1. Define the path in an HTTP request
2. Define the env variable in an HTTP request
3. Use the path method to filter HTTP requests
4. Describe GET parameters and how they allow web applications to handle user queries

An HTTP Request that our browser sends to the server contains two main sections of information. One is headers and the other section is the resource or path requested (for example, /search or /profile_name). Let's break down the second section since there's a lot of information that can be stored in it!

The path that is requested is the resource that the client wants. Since your server can contain a lot of functionality, the path signifies which specific part of your server it wants. If we were creating a simple shopping cart application, for example, we can think of a few different paths that are required:

How would we implement this in our Rack app? The path lives in the HTTP request, and to get to it we have to inspect the env part of our #call function. In the env variable is all of the information contained in the request. Thankfully, Rack has a great way of parsing all this information for us. It looks like this:

class Application

  def call(env)
    resp = Rack::Response.new
    req = Rack::Request.new(env)
    resp.finish
  end
  
end

This Rack::Request instance now has a ton of useful methods. If we look through the documentation, it has a method called #path. This will return the path that was requested.

Before we inspect the #path, let's set up what we would do if someone wanted to see all of our items:

class Application

  @@items = ["Apples","Carrots","Pears"]

  def call(env)
    resp = Rack::Response.new
    req = Rack::Request.new(env)
    
    @@items.each do |item|
      resp.write "#{item}\n"
    end

    resp.finish
  end
end

If you're on a local environment, you can take it a step further and customize the response based on which specific path you enter. For example, right now the code above will list all of your items no matter what path you put in. You can give it a try by using the code to create your Rack file. Type localhost:9292/items, localhost:9292/cart, localhost:9292/flatiron/is/awesome. All of those URLs work since we're not filtering for path. No matter what the request is, we end up sending the same response. Let's filter so that this only works for the /items path using the #path method of our Rack::Request object:

class Application

  @@items = ["Apples","Carrots","Pears"]

  def call(env)
    resp = Rack::Response.new
    req = Rack::Request.new(env)

    if req.path.match(/items/)
      @@items.each do |item|
        resp.write "#{item}\n"
      end
    else
      resp.write "Path Not Found"
    end

    resp.finish
  end
end

Great! With this we can now do different things depending on the path.

What if users wanted to check and see if we have Apples available in our list of items? How do other websites handle getting user queries? If we go to GitHub and type in "apples" in the search query, we get a URL that looks like this: https://github.com/search?q=apples. We have a domain of github.com, path of search, and then a ? character. After that character comes q=apples. There is our search!

The section after the ? is called the GET parameters. If you notice in the above example, GET params come in key/value pairs. The key in GitHub's case would be q and the value is apples. The matching Ruby data structure that is also a key/value store would be a Hash! Thankfully, Rack provides the mechanism to parse the GET params and return them to us in a standard Hash. If we wanted to implement a /search route that accepted a GET param with the key q it would look something like this:

class Application

  @@items = ["Apples","Carrots","Pears"]

  def call(env)
    resp = Rack::Response.new
    req = Rack::Request.new(env)

    if req.path.match(/items/)
      @@items.each do |item|
        resp.write "#{item}\n"
      end
    elsif req.path.match(/search/)

      search_term = req.params["q"]

      if @@items.include?(search_term)
        resp.write "#{search_term} is one of our items"
      else
        resp.write "Couldn't find #{search_term}"
      end

    else
      resp.write "Path Not Found"
    end

    resp.finish
  end
end

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