Why 502 Bad Gateway?

Question

PHP

There was Apache prefork, there was not enough memory under huge loads, I installed nginx + php 7.2 fpm.
Now the server waits a long time without responding and then issues a 502 Bad Gateway.
All default settings installed via apt-get.
Scripts run for a long time, 10 seconds, photos are processed, several thousand requests per second.
Apache worked well, but crashed at peak load. So I decided to replace it.

Answer 1

If fpm does not seem to have enough workers, increase their number in the fpm config. In any case, the 502 error is not because nginx has crashed, but because the third-party application has not responded, in your case it's puff.