I'm currently struggling into getting the correct configuration for my M2 shop and i can't figure out on how to troubleshoot this.
On some images, varnish backend is randomly returning 503 backend fetch, sometimes it's appearing, sometimes it's not.
I'm using Ubuntu 18.4 with Nginx / Varnish 5.2.1 and PHP 7.3.16.
Here is the content of my default.vcl: https://pastebin.com/HJ2B7pvf
Then, my nginx conf: https://pastebin.com/GDytNsht
First thing that i struggled, i could not enable the health_check no matter what i was putting on it. Each time i got a 503 backend fetch failed on this so i disabled it (tried to remove pub/). Second funny thing, i'm not able to setup MAGE_ROOT to my pub directly, each time got a 404 error.
I'm kind of stuck in there, any help would be appreciated.