Replace rackup with cabup and files are automatically reloaded for you:

$ cabup
Puma starting in single mode...
* Version 3.11.4 (ruby 2.4.1-p111), codename: Love Song
* Min threads: 0, max threads: 16
* Environment: development
* Listening on tcp://localhost:9292
Use Ctrl-C to stop

• Functionality
• Usage
• Public API
• Versioning
• Other alternatives
• License

• cabup: Command-line tool which replaces rackup
• Made for applications that use explicit require/require_relative
• Tracks all required files
• Tracks all defined classes/modules
• #run checks if any of the files have changed (modification time), and reloads the files
• All defined classes/modules are un-defined before the file is loaded again
• Does not track autoloaded files
• Simple: 150 lines of code, no dependencies
• Cross platform: Uses only plain Ruby features

Reloading Ruby projects is rather complicated since any file can execute arbitary code, and before you can load the new, modified file, Cab needs to unload the previous file. Cab doesn't aim to magically support all complicated use cases, but instead requires you to write Ruby files which are reloadable. This shouldn't be a big problem since files that are easy to reload turns out to be files that are easy to reason about.

As long as you follow these rules you should be fine:

• Rule 1: Files that only define their own classes/modules will always work.
• Rule 2: Files should require (or require_relative) their dependencies.
• Rule 3: Files that execute code when they load must be idempotent (see below).
• Rule 4: Not everything can be reloaded and changes in some files (e.g. initializers) might require you to restart the server. That's okay. Cab strives to be useful, not perfect.

If you're trying out Cab with Sinatra you will quickly discover that reloading doesn't appear to work:

require 'sintra'

get '/' do
  'Hello world!'
end

Here's what happens:

• You start the application with cabup
• The application is loaded and one route is defined
• You modify the application
• Cab tries to unload the file, but there's nothing to do since there are no classes/modules
• Cab then loads the file again
• The Sinatra application now has two routes defined, but the first one will always shadow the later one

There are two ways to handle this. Solution 1 is to start the file with cleaning up from the previous time it was loaded:

require 'sinatra'
Sinatra::Application.reset!

get '/' do
  'Hello world!'
end

Solution 2 (the preferred way) is to use a class instead:

require 'sinatra'

class MyApp < Sinatra::Application
  get '/' do
    'Hello world!'
  end
end

• dir: Only track files inside this directory
• blk: The block used by #run
• returns: Cab

cab = Cab.new(dir: __dir__) do
  require_relative 'app'
  App.do_something
end

• returns: The return value of the block

Checks if any of the tracked files has changed, reloads them if needed, and then executes the block and returns its value:

cab = Cab.new(dir: __dir__) do
  require_relative 'app'
  1 + 1
end

cab.run  # => 2

• blk: A block which should return a Rack app
• returns: A Rack app which automatically reloads on all requests

app = Cab.rack do
  require_relative 'app'
  Sinatra::Application
end

# If we're in a config.ru:
run app

Cab is currently not yet released.

The Roda framework has a summary of various code reloading libraries and techniques: "Code Reloading".

Cab is is available under the 0BSD license:

Copyright (C) 2018 Magnus Holm judofyr@gmail.com

Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted.

THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.