Humid

A few helpers for Javascript server-side rendering on Rails with mini_racer

Humid is a set of helper functions for using mini_racer for Server Side Rendering (SSR). There are only 2 pure public functions and a configure to set default args. mini_racer does the heavy lifting, Humid just provides a few conveniences.

While it was built with React in mind, it can work with any JS function that returns an HTML string.

Design

Humid is designed with 2 goals in mind:

1. Its for the common case where all data is gathered before rendering. Your application fetches everything needed, passes it as props, and Humid returns the rendered HTML in a single synchronous call. It does not support streaming or async data fetching during render.

2. Its a stepping stone for when you want to scale on the edge using Cloudflare V8 isolates .mini_racer is a bare V8 environment, if your JS bundle works with mini_racer, it'll work on Cloudflare V8 isolates.

Note

See the sample for a complete working example.

Installation

Add Humid to your Gemfile.

gem 'humid'

For source-map support, also add

yarn add source-map-support

Configuration

Add an initializer to configure the default options for Humid.render. These are overridable on Humid.render.

# config/initializers/humid.rb

Humid.configure do |config|
  # Path to your build file located in `app/assets/builds/`. You should use a
  # separate build apart from your `application.js`.
  #
  # Required
  config.application_path = Rails.root.join("app/assets/builds/server_rendering.js")

  # Path to your source map file
  #
  # Optional
  config.source_map_path = Rails.root.join("app/assets/builds/server_rendering.js.map")

  # Raise errors if JS rendering failed. If false, the error will be
  # logged and Humid.render will return an empty string.
  #
  # Defaults to true.
  config.raise_render_errors = Rails.env.local?

  # The logger instance.
  # `console.log` and friends (`warn`, `error`) are delegated to
  # the respective logger levels on the ruby side.
  #
  # Defaults to `nil`
  config.logger = Rails.env.local? ? Rails.logger : nil
end

if Rails.env.local?
  # Use single_threaded mode for dev and test environments.
  # This will also work for mini_test parallel tests
  MiniRacer::Platform.set_flags! :single_threaded
  MINI_RACER_SSR = { context: MiniRacer::Context.new(timeout: 1000, ensure_gc_after_idle: 2000) }

  # Reload the context when the SSR bundle changes
  ssr_checker = ActiveSupport::FileUpdateChecker.new([Humid.config.application_path.to_s]) do
    MINI_RACER_SSR[:context].dispose
    MINI_RACER_SSR[:context] = MiniRacer::Context.new(timeout: 1000, ensure_gc_after_idle: 2000)
  end

  Rails.application.reloaders << ssr_checker
  Rails.application.reloader.to_run do
    ssr_checker.execute_if_updated
  end
end

Usage

Set a renderer

In your entry file, e.g, server_rendering.js (specified in config.application_path), pass your HTML render function to setHumidRenderer. There is no need to require the function, its included in the environment.

// Set a factory function that will create a new instance of our app
// for each request.
setHumidRenderer((json) => {
  const initialState = JSON.parse(json)

  return ReactDOMServer.renderToString(
    <Application initialPage={initialState}/>
  )
})

If you'd like support for source map support, you will need to add the following to the same file and set config.source_map_path like the configuration above.

require("source-map-support").install({
  retrieveSourceMap: filename => {
    return {
      url: filename,
      map: readSourceMap(filename)
    };
  }
});

Your webserver

On production, keep in mind that mini_racer is thread safe, but not fork safe. When using with web servers that employ forking, create the context on worker boot. There should be no context created on the master process.

For example with puma:

# config/puma.rb
on_worker_boot do
  ctx = MiniRacer::Context.new(timeout: 1000, ensure_gc_after_idle: 2000)
  MINI_RACER_SSR = { context: Humid.prepare(ctx) }
end

on_worker_shutdown do
  MINI_RACER_SSR[:context].dispose
end

Humid.prepare will prepare the context's environment.

You can also override config options per-context:

ctx = Humid.prepare(
  MiniRacer::Context.new(timeout: 1000),
  application_path: Rails.root.join("other_bundle.js"),
  logger: nil
)
MINI_RACER_SSR = { context: ctx }

Note

If you pass a context that was already prepared, prepare will noop and return the context back to you.

Call Humid.render

And finally call render from ERB.

<%= Humid.render(MINI_RACER_SSR[:context], json).html_safe %>

Instrumentation is included:

Completed 200 OK in 14ms (Views: 0.2ms | Humid SSR: 11.0ms | ActiveRecord: 2.7ms)

The mini_racer environment

Functions not available

The following functions are not available in the mini_racer environment

• setTimeout
• clearTimeout
• setInterval
• clearInterval
• setImmediate
• clearImmediate

console.log

console.log and friends (info, error, warn) are delegated to the respective methods on the configured logger.

All arguments are passed through — MiniRacer converts JS objects to Ruby hashes and arrays automatically. A log_formatter proc controls how these arguments are formatted into a single string for the logger:

Humid.configure do |config|
  config.logger = Rails.logger

  config.log_formatter = proc { |level, message, *rest|
    parts = [message]
    parts += rest.map { |a| a.is_a?(String) ? a : JSON.pretty_generate(a) }
    parts.join("\n")
  }
end

The formatter receives (level, message, *rest) where:

• level — the log level as a symbol (:debug, :info, :warn, :error)
• message — the first argument passed to console.log/info/warn/error
• rest — any additional arguments (objects come through as Ruby hashes/arrays)

The default formatter returns message unchanged.

Server-side libraries that detect node.js envs.

Some libraries check for Node.js or browser globals to decide which code path to use. In mini_racer, neither environment is detected. You may need to configure your bundler to alias server-friendly versions. For example, in esbuild:

// build_ssr.mjs
await esbuild.build({
  // ...
  alias: {
    'html-dom-parser': 'html-dom-parser/lib/html-to-dom-server'
  }
})

Writing universal code

Vue has a resource on how to write universal code. Below are a few highlights that are important to keep in mind.

State

Humid uses a single context across multiple request. To avoid state pollution, we provide a factory function to setHumidRenderer that builds a new app instance on every call.

This provides better isolation, but as it is still a shared context, polluting global is still possible. Be careful of modifying global in your code.

Missing browser APIs

Some libraries that depend on browser APIs will fail in the mini_racer environment because of missing browser APIs. Account for this by moving the require to useEffect in your component.

  useEffect(() => {
    const svgPanZoom = require('svg-pan-zoom')
    //...
  }, [])

Polyfills

React SSR may import node.js dependencies that you need to polyfill for. See a sample esbuild build script and a shim.js to get around these issues.

Telemetry

The MiniRacer::Context gives you access to V8 heap statistics for monitoring memory usage over time.

MINI_RACER_SSR[:context].heap_stats
# {:total_heap_size=>3100672,
#  :total_heap_size_executable=>4194304,
#  :total_physical_size=>1280640,
#  :total_available_size=>1501560832,
#  :used_heap_size=>1205376,
#  :heap_size_limit=>1501560832,
#  ...}

You can combine humid's instrumentation and OpenTelemetry to track heap growth per worker:

meter = OpenTelemetry.meter_provider.meter("humid")
render_histogram = meter.create_histogram("humid.render.duration", unit: "ms", description: "SSR render duration")
heap_gauge = meter.create_gauge("humid.heap.used_bytes", unit: "By", description: "V8 heap used bytes")

ActiveSupport::Notifications.subscribe("render.humid") do |event|
  stats = MINI_RACER_SSR[:context].heap_stats
  attributes = { "worker.pid" => Process.pid.to_s }

  render_histogram.record(event.duration, attributes: attributes)
  heap_gauge.record(stats[:used_heap_size], attributes: attributes)
end

A steadily climbing used_heap_size across requests indicates a memory leak in your JavaScript bundle.

Contributing

Please see CONTRIBUTING.md.

License

Humid is Copyright © 2021-2026 Johny Ho. It is free software, and may be redistributed under the terms specified in the LICENSE file.

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