HTTP status codes

Five. 1xx is informational, 2xx means success, 3xx is redirection, 4xx covers client errors where the request was faulty, and 5xx covers server errors where the request was fine but the server could not fulfil it. The first digit tells you who to blame before you read anything else.

The 5xx family, and 502, 503 and 504 in particular. These mean the server or something in front of it could not deliver a response. Cloudflare adds its own 520–526 range for failures between its edge and your origin, which are among the most useful to recognise on sight.

4xx means the problem is attributed to the request — wrong URL, missing authentication, malformed body. 5xx means the request was acceptable and the server failed anyway. A surge of 5xx is an outage; a surge of 4xx is usually a broken client or a bad link.

Yes, and it is the failure mode uptime checks miss most often. An application can serve an error page, an empty template, or a stale cache with a 200 status. Keyword monitoring — checking the response body actually contains expected text — is what catches this.