Concurrency can be significantly less than the number of dynos

from James Somers (Ft. Andrew Warner, ATodd, Chrissy & LEMON) – Heroku's Ugly Secret on Rap Genius

Meaning

Another nasty consequence being that Heroku’s 30s request time limit, which used to be based on request time alone, is now based on request AND queue time. Which means that a fast request, taking say 2s, could still trigger an “H12” timeout error if it gets caught in a large queue at a dyno.

Which is exactly what happens — in our case, more than than a thousand times an hour.

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