby on Rails application to a constellation of microservices. I’ve told the technical side of this story multiple times, both in presentations, and as a multi-part series for SoundCloud’s engineering blog. These engineering bits are what people are most interested in hearing about, but recently I realised I never explained to a wider audience how we ended up using microservices to begin with.

I am sorry to disappoint my fellow techies, but the reason we migrated to microservices had to do much more with productivity than pure technical matters. I’ll explain.

Note: This post definitely has a lot of revisionism, and, in trying to make it easier to understand, oversimplifies a fairly chaotic chain of events into a linear timeline. Nevertheless, I believe it paints a pretty good picture of my first couple of years at SoundCloud…”

Job queues, message queues and other queues. Almost all of them in one place

“There are many queueing systems out there. Each one of them is different and was created for solving certain problems. This page tries to collect the libraries that are widely popular and have a successful record of running on (big) production systems.

The goal is to create a quality list of queues with a collection of articles, blog posts, slides, and videos about them. After reading the linked articles, you should have a good idea about: the pros and cons of each queue, a basic understanding of how the queue works, and what each queue is trying to achieve. Basically, you should have all the information you need to decide which queue will best fit your needs.

The whole site is open source and is updated quite frequently. If you find any mistakes or see something missing, feel free to post an issue or send a pull request. All contributions are welcome! Thanks…”


“There’s many queueing systems out there and each one of them is different and was made for solving certain problems. This page collects all of the most popular ones, along with links to articles or videos about them, in one place…”

A study on scripting language APIs

A study on scripting language APIs

“Applications written in two programming languages, in order to optimize parts where performance is critical or to obtain extensibility through user-written scripts, are common-place nowadays. There are several ways to obtain this kind of interoperability; ideally, however, a language should provide a foreign language interface (FLI), allowing programmers to send and receive both data and function calls to code written in another language. This work discusses the main issues involving the design of APIs for integration of language environments within C applications. We present the main problems faced in the interaction between code executed in an environment with inherently dynamic characteristics such as a scripting language and C code. We compare approaches employed by five languages when handling communication between the data spaces of C and embedded runtime environments and the consequences of these approaches in memory management, as well as sharing of code between the C application and that from the scripting language. We illustrate the diferences of the APIs of those languages and their impact in the resulting code of a C application through a case study. Diferent scripting languages were embedded as plugins for a library, which on its turn exposes to client applications a generic scripting API. This way, the code of each plugin allows us to observe in a clear and isolated way the procedures adopted by each language for function calls, registration of C functions and conversion of data between the environments.”


“Goliath is an open source version of the non-blocking (asynchronous) Ruby web server framework powering PostRank. It is a lightweight framework designed to meet the following goals: bare metal performance, Rack API and middleware support, simple configuration, fully asynchronous processing, and readable and maintainable code (read: no callbacks).

The framework is powered by an EventMachine reactor, a high-performance HTTP parser and Ruby 1.9 runtime. One major major advantage Goliath has over other asynchronous frameworks is the fact that by leveraging Ruby fibers introduced in Ruby 1.9+, it can untangle the complicated callback-based code into a format we are all familiar and comfortable with: linear execution, which leads to more maintainable and readable code.

Each Goliath request is executed in its own Ruby fiber and all asynchronous I/O operations can transparently suspend and later resume the processing without requiring the developer to write any additional code. Both request processing and response processing can be done in fully asynchronous fashion: streaming uploads, firehose API’s, request/response, and so on…”