server.rb

class PostsController

  def create
    @post = Post.new
    @post.title = params[:title]
    @post.save

    redirect "/posts/#{@post.id}" # The show action below.
  end

  def show
    # What is @post in the context of this action 
    # 1. <#Post> - the same one created by #create
    # 2. Nothing.
  end

end

# Imagine someone submits a POST request to your App at "/posts"
# Inside Sinatra/Rack/Rails, Imagine some code like this:
request = HTTPRequest.new(:post, "/posts")
# So now we have the raw HTTP Request. 
# We want that to be handled by PostsController#create

# This will be our response
response = PostsController.new

if request.path "/posts" && request.method == :post
  # Pass the request to the instance of the PostsController
  # that will handle the request.
  html = response.create(request)
  
  # After calling that we have the HTML
  # and we send it back to the request object
  # ending the request cycle.
  request.respond_with(html)
elsif request.path.match(/posts\/+d/) && request.method == :get
  html = response.show(request)
  request.respond_with(html)
end


# In order for @post to exist within both calls to #create and #show
# those methods would have to be called on the exact same instance of 
# the response instance.

# However, how many requests are actually made? 1 or 2? If it's 2, wouldn't our
# server application have instantiated 2 instances of PostsController, 1 to
# handle each request? Thus the instance variables can't be shared between the
# methods.