NGINX Error Code 502 / 503 - nginx

Question

I want to ask about the nginx web server, when accessing the web a lot, then the server becomes down and get an error code 502/504, I use varnish 4 in the web server with port 8000, the physical server has the following specifications:
8 CPU Cores
16GB RAM
For nginx configuration is as follows:
user nginx;
worker_processes 8;
error_log /var/log/nginx/error.log;
#error_log /var/log/nginx/error.log notice;
#error_log /var/log/nginx/error.log info;
pid /run/nginx.pid;
events {
worker_connections 1024;
}
http {
include /etc/nginx/mime.types;
default_type application/octet-stream;
log_format main '$remote_addr - $remote_user [$time_local] "$request" '
'$status $body_bytes_sent "$http_referer" '
'"$http_user_agent" "$http_x_forwarded_for"';
access_log /var/log/nginx/access.log main;
sendfile on;
#tcp_nopush on;
fastcgi_buffers 256 16k;
fastcgi_buffer_size 256k;
fastcgi_connect_timeout 300;
fastcgi_send_timeout 300;
fastcgi_read_timeout 300;
fastcgi_max_temp_file_size 0;
......
}
While in php-fpm is as follows:
pm = dynamic
pm.max_children = 246
pm.start_servers = 32
pm.min_spare_servers = 32
pm.max_spare_servers = 64
Please help me feel confused, I followed some of the recommendations that I found from several sources, but still failed, thanks
Regards,
Janitra Panji

Answer 1

The HTTP status codes you are asking about are all defined on the related Wikipedia page
502 Bad Gateway
The server was acting as a gateway or proxy and received an invalid response from the upstream server.
503 Service Unavailable
The server is currently unavailable (because it is overloaded or down for maintenance). Generally, this is a temporary state.
504 Gateway Timeout
The server was acting as a gateway or proxy and did not receive a timely response from the upstream server.
The all indicate that the service behind your Nginx reverse proxy is down for some reason or another. You should study the tuning of your backend server. The issue quite possibly there, and not with Nginx.