Question 1

What is the difference between 401 and 403?

Accepted Answer

401 Unauthorized means the request lacks valid authentication credentials. The user is not logged in, or the token is expired. 403 Forbidden means the user is authenticated but does not have permission to access the resource. Think of 401 as 'who are you?' and 403 as 'I know who you are, but you cannot come in.'

Question 2

When should I use 404 versus 410?

Accepted Answer

Use 404 Not Found when a resource does not exist or you cannot confirm whether it ever existed. Use 410 Gone when you know the resource existed and has been permanently removed with no replacement. Search engines treat 410 as a stronger signal to deindex the URL immediately, while 404 pages may be recrawled.

Question 3

What is the difference between 301 and 302?

Accepted Answer

301 Moved Permanently tells browsers and search engines that the resource has moved forever. Search engines transfer ranking signals to the new URL. 302 Found indicates a temporary move and search engines keep indexing the original URL. Use 301 for permanent migrations and 302 only when the original URL will return later.

Question 4

Why does 429 include a Retry-After header?

Accepted Answer

The Retry-After header tells the client exactly how many seconds to wait before making another request. Without it, clients often retry immediately and continue triggering the rate limit. A well-behaved client reads Retry-After and backs off, reducing load on your server and giving the client a clear path to recovery.

Question 5

What causes a 502 Bad Gateway versus a 504 Gateway Timeout?

Accepted Answer

Both come from a reverse proxy like Nginx or a load balancer. A 502 Bad Gateway means the upstream server returned an invalid or corrupt response. A 504 Gateway Timeout means the upstream server did not respond before the gateway's timeout threshold. 502 often means the backend crashed or returned garbage, while 504 means it was too slow.

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