Rack is used by many Ruby web frameworks and libraries, such as Ruby on Rails and Sinatra. It is available as a Ruby Gem. Many Ruby applications are called "rack-compliant".[2]
The characteristics of a Rack application is that the application object responds to the call method. The call method takes in the environment object as argument and returns the Rack response object.
The URL information(information that would direct to the application, information that directs to the actual location in the application, Query string)
Server information like the server name and server port
The HTTP metavariables that are received from the client
b) Rack specific information
This includes the information like
The version of the Rack application that is running
The URL scheme that is used, that is, if the request that is received is http or https.
The raw HTTP data.
A Ruby object for reporting errors.
Information like if the application object is simultaneously invoked from another thread or process.
Information on the server expectations and capabilities (capability of the server for connection hijacking).
In case the application is being used as a middleware, the environment can have objects that would provide session information, logging capabilities, information on the size of the data that can be used for read and writes etc. In addition to these, the server can store their own data in the environment.
The header contains the response for each and gives the key-value pairs. The keys have to be strings.
Body contains the final data which is sent by the server to the requester.
Rack::Response provides a convenient interface to create a Rack response. The class Rack::Response is defined in lib/rack/response.rb. To use the Response class, instantiate it from the middleware layer down the stack. It can be used to modify the cookies.
Rack makes it easy to add a chain of middleware components between the application and the web server. Multiple middleware components can be used in the rack which modifies the request/response before handing it over to the next component. This is called middleware stack.
The Rack server adds multiple middle middleware by default for the functionalities like showing exception with all the details,[8] validating the request and responses according to the Rack spec[9] etc.
# helloWorld.ru# The application that has the call method defined.classHelloWorld# Call method that would return the HTTP status code, the content type and the content.defcall(env)[200,{"content-type"=>"text/html; charset=utf-8"},["Hello World"]]endendrunHelloWorld.new
The server for the above code can be initiated using "rackup helloWorld.ru" and can be accessed at http://localhost:9292/ The default port used by the Rack application is 9292.