A concise introduction to HTTP

What is HTTP?

The HTTP(Hyper Text Transfer Protocol) is a protocol that is responsible communication between web servers & clients. The client makes a HTTP request to a server. The server then returns a response to the client.

Why learn about HTTP?

When you visit a website, your browser make many requests to the server in the background. You can view these requests in the Chrome browser by clicking on the Network tab in Chrome Dev Tools. For example, in the image below, we can see that 35 requests were made when loading Wikipedia.

Learning about how HTTP requests allows us to optimise our server to better respond to HTTP requests.

HTTP requests

In HTTP, every request consists of a request line, headers and optionally, a body.

Request Line

The request line is the first line in the request message. It indicates the type of method, a URL address and the HTTP version number.

Example:

GET https://www.google.com/ HTTP/1.1

Headers

HTTP headers are used to send additional parameters along with the request or response. Each HTTP header is made up of a name and a value.

Example of a request header:

Host: abc.com

User-Agent: Chrome/5.0 (Windows 10)

Accept-Language: fr, de

In the above example, you’re submitting a GET request to the abc.com host for a specific resource in French or German on the Chrome browser.

Example of a response header:

Date: Tue, 24 Jan 2023 12:30:12 GMT

Server: Apache/2.2.14 (Win32)

Last-Modified: Wed, 22 Jul 2022 19:15:56 GMT

Content-Length: 50

Content-Type: text/html

Connection: Closed</code.

The response header tells us what web server and OS the host uses. If you’re requesting a file, the header will also show information about its latest modification date, the length and content type of the file. The final line indicates that the connection is closed since the request is complete.

Message Body

The message contains the data that you’re either sending or receiving, depending the request method used. For example, if you're requesting a HTML file with the GET method, the request may look like this:

Date: Tue, 24 Jan 2023 12:30:12 GMT

Server: Apache/2.2.14 (Win32)

Last-Modified: Wed, 22 Jul 2022 19:15:56 GMT

Content-Length: 50

Content-Type: text/html

Connection: Closed</code.

<!DOCTYPE html>
<html>
    <head>
        <title>Title of the document</title>
    </head>

    <body>
        The content of the document......
    </body>
</html>

The body of a HTTP request or repsonse comes right after the header.

HTTP Methods

Now that we have learnt about how a HTTP request or response is made, it's time to take a look at different HTTP methods. Some commonly used HTTP methods are:

Method	Description	Example
GET	retreive data from server	loading a HTML page, JSON data, images, etc.
POST	submit data to the server	submitting contact form
PUT	update data that is already on ther server	editing a blog post
DELETE	delete data from the server	deleting a file

HTTP Response Status Codes

Every HTTP response message contains a HTTP status code, which tells us about the result of the request. Responses can be classified into five groups:

Status Codes	Type
100 – 199	Informational
200 – 299	Successful
300 – 399	Redirection
400 – 499	Client Error
500 – 599	Server Error

Common status codes:

Status Code	Description
200 OK	Request successful
201 Created	Request successful & resource created
301 Moved Permanently	Location of requested resource has been moved to another URL
400 Bad Request	Client Error (eg. wrong request syntax)
401 Unauthorized	No valid authentication credentials for the requested resource
404 Not Found	Server cannot find requested resource
500 Internal Server Error	Server encountered something unexpected

HTTPS

HTTPS (Hypertext Transfer Protocol Secure), in short, is the secure version of HTTP. It is not a separate protocol from HTTP. It is simply using TLS/SSL encryption over the HTTP protocol, which enables clients to safely transmit sensitive data to the server.

Any website, especially those that require login credentials, should use HTTPS. Modern web browsers such as Chrome and Firefox flag all non-HTTPS websites as not secure.

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