There's a lot of open issues
Mayu Live is a live streaming server side VirtualDOM framework for Ruby, inspired by modern frontend tools that exist in the JavaScript ecosystem.


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 Project Readme

Mayu Live

Tests Release GitHub commit activity License AGPL-3.0 Status: Experimental



Mayu is a live streaming server side component-based VirtualDOM rendering framework written in Ruby.

Everything runs on the server, except for a tiny little runtime that deals with the connection to the server and updates the DOM.

It is very early in development and nothing is guaranteed to work. Still trying to figure out how to make a framework that is both easy to use and fun to work with.

Some parts are quite messy and some files are very long. This is fine. I like to paint with a broad brush until things are put in place and things feel right.

A starter kit is available at! If you have all dependencies installed, you will be able to deploy an app to within a few minutes without having to configure anything!

Core features:

  • 100% Ruby
  • 100% Server Side
  • 100% Async
  • Interactive web apps without JavaScript
  • Hot-reloading in dev
  • Automatic asset handling
  • Built-in metrics
  • File-system based routing inspired by Next.js
  • Designed for edge deployments
  • Powerful and compact templating using Haml
  • One component per file
  • One file per component
  • Lazy loading, prefetch hints, HTTP/2, caching

Table of contents

  • Description
  • Getting started
    • Install dependencies
    • Start the example app
    • Run the tests
  • Features
    • 100% server side
    • 100% async
    • Components
    • Scoped CSS
    • State management
    • Path based routing
    • Hot reloading
    • Optimized data transfer
    • Realtime metrics
    • Haml
  • Implementation notes
    • Tests
    • Virtual DOM
    • Server
      • Development server
      • Production server
    • Static typing
  • Contributing

Getting started

Install dependencies

Make sure that you have installed Ruby and NodeJS. The required versions are specified in the file .tool-versions in the project root.

ImageMagick and libwebp are also required for resizing images.

Install Ruby dependencies:

bundle install

Install node dependencies:

npm install

Build browser runtime:

npm run build

Start the example app

cd example
bundle install
bin/mayu dev

Now, open https://localhost:9292/ in your browser.

HTTP/2 requires HTTPS to work, therefore in development mode, Mayu will use the localhost gem to generate a self-signed certificate for localhost.

Depending on your system/browser you might need to do one of the following:

MacOS: Add the certificate to the keychain
  1. Open ~/.localhost/localhost.crt with Keychain Access.
  2. Choose Get Info and open Trust then choose `Always trust`.
  3. Restart your browsers.
Chrome: Enable self-signed certs for localhost
Go to chrome://flags/#allow-insecure-localhost and enable the setting. This will allow requests to localhost over HTTPS even when an invalid certificate is presented.
Firefox: Add an exception for the certificate
Firefox will show Warning: Potential Security Risk Ahead. Click Advanced, then Accept the Risk and Continue to add an exception for this certificate.

Run the tests

rake test


Most of these features are implemented, however, there are lots of sneaky bugs. Contributions in such as bug reports or pull requests are appreciated! 💚

The example app is deployed to as a proof of concept.

100% server side

Mayu keeps all state on the server and all HTML is being rendered on the server.

There is no need to implement an API, you access databases and private APIs directly in your callback handlers.

Mayu detects changes in components and sends instructions on how to patch the DOM to the browser using the Streams API. Client stream implementation.

Callbacks are regular POST-requests to /__mayu/session/#{session_id}/#{callback_id}, where the body contains the serialized event data.

100% async

socketry/async makes it possible to do all this without blocking.


  mount do
    loop do
      sleep 0.5
%p= state[:time]

This will print the current server time.

The component will render once every second even though it updates twice per second, since the time string only changes once per second.

Analog SVG clock component

Async loading

It's also easy to defer rendering until some action has happened, for example, the form demo loads tab content asynchronously when the mouse enters the tab header, so when the user clicks the tab, the content is already loaded.

Tabs implementation


Components are the building blocks of a Mayu application. They contain logic and return other components or HTML elements.

You might be familiar with ReactJS and other component based rendering libraries. This is the same thing, but in Ruby.

Scoped CSS

All class names and element names are given an unique name, inspired by CSS Modules.

Class names are applied automatically.


  .box {
    padding: 1px;
    border: 1px solid #000;

  .hello {
    font-weight: bold;

  button {
    background: #0f0;
    color: #fff;
  %p.hello Hello world
  %button Click me!

This would generate the following HTML:

<div class="/app/components/">
  <p class="/app/components/Example.hello?MjQSEK">Hello world</p>
  <button class="/app/components/Example_button?MjQSEK">Click me!</button>

Those are valid class names, as long as the characters are escaped in the CSS-file. Specification.

This will be inserted into <head>:


The browser will also be made aware of the assets used on a page via the Link-header, so that they can load even before the browser starts parsing the HTML.

$ curl -i https://localhost:9292 -k
HTTP/2 200
content-length: 5260
content-type: text/html; charset=utf-8
link: </__mayu/static/jkA-D11H90ChVHYqIOKn8I_A4w2MJ4nG-UEVP19UGqg=.js>; rel=preload; as=script; crossorigin=anonymous; fetchpriority=high, </__mayu/static/NtXGjOdgHqDJUnAhmk3NwuzFnkk8Z1NlBCE_XykVE-8=.css>; rel=preload; as=style, </__mayu/static/u6rK2NKHRcFKribL1sMcDdr1gXHbgYIVznfN5RJEKCA=.css>; rel=preload; as=style, </__mayu/static/shJPApqH5hptQERL4DivMTX42leUQRht9vGW4X_Rr84=.css>; rel=preload; as=style, </__mayu/static/ZStAGN7uGe7CU3cxSgAIOL550d1VDqVDUzdiQuFOXo8=.css>; rel=preload; as=style

Separate CSS-files

If you have very complex CSS, or maybe generate CSS-files, you can create a .css-file right next to your component.

This does the same thing as the previous example:


.box {
  padding: 1px;
  border: 1px solid #000;

.hello {
  font-weight: bold;

.button {
  background: #0f0;
  color: #fff;


  %p.hello Hello world
  %button.button Click me!

You can also mix the two styles.

Source maps

You can debug the sources of your CSS using source maps.

Source maps screenshot

State management

Mayu comes with some basic state management inspired by Redux Toolkit.

This is implemented but not yet integrated into the VDOM logic.

Example store

Ideally I would want something like XState, but I'm not experienced with it so I can't make anything like it.

Path based routing

Routing is inspired by the Next.js Layouts RFC.

It's a simple and straight forward approach, and it's super easy to locate files using a fuzzy finder plugin.

Here's the structure of a blog app:

├── root.haml
├── root.css
└── pages
    ├── page.haml
    ├── layout.haml
    ├── layout.css
    ├── about
    │   ├── page.haml
    │   └── page.css
    └── posts
        ├── page.haml
        ├── layout.haml
        └── :id
            └── page.haml

This would create the following routes:

path component layouts
/ app/pages/page.haml app/pages/layout.haml
/about/ app/pages/about/page.haml app/pages/layout.haml
/posts/ app/pages/posts/page.haml app/pages/layout.haml app/pages/posts/layout.haml
/posts/:id/ app/pages/posts/[id]/page.haml app/pages/layout.haml app/pages/posts/layout.haml
/* app/pages/404.haml app/pages/layout.haml

For a real-world example, check out example/app/pages/.

Hot reloading

There is a resource system inspired by JavaScript bundlers that loads all types of files.

Development mode

Components and styles update immediately in the browser as you edit files. No browser refresh needed.

Production mode

Before a server shuts down (when receiving the SIGINT signal), it will pause all sessions, serialize and encrypt them, and send them to the client and close the connection.

The client will then reconnect and post the encrypted session which will be decrypted, verified, deserialized and resumed.

I don't know how stable this is at the moment. Sometimes it seems like it can't restore components properly, maybe when their implementation has changed.

Whenever Issue #20 has been fixed, it would be quite easy to serialize the browser DOM and send it along with the encrypted state when resuming, and then use a DOM-diffing algorithm (like morphdom) on the browser DOM vs the DOM generated by the VDOM.

Optimized data transfer

Everything is minified and optimized and deliviered over HTTP/2. Images are scaled into different versions, non-binary assets are compressed with Brotli.

Asset filenames are based on their content hash so that they can be cached easily without having to worry about expiring them when they change.

The message stream uses DecompressionStream with the deflate-raw format. Browsers that don't support DecompressionStream will download a replacement based on fflate.

Messages are packed with MessagePack, which is supposed to be very efficient, although it's also the largest dependency at the moment. A good thing with MessagePack is that it can send binary data, which is useful when transferring state.

First page load with Slow 3G throttling (no cache):

Request waterfall

Second page load with Slow 3G throttling (cache):

Request waterfall

Realtime metrics

Mayu exposes a Prometheus-endpoint for metrics so you can see how your app performs.

Screenshots from Grafana on

Active sessions Patch times and counts


Mayu uses Haml, it's pretty convenient.

Check out the Haml Reference. Mayu has some differences with regular Haml to make it work better with a virtual DOM, you can read more about that in the documentation.

Look at this example:


That above code will be transformed into something like this:

# frozen_string_literal: true
Self =
    assets: ["0tyaKLqdvUGGcwZkdPOdMiMoMZoO74sMmtyRTuksjaQ=.css"],
    styles: {
      __Card: "example/app/pages/Counter_Card?7d89edff",
      __article: "example/app/pages/Counter_article?7d89edff",
      __output: "example/app/pages/Counter_output?7d89edff",
      __button: "example/app/pages/Counter_button?7d89edff",
  Card = import("/app/components/UI/Card")
  def self.get_initial_state(initial_value: 0, **) = { count: initial_value }
  def decrement_disabled = state[:count].zero?
  def handle_decrement
    update do |state|
      count = [0, state[:count] - 1].max
      { count: }
  def handle_increment
    update do |state|
      count = state[:count] + 1
      { count: }
public def render
          { class: :__button },
          { title: "Decrement" },
            onclick: mayu.handler(:handle_decrement),
            disabled: decrement_disabled,
        **mayu.merge_props({ class: :__output })
          { class: :__button },
          { title: "Increment" },
          { onclick: mayu.handler(:handle_increment) },
      **mayu.merge_props({ class: :__article })
    **mayu.merge_props({ class: :__Card }, { class: :card })

Check out more examples in the tests

Implementation notes


Tests are located in the lib/-directory next to their implementation. So for lib/mayu/state.rb the test would be located in lib/mayu/state.test.rb.

This pattern is quite common in JavaScript (Jest does this), and it's quite convenient to have things that are related close to each other, rather than to have a separate tree for tests.

It's also preferred to test things on a higher level, and only write unit tests for specific edge cases and trickier situations. Sorbet is pretty good at finding errors. If the higher level tests pass, then everything works as expected.

The example app could also be considered to be a test. It should always work and be updated to use the latest features.

Virtual DOM

Components return a VDOM::Descriptor which has a type, props and a key, similar to React, and props can also contain children. VDOM::VTree is responsible for keeping track of the VDOM::VNodes that make up the application. A VNode has a Descriptor and children which is an array of VNode objects. It can also have a component, in that case it would call the appropriate lifecycle methods of that component and pass its descriptors' props to the component before rendering.

The child diffing algorithm is quite inefficient. I have tried to implement the algorithm in snabbdom/preact/million several times, but they rely on DOM-operations for ordering (node.insertBefore) and the algorithm has to take care of that and make sure that the order is exactly the same in the VDOM as in the DOM after all patch operations have been applied.

The child diffing algorithm makes a few unnecessary moves, and there's lots of room for improvement, but at least the order is correct.


The server is configured in mayu.toml in the project root.


For development you probably want these settings:

count = 1
hot_swap = true
self_signed_cert = true


The production server depends on the output from a build step that parses all inputs and generates static files.

hot_swap = false
self_signed_cert = false

Static typing

Most files are strictly typed with Sorbet.

Some aren't strictly typed yet, but the goal is to enable strict typechecking everywhere.


Bug reports and pull requests are welcome on GitHub at This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the code of conduct.