HTTP is a network protocol

• Mental model: calling a function across a network
  1. Client is the caller
  2. Server is where the function exists
  3. Client receives the result

Let’s use telnet to make such a ‘call’

<%= source_begin “language-bash” %> $ telnet example.com 80 $ GET / HTTP/1.1 $ Host: example.com <%= source_end %>

Review: What does HTTP do?

• Expects a “request” message which is answered by a “response”
• Request message
  • Defines a “method”
  • Defines the “path”
  • Supplies information about the format and lots of other stuff
• Response message
  • Defines a status code
  • Defines format of the response
  • Defines the response body

Deconstructing the call

• “Call” includes:
  1. target host (example.com)
  2. target port on the host (80)
  3. HTTP verb: “GET” (a.k.a. method) (ref: HTTP Method Definitions)
  4. address: (the path or “/”)
  5. Version of HTTP protocol (1.1) 1.Datatype of required response (aka mime-type)
• There are lots of other “parameters” that are allowed in this call

Deconstructing the returned data from that call

• (Reference: HTTP Response Fields)
• “Response” includes:
  1. Status code: 200 (see HTTP Status Codes)
  2. Content-Type: text/html Tells recipient how to parse the result)
  3. Content-Length: 1270 (Content following this is 1270 bytes)
• There are lots of other “parameters” that are allowed in this call
  • Accept-Ranges: bytes (lets the server tell the caller that it has that capability)
  • Cache-Control: max-age=604800, how long this response may be cached)
  • Date: Thu, 27 Feb 2014 00:33:26 GMT The current date time on the server)
  • Etag: “359670651” kind of a unique-id for caching purposes)
  • Expires: Thu, 06 Mar 2014 00:33:26 GMT (says when the page is required to expire)
  • Last-Modified: Fri, 09 Aug 2013 23:54:35 GMT (When this page was edited)
  • Server: ECS (ewr/1584) (Software running the server)
  • X-Cache: HIT (Page came out of a cache)
  • x-ec-custom-error: 1 (There was no error in the cache)
• All this is followed by data that we know should be interpreted as the Content-Type.

What are the HTTP Methods

• GET, PUT, POST, DELETE (there are more, those are the most common ones)
• Think of them like a required first argument to the call
• The interpretation of the

Simplified view of the world

• Think of the relationship between the client (the user in a browser) and the server (the rails server) as a program calling a function!
  1. Call: CallUsingHTTP(“GET”, “www.brandeis.edu”, “/index.html”)
  2. Returns: Status code, Content type, Content, and potentially more