The worst class of outage is the silent wedge: the TCP listener accepts connections, the process is alive, the startup banner re-emits on retry, but no handler ever responds. Curl returns HTTP 000 instead of a useful error. Logs cannot distinguish 'just started' from 'stuck for an hour.' Only an external probe with a response deadline catches it. If your monitoring reports 'process up, port open,' you have not actually checked that the service is serving.
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