HTTP

1. Headers

Definition: HTTP headers are key-value pairs sent in both request and response messages, providing crucial information about the request or the response.
Types:
- Request Headers: Sent by the client to the server. Examples include:
  - User-Agent: Identifies the client software.
  - Accept: Specifies the media types acceptable for the response.
  - Authorization: Contains credentials for authenticating the client.
- Response Headers: Sent by the server to the client. Examples include:
  - Content-Type: Indicates the media type of the resource.
  - Cache-Control: Provides directives for caching mechanisms.
  - Location: Used with redirections, specifies the URI of the resource.
General Headers: These can be used in both requests and responses but don't apply to the content itself, such as:
- Date: The date and time at which the message was sent.
- Connection: Control options for the current connection.

Header Name	Description
Date	The date and time the message was sent.
Connection	Control options for the current connection.
Transfer-Encoding	Indicates the form of encoding used to safely transfer the payload body.
Accept	List of acceptable media types for the response.
Accept-Charset	Character sets that are acceptable.
Accept-Encoding	Specifies the content encodings that are acceptable.
Authorization	Credentials for authenticating the client with the server.
Cookie	Contains stored HTTP cookies.
Host	The domain name of the server (useful for virtual hosting).
User-Agent	Identifies the client software.
Referer	The URI of the resource from which the request was initiated.
Content-Type	The media type of the resource being sent in the response.
Content-Length	The length of the response body in octets (8-bit bytes).
Cache-Control	Directives for caching mechanisms in both requests and responses.
Expires	The date/time after which the response is considered stale.
Location	The URL to redirect the client to.
Server	Information about the software used by the origin server.
Set-Cookie	Used to send cookies from the server to the user agent.

Definition: HTTP Status Codes are standardized responses from a server to a client's request made to the server. They indicate whether a specific HTTP request has been successfully completed.
Categories:
- 1xx: Informational (e.g., 100 Continue, 101 Switching Protocols)
- 2xx: Success (e.g., 200 OK, 201 Created, 204 No Content)
- 3xx: Redirection (e.g., 301 Moved Permanently, 302 Found, 304 Not Modified)
- 4xx: Client Errors (e.g., 400 Bad Request, 401 Unauthorized, 404 Not Found)
- 5xx: Server Errors (e.g., 500 Internal Server Error, 502 Bad Gateway, 503 Service Unavailable)
Structure: Each code consists of a three-digit number, where the first digit denotes the response type, and the last two digits provide additional context.

Informational Codes (1xx) indicate that the request has been received and is being processed; this is essential for scenarios involving longer operations where client feedback is necessary.
Success Codes (2xx) confirm satisfactory completion of requests, vital for web applications to operate without interruption.
Redirection Codes (3xx) guide clients to different resources or locations, supporting URL management and resource routing.
Client Error Codes (4xx) indicate issues with the client’s request; understanding these effectively can enhance user experience and debugging processes.
Server Error Codes (5xx) signal problems on the server side, helping developers and administrators pinpoint and resolve internal failures.