The upstream timeout signals some problems with our java machines. But at
the same time the debian9 nginx/loadbalancer is running fine and has no
problems connecting to any of the upstream servers.
And the problems with letsencrypt and SSL_write are signaling to me some
problems with nginx or TCP or whatever.
I really don't know how to debug this situation. But we can reliable
reproduce it most of the times we encounter high load on debian10 servers
and did never see it on debian 9.
Then I installed the stable version nginx 1.16 on debian10 to see if this is
a bug in nginx which is already fixed:
nginx version: nginx/1.16.1
built by gcc 8.3.0 (Debian 8.3.0-6)
built with OpenSSL 1.1.1c 28 May 2019 (running with OpenSSL 1.1.1d 10 Sep
TLS SNI support enabled
configure arguments: ...
But it didn't help.
Can somebody help me and give me some hints how to start further debugging
of this situation?