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 any type of a resource
    • mailto:info@example.com (an email address)
    • https://www.example.com/index.html#date (A section of a website)

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

Note: older version in the backend-basics repo