Web Server Protocals

    Web Server Software, we mentioned that "To consider a software to be web server, the software must implement the HTTP protocol". So, to understand how web server works, we need to understand the HTTP (Hypertext Transfer Protocol) and by understanding HTTP, you will have a better sense of troubleshooting any issues appear between Web Servers (Apache HTTP as an example) and Web Clients (Web Browsers). So

1.  What is HTTP?

The HTTP (Hypertext Transfer Protocol) is an application-level protocol for distributed, collaborative, hypermedia information systems. HTTP is used to deliver data between web servers and web clients, HTTP is a TCP/IP protocol which means that it's a reliable protocol. It guarantees that your data will not be damaged in transit. The default HTTP port is TCP "80", but other ports can be used as well.

2.  How HTTP Works?

Once you enter a web address (https://middlewaretechnologiesbyashok.blogspot.in) in your web browser (Firefox, Chrome, etc), the HTTP fetches information from the web server (Apache HTTP) and responses back that requested web page to the client (Firefox). Once upon a time, "https://" needed to      be before any web address to tell the browser that you use http protocol but those days you don't need to write "https://" because the web clients become smarter enough to understand that you mean http

            image 2.5.1

When you type a web page address as " https://middlewaretechnologiesbyashok.blogspot.in " , the web browser sends an HTTP request to the web server that stores middlewaretechnologiesbyashok.blogspot.in files , the web server tries to find the desired object ("index.html") and if the web server finds the object , web server sends the object to the client in an HTTP response, along with the type of the object (in this case , HTML format) , the length of the object and other information which we will talk about soon.

3.  Web Resources

Web resource is the source of web content, web resource could be static files such as html, pdf, jpeg and mp4, dynamic content which generates on the fly such as stock prices, weather and news. Dynamic contents are usually data that updated frequently or data retrieved from databases upon your request

4.  Uniform Resource Identifier (URI)

I believe that you know the purpose of postal addresses, right?  You can go to your destination by knowing the postal address of the destination. The same concept happened on the Internet, to browse any web site (your destination), you need to know its address (postal address) and we call this address URI (Uniform Resource Identifier).

Here's a URI for an image resource on middlewaretechnologiesbyashok.blogspot.in web server https://middlewaretechnologiesbyashok.blogspot.in

5.  Uniform Resource Locator (URL)

Uniform Resource Locator (URL), commonly informally termed a web address. URL describe the specific location of a web resource on a server. A URL has two main components

-   Protocol identifier: For the URL https://middlewaretechnologiesbyashok.blogspot.in/, the protocol identifier is http.

-   Resource name: For the URL https://middlewaretechnologiesbyashok.blogspot.in/, the resource name is "logo.png"

6.  Media Types (MIME)

Web Servers attach a MIME type to all HTTP object data. When a web browser gets an object back from a server, it looks at the associated MIME type to see if it knows how to handle the object. Most browsers can handle hundreds of popular object types: displaying image files, parsing and formatting HTML files, playing audio files through the computer's speakers, or launching external plug-in software to handle special formats.

7.  HTTP Request Methods

HTTP defines a set of request methods to indicate the desired action to be performed for a given resource. The method tells the server what action to perform, this action could retrieve a web page or delete a file

Method	Description
GET	Used to retrieve information from the given server using a given URL. GET retrieve data only and don't have other effect on the data.
HEAD	Same as GET, but transfers the status line and header section only without the response body

POST	Used to submit data to be processed to a specified resource
PUT	Replaces all current representations of the target resource with the uploaded content
DELETE	Removes all current representations of the target resource given by a URL
CONNECT	Establishes a tunnel to the server identified by a given URL
OPTIONS	Describes the communication options for the target resource

Table 5.2.1

8.  HTTP Status Codes

Every HTTP response message comes back with a status code. The status code is a three-digit numeric code that tells the client if the request succeeded or if other actions are required. Understanding the HTTP Status Codes help you to troubleshoot the web server. Common status codes shown in Table 5.2.2

Status Code	Description
200	Success, Document Returned correctly
301	The requested page has moved to a new url
404	The server cannot find the requested page
500	The requested was not completed, the server me an unexpected condition

Table 5.2.2

For full HTTP status codes, please visit "https://httpstatuses.com/"

Httpstatus.com is an easy to reference database of HTTP Status Codes with their definitions and helpful code references all in one place.

9.  HTTP Protocol Versions

There are several versions of the HTTP protocol, currently HTTP/1.1 is                                          the most widely used http protocol, the new HTTP/2 was published in February 2015 and someday it will replace the HTTP/1.1, here's a list of HTTP Protocol Versions

-     HTTP/0.9

HTTP/0.9 supports only GET method, and it doesn't support MIME types, HTTP headers or version numbers

-   HTTP/1.0

HTTP/1.0 added version numbers, HTTP headers, additional methods and multimedia object handling. HTTP/1.0 made it practical to support graphically appealing web pages and interactive forms.

-   HTTP/1.1

HTTP/1.1 focused on correcting architectural flaws in the design of HTTP, specifying semantics, introducing significant performance optimizations and removing mis-features

-HTTP/2.0

HTTP/2 is a replacement for how HTTP is expressed “on the wire.” It is not a ground-up rewrite of the protocol; HTTP methods, status codes and semantics are the same, and it should be possible to use the same APIs as HTTP/1.x (possibly with some small additions) to represent the protocol. The focus of the protocol is on performance; specifically, end-user perceived latency, network and server resource usage. One major goal is to allow the use of a single connection from browsers to a Web site.