Rack is a modular interface between web servers and web applications developed in the Ruby programming language. It provides a minimal, modular, and adaptable interface for building web applications by wrapping HTTP requests and responses in the simplest way possible. This unifies and distills the API for web servers, web frameworks, and middleware into a single method call[2][4][5].
Key Features of Rack:
Interface Definition:
Rack defines a simple interface that any Ruby web application can implement. The core requirement is that the application must respond to a call method, which takes an environment hash as a parameter and returns an array with three elements: the HTTP status code, a hash of headers, and the response body[1][2][3][6].
Middleware:
Rack allows the use of middleware, which are components that sit between the client and the server, processing inbound requests and outbound responses. Middleware can modify the request/response, perform logging, handle sessions, and more. Middleware components are typically Ruby classes that implement the call method[1][2][4][6].
Rack Applications:
A Rack application is any Ruby object that responds to the call method. This method takes an environment hash containing information about the HTTP request and returns an array with the status, headers, and body of the response[2][3][6].
Rackup:
Rack provides a command-line tool called rackup to start a Rack-based application. The config.ru file is used to configure the Rack application and middleware stack. Running rackup starts the server and serves the application[1][4][5].
Compatibility:
Rack is supported by many Ruby web frameworks and servers, such as Ruby on Rails, Sinatra, Puma, Unicorn, and others. This compatibility allows developers to switch between different web servers without changing the application code[2][5][6].
Example of a Simple Rack Application:
class HelloWorld
def call(env)
[200, {"Content-Type" => "text/html"}, ["H...
