What HTTP response codes do you know? How do 401 and 403 differ? What does 503 indicate? What does 400 mean and how to find the cause of the error?

HTTP response codes are divided into five groups:

• 1xx — informational
• 2xx — successful (e.g., 200 OK)
• 3xx — redirection
• 4xx — client errors
• 5xx — server errors

Difference between 401 and 403:

• 401 Unauthorized — means the request requires authentication. The client has not provided or has provided incorrect credentials.
• 403 Forbidden — the client is authenticated but does not have permission to access the resource.

Code 503 Service Unavailable indicates that the server is temporarily unavailable, usually due to overload or maintenance.

Code 400 Bad Request means the server cannot process the request due to a syntax error or invalid data.

To troubleshoot a 400 error:

• Check the correctness of the request format (JSON, URL parameters, headers).
• Ensure all required fields are present.
• Check server logs for detailed information.
• Use debugging tools (Postman, curl) to reproduce the request.