Domains periodically unavailable for some time: (111: Connection refused) while connecting to upstream

Symptoms

• The following errors can be found at Domains > example.com > Logs:

  [error] 15231#0: *12218 connect() failed (111: Connection refused) while connecting to upstream, client: 203.0.113.2, server: example.com, request: "GET / HTTP/1.1", upstream: "https://192.0.2.2:7081/", host: "www.example.com"

• Apache graceful restart was enabled according the article How to enable/disable graceful restart for Apache? but errors are still appeared.

• Apache graceful restart takes few seconds before the new master process is deployed as can be seen in /var/log/apache2/error_log or /var/log/httpd/error_log:

  [mpm_prefork:notice] [pid 11522] AH00170: caught SIGWINCH, shutting down gracefully
  [mpm_prefork:notice] [pid 16650] AH00163: Apache/2.4.6 (CentOS) OpenSSL/1.0.2k-fips Apache mod_fcgid/2.3.9 configured -- resuming normal operations

Cause

When a subscription/domain is created Plesk restarts Apache and it causes a temporary downtime of Apache. 

Resolution

Configure Nginx to process all website's content:

1. Log into Plesk
2. Navigate to Domains > example.com > Apache & nginx Settings and disable Proxy mode

Warning: If Proxy mode is disabled, Apache rewrite rules and .htaccess files will not be processed. Apache still needs to be installed and running on a server.

Additional information

Apache restarts frequently on Plesk running server: Caught SIGTERM, shutting down