webdev-basics/2-http.md

marp	paginate	math	theme	title
true	true	mathjax	buutti	2. HTTP

HTTP

Contents

1. What is HTTP?
2. HTTP Requests
3. HTTP Responses
4. IP address and ports

What is HTTP?

• HTTP (Hypertext Transfer Protocol) is a TCP/IP based communication protocol, which provides a standardized way for computers to communicate with each other
• The communication happens between a client (browser, application…) and a server
  • The client sends a request, and the server returns a response
• HTTP is connectionless:
  • The client and server are only aware of each other during the request-response phase
  • After that, each new request is sent through a new connection

But what is TCP/IP?

• It is known as the Internet protocol suite
• Framework used for organizing communication protocols over the Internet
  • TCP = Transmission Control Protocol
    • Controlled stream of bytes between applications communicating via an IP network
  • IP = Internet Protocol
    • Delivering packets from host to destination with IP addresses

HTTP structure

• The HTTP protocol acts as a communication bus between the web client (e.g. web browser) and the web server
• There can be multiple clients connected to the server simultaneously
• The clients are not connected to each other but only to the server

HTTP requests

• HTTP defines a set of request methods
• The most common requests are GET, POST, PUT and DELETE
  • These are needed to create a basic CRUD app (Create, Read, Update, Delete)
• More requests are listed in the mdn web docs

HTTP Method	Usage	Example Use
GET	Retrieve content	Browser fetching a webpage
POST	Add new data	Submitting a form to create a new account
PUT	Update existing data	Updating user profile information
DELETE	Remove existing data	Deleting a user account

Note: URI vs URL

• The website address is more formally known as the URL (uniform resource locator)
  • https://maps.google.com
  • https://maps.google.com/maps?q=Oulu (includes a query)
• It is a subset of an URI (uniform resource identifier)
  • This identifies a resource on a webpage

HTTP request format

• Request line: <request method> <request URI>
  • Example: GET https://example.com/products
• Headers (optional)
  • Example: Date: Mon, 07 Sep 2020 08:39:33 GMT
  • Example: Content-Type: application/json
• Body (optional)
  • Example in the JSON format:

    {
      "date": "2020-09-08T11:39:34.2837813+03:00",
      "temperatureC": 11,
      "temperatureF": 51,
      "summary": "Cool"
    }
    

HTTP request examples

Method	URI	Headers	Body
GET	http://server.com/
GET	http://server.com/10
POST	http://server.com/users	Content-Type: application/json	{"name": "Dan"}
PUT	http://server.com/users/2	Content-Type: application/json	{"name": "Danny"}
DELETE	http://server.com/users/2

HTTP response format

• HTTP responses have the following format:
  • Status line: <HTTP version> <status code> <reason phrase>
    • Example:HTTP/1.1 200 OK
  • Headers (optional)
    • Date: Mon, 07 Sep 2020 08:39:33 GMT
    • Content-Type: application/json
  • Body (optional)

    {
      "date": "2020-09-08T11:39:34.2837813+03:00",
      "temperatureC": 11,
      "temperatureF": 51,
      "summary": "Cool"
    }
    

HTTP status codes

• The status line of HTTP a response sent by an API should accurately describe the status of what happened on the server after each request:
  • Did the operation succeed? (2xx status codes)
  • Was there an error with the request? (4xx status codes)
    • E.g., the request line was malformed, or the server doesn't support it
  • Did a server-side exception occur? (5xx status codes)
    • This is never the client's fault!

Status codes for a CRUD app

• Your APIs should use the following status codes for responses to CRUD operations
• See Restfulapi.net and Wikipedia HTTP status code listings

2xx SUCCESS	4xx CLIENT ERROR	5xx SERVER ERROR
200 - OK	400 - Bad Request	500 - Internal Server Error
201 - Created	401 - Unauthorized
204 - No Content	403 - Forbidden
	404 - Not Found
	409 - Conflict

IP and ports

IP addresses

• Every node, or host on a computer network needs an IP (Internet Protocol) address
  • These can be public or private
  • Serves two functions: identifies and provides the location of the host
• The old 32-bit IPv4 addresses are of the form 192.0.2.1
• The 128-bit IPv6 addresses are of the form 2001:0db8:0000:0000:0000:8a2e:0370:7334 (can be shortened to 2001:db8::8a2e:370:7334)

Port numbers

• The IP address is often followed by a port number separated by a colon: 192.0.2.1:8080
• The port number (16-bit unsigned integer) defines the port through which communication to your application happens
• Ports allow multiple applications to be located on the same host
• The client connects to this port in some server (defined by an IP address), so the client and the server are able to communicate
• Computers have several different ports at their disposal

Reserved ports

• Some of them are reserved and your application might not be able to assign those ports to itself
• Ports from 0 to 1023 are system ports
  • There are restrictions in manually assigning applications to use them
• Generally, ports from 1024 to 65535 are available for the user to assign for applications