# Backend-Basics

# URLs

A URL(UniformResourceLocator) or URI (UniformResourceIdentifier) is the web address that a client uses to request a document (or JSON data in the case of REST APIs).

Parts of a URL:

htttps://example.com:443/cars?color=red&brand=bmw#specs

Part	example	description
scheme	`https://`	Protocol to use for the request. `http` `https` `ftp`
domain	`example.com`	where to send the reuest. for HTTP(s) requests, this is the Host header `Host: example.com`
port	`:443`	Defaults to 80 for HTTP and 443 for HTTPS (defult-port is not displayed in the address-bar of the browser)
path	`/cars`	Path to ask the server for
query parameters	`?color=red&brand=bmw`	usually used to ask for a different version of a page
fragment id	`#specs`	not sent to the server. Used to jump to an HTML-id or by JS on the page

#

allways have a port! If you don't enter a port, the browser uses a default port!

80 - http
443 - https
3000 - ok for developement

# URL Encoding

URLS can not have special characters. To use the in an URL, encode them as:

% + hex representation of ASCII value

space: %20

%: %25

W3Schools URL encoding reference (opens new window)

URL wikipedia (opens new window)

# Javascript `new URL()`

https://developer.mozilla.org/en-US/docs/Web/API/URL/URL (opens new window)

instead of

const url = 'https://myurl.io/api'
+'?height=' + encodeURIComponent(height)
+'?width=' + encodeURIComponent(width)

use:

const url = new URL('https://myurl.io/api')
url.searchParams.set('width', width)
url.searchParams.set('height', height)
url.toString

# localhost

127.0.0.1 oder localhost -> the computer itself !!

# Headers

MDN Headers (opens new window)

# The HTTP Protocol

# Request and Response

client application wants to get data from a RESTful web service:

it sends an HTTP request.
the server then sends the data via an HTTP response.

If the client wants to send data to the server:

The data is stored in the body of the request.
it sends an HTTP request.
The server then sends a response to acknowledge whether or not the storing of the data was successful.

Request and response are standardized In the HTTP protocol.

It is defined:

how many text fields exist
in which sequence, and
how much text is allowed in that field.

# Request

Under the hood, HTTP uses only text.

In the message body, there can be binary data as well, e.g for an image file
The header, however, only contains text.

The request head looks something like this:

Method - URL - HTTP Version
Additional Headers
Empty Line
Message Body

Example:

GET / HTTP/1.1
Host: www.spiegel.de
Connection: keep-alive
Purpose: prefetch
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_0) AppleWebKit/537.36 (KHTML...
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/
Accept-Encoding: gzip, deflate
Accept-Language: de
Cookie: __gads=ID=b983721bda83d7ae:T=1487252257:S=ALNI_MYlth5Tx71QzpXnN3I0AoPWAu4Uog;...

# Response

HTTP Version - Status Code - Status Name
Response Headers
Empty Line
Message Body