Error Rate

Error rate is the share of requests that returned an error, out of all requests in a period; a basic indicator of a service's technical health.

Error rate looks simple but its denominator decides everything: the error rate over all requests and the error rate over a critical endpoint's requests are two entirely different stories, and one overall number can hide a single broken point under a large volume of healthy requests. A second trap is that “error” must be defined precisely: a 500 server error is your fault, but a 404 may be normal behavior and a 401 is no problem at all. A key measurement point is that the error rate from the server's view differs from the user's view; an error happening in the user's browser (a failed JavaScript request, say) is not seen in the server log and vice versa. This is why a reliable error rate usually comes from server-side or backend data, not merely a browser script.