Asked about The distributions you will actually meet
Why report p95 or p99 latency rather than the mean?
An answer that works
Latency is heavy-tailed. The mean is pulled up by a few very slow requests while describing few real ones, and it hides the tail entirely. High percentiles describe the experience of your unluckiest users, which is where problems show.
Then they push further
A request fans out to 10 backend calls in parallel and waits for all. How does the tail affect it?
It waits for the slowest of 10. If each call is slow 1% of the time, the request is slow about 10% of the time, so a backend's p99 becomes the request's p90. Fan-out magnifies tails.
What gets marked up
Marked up for the fan-out argument, which shows real systems experience.
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