HTTP Status Codes

Every status code from 100 to 511, what it actually means, and — more usefully — what tends to cause it in practice. Bookmark it for the next time a load balancer starts throwing 502s at 3am and you need to remember whether that's your fault or upstream's.

The five families

• 1xx — informational, in-progress. You'll rarely see these directly.
• 2xx — success. 200 is the default; 201 specifically means something was created.
• 3xx — redirection. 301 is permanent and search engines update their index to match; 302 is temporary and they keep indexing the original.
• 4xx — the client got something wrong. Bad request, missing auth, no permission, resource doesn't exist.
• 5xx — the server got something wrong. This is the one that should page someone.

The ones you'll actually debug

• 401 vs 403 — 401 means "we don't know who you are," 403 means "we know exactly who you are, and the answer is no." Mixing these up in API responses genuinely confuses client developers.
• 502 Bad Gateway — your proxy or load balancer got an invalid response from whatever's behind it. Check the upstream service, not the load balancer itself.
• 503 Service Unavailable — the server is up but deliberately not serving right now (overload, maintenance). Pair it with a Retry-After header and Googlebot will politely come back later instead of treating the page as gone.
• 504 Gateway Timeout — upstream took too long to respond. Different problem from 502: this one's usually about a slow query or an overloaded backend, not a broken connection.

Redirects and SEO, in one paragraph

301 passes the vast majority of link equity to the new URL — use it for anything permanent, like a domain migration. 302 doesn't, because it's telling search engines the move is temporary and the old URL is still the canonical one. Using 302 for something that's actually permanent is a common and quietly costly mistake.