Rack and Rack Middleware Exception Handling

In this post we saw how to construct a very simple Rack application with some Middleware.

Now, we’re going to raise an exception in MessageApp and rescue it in the MessageSetter middleware which sits one level up in the application stack.

#./config.ru
require_relative './message_app'
require_relative './middleware/message_setter'

use MessageSetter
run MessageApp.new

#./message_app.rb
class MessageApp
  def call(env)
    raise
    [200, {}, [env['MESSAGE']]]
  end
end

#./message_setter.rb
class MessageSetter
  def initialize(app)
    @app = app
  end

  def call(env)
    if query_string_empty?(env)
      env['MESSAGE'] = 'Hello, World!'
    else
      env['MESSAGE'] = env['QUERY_STRING']
    end

    begin
      @app.call(env)
    rescue
      puts "CAUGHT"
      Rack::Response.new([env['MESSAGE'], ' (caught an exception along the way!)'], 200, {})
    end
  end

  private

  def query_string_empty?(env)
    env['QUERY_STRING'] == ""
  end
end

When running the application now you get “CAUGHT” printed to your server readout and “Hello, World! (caught an exception along the way!)” rendered in the viewport.

sdgf