I'm not sure if it is relevant but I have a tiny app on Heroku on free dynos and it requires extra time to spin up when requested (~20 sec).
My logs have a number of H27
/ HTTP 499
warnings in heroku/router
on requests originating from ASN IPs registered to Facebook and Cogent. Requests from ASNs belonging to Yandex do not result in HTTP 499
s and neither do request from humans.
The traffic isn't unexpected as the app's been shared on Facebook. My hypothesis is social sites check shared posts for update and pull in a preview image periodically.
Is my periodic update assumption is correct? Regardless, why do the result in 499
s?