Xeus-Lite
JupyterLite + Xeus = ❤️️
JupyterLite is a JupyterLab distribution that runs entirely in the web browser, backed by in-browser language kernels. Xeus is C++ library for writing Jupyter kernels. In this blogpost we show how these two can work together.
JupyterLite
With vanilla Jupyter, kernels are running in separate processes. The communication between the kernels and the client is achieved with ZeroMQ. In contrast to this, JupyterLite runs entirely in the browser, including the language kernels. The following kernels are already implemented in the JupyterLite framework:
- JavaScript Kernel
- P5 Kernel: Since P5 is a JavaScript library, it is easy to provide a P5 kernel running in the browser.
- A Python kernel based on pyodide called pyolite: While the two JavaScript based kernels are trivial to implement as in-browser kernels, pyodide is a Python interpreter compiled to WebAssembly via Emscripten. Not only Python itself has been compiled to WebAssembly, but also the whole Scientific Stack including packages like: Numpy, Scipy, Pandas and many more. An overview of packages can be found here. Furthermore any pure Python package can be installed from within pyolite.
Since JupyterLite runs entirely in the browser, it is very easy to try it out!
Xeus
Xeus is C++ library for writing Jupyter kernels. It is not a kernel but a library that simplifies the process of writing kernels. Xeus implements the Jupyter messaging protocol, so that kernel authors only need to implement a handful of methods related to the actual interpreter, like executing pieces of code and auto-completing.
With the help of Xeus, it is very easy to write Jupyter kernels . In fact, Xeus has already been used for making several language kernels:
- xeus-cling is a Xeus-based C++ kernel.
- xeus-python is a Xeus-based Python kernel.
- xeus-lua is a Xeus-based Lua kernel.
- xeus-sql / xeus-sqlite are Xeus-based kernel for SQL / SQLite databases.
- xeus-robot is a Jupyter kernel for the Robot Framework .
- LFortran is a modern interactive Fortran compiler with a Xeus-based Jupyter kernel.
Since it is very easy to write Xeus based kernels for Jupyter, we also want to leverage Xeus for writing JupyterLite kernels. This has been the main motivation for Xeus 2.0.
Xeus Architecture
Xeus is has three extension points as depicted below
- The Server is the middleware component responsible for receiving and sending messages to the Jupyter client. The default server it is built upon ZeroMQ and handles the concurrency model of the application.
As we will see later, a custom server is needed to connect Xeus with JupyterLite. - The Interpreter provides the interface that kernel authors must implement.
- The Debugger provides the interface that can to be implemented when the kernel shall have debug capabilities, but we do not yet support this for Xeus-JupyterLite-Kernels.
JupyterLite Xeus-Kernels
Traditionally, the Jupyter kernel and the Client to the Jupyter kernel run in separate processes as depicted below:
The communication between the kernels and the client (ie the Jupyter-server) are done with inter-process communication via ZeroMQ. But with JupyterLite the kernel runs directly in the browser. In that case, ZeroMQ is not required anymore to communicate with the frontend. This was the motivation for Xeus 2.0, where it is possible to build Xeus without ZeroMQ. To implement JupyterLite Kernels with Xeus, we need to implement a custom Server which handles the Emscripten/Embind-based communication layer
The overall architecture is best described with the picture below: The C++ Xeus-Kernel is compiled to WebAssembly with Emscripten / Embind. The WebAssembly compiled code runs in a dedicated WebWorker thread. The WebWorker communicates via Message Passing with a thin layer of TypeScript which implements the IKernel API of JupyterLite. The communication between JupyterLite and the JupyterLite k ernel is done entirely in JavaScript.
The C++ code needed to compile an existing xeus-kernel to WebAssembly is very minimal:
Furthermore one needs a thin layer of TypeScript / JavaScript to implement the JupyterLite IKernel API. A good example of this code is given in the JupyterLite Xeus-Lua repository. While most of the repositories code is boilerplate, one needs to provide a simple Dockerfile which compiles the Xeus-Kernel to WebAssembly.
Xeus-Lua
We recently released a Xeus based Lua kernel. Even though its worthwhile to have Xeus-Lua kernel for vanilla Jupyter, the main purpose of Xeus-Lua was to experiment with Xeus-based JupyterLite kernels.
We choose Lua as the first language to try out Xeus-based JupyterLite kernels since Lua is easy to compile to WebAssembly.
A JupyterLite instance with Xeus-Lua included can be found here
Xeus-Wren
Wren is relatively new “Classy Little scripting Language” created by the author of the great book “Crafting Interpreters”. Wren can be described with the following:
We choose Wren as a second language to experiment with Xeus + JupyterLite since it is small, simple to compile to WebAssembly and easy to embed in C++. A JupyterLite instance with Xeus-Wren included can be found here.
Outlook
There are many languages which can be compiled to WebAssembly like:
- Julia: https://github.com/Keno/julia-wasm
- Octave: https://news.ycombinator.com/item?id=22926120
- C++: https://github.com/binji/wasm-clang
- Python: https://github.com/pyodide/pyodide
Also databases as sqlite can be compiled to WebAssembly:
Our goal is to provide Xeus-based JupyterLite kernels for these projects to make the JupyterLite ecosystem even more powerful.
Acknowledgements
We like to thank
- Jeremy Tuloup, Madhur Tandon, Martin Renou, and Nicholas Bollweg for their work on JupyterLite.
- Johan Mabille for Xeus and his help implementing the Xeus-Emscripten server.
About the Author
Thorsten Beier is a Scientific Software Engineer at QuantStack. Before joining QuantStack, he graduated in computer science at the University of Heidelberg and worked at the EMBL. As an open source developer, Thorsten worked on a variety of projects, from nifty and vigra in C++ to inferno, kipoi and ilastik in Python.
