What are HTTP headers?

HTTP headers are service information sent in an HTTP request and HTTP response that helps the client and server understand each other correctly.

In simple terms: HTTP headers are the "explanations" attached to a request or response.

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#Why HTTP headers are needed

Headers are needed to communicate:

• what data is being sent
• in what format
• how to process it
• who the sender is
• whether authorization is required
• whether the response can be cached

Without headers, the server and client would see just a set of bytes and would not understand their meaning.

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#Where HTTP headers are located

Headers exist:

• in the HTTP request
• in the HTTP response

They come:

• after the start line
• before the message body

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#Examples of HTTP headers (simple and important)

#Content-Type

Shows what data is being sent:

• text/html
• application/json
• image/png

The server and client understand how to interpret the data.

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#Authorization

Used for authentication:

• tokens
• access keys

Tells the server who is making the request.

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#Accept

Shows what data formats the client is willing to accept:

• JSON
• HTML
• XML

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#User-Agent

Information about the client:

• the browser
• the operating system
• the application

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#Cache-Control

Controls caching:

• whether the response can be stored
• how long it stays valid

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#Request headers and response headers

#Request headers

Describe:

• the client
• the request's format
• access conditions

An example of what they mean:

"I am a browser, I want JSON, here is my token"

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#Response headers

Describe:

• the response's data
• the format
• processing rules

An example of what they mean:

"Here is the data, it's JSON, you can cache it for 5 minutes"

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#What matters for an interview

• headers are not the request body
• they do not contain the main data
• they control how the transfer behaves
• one request or response can have many headers

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#Real-life example

HTTP headers are like stickers on a package:

• "Fragile"
• "Handle with care"
• "Urgent"
• "Where to deliver"

The package itself is the body, and the stickers are the headers.

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#Short answer for an interview

Remember this wording:

HTTP headers are the service fields of an HTTP request and response that carry meta-information about the data, the client, and the rules for processing the message.