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

Symptoms

• Apache service frequently restarts. One of the following messages is logged in Apache logfile (CentOS/RHEL-based: /var/log/httpd/error_log, Debian/Ubuntu-based: /var/log/apache2/error.log):

  CONFIG_TEXT: [mpm_event:notice] [pid 3239834:tid 3239834] AH00493: SIGUSR1 received. Doing graceful restart

  ---

  CONFIG_TEXT: [notice] caught SIGTERM, shutting down

  ---

  CONFIG_TEXT: [mpm_event:notice] [pid 665:tid 140662910175360] AH00492: caught SIGWINCH, shutting down gracefully

  ---

  CONFIG_TEXT: [mpm_prefork:notice] [pid 12450] AH00169: caught SIGTERM, shutting down

  ---

  CONFIG_TEXT: [error] 34053#0: *190501 upstream prematurely closed connection while reading response header from upstream, client: 203.0.113.101, server: example.com, request: "POST /wp-admin/admin-ajax.php HTTP/2.0", upstream: "https://203.0.113.2:7081/wp-admin/admin-ajax.php", host: "example.com", referrer: "https://example.com/"

• System journal for Apache service contains multiple messages about Apache restart:

  CONFIG_TEXT: Nov 27 08:13:00 servername systemd[1]: Stopping The Apache HTTP Server...
  Nov 27 08:13:03 servername systemd[1]: Stopped The Apache HTTP Server.
  Nov 27 08:13:03 servername systemd[1]: Starting The Apache HTTP Server...
  Nov 27 08:13:05 servername systemd[1]: Started The Apache HTTP Server.
  Nov 27 08:13:54 servername systemd[1]: Stopping The Apache HTTP Server...
  Nov 27 08:13:57 servername systemd[1]: Stopped The Apache HTTP Server.
  Nov 27 08:13:58 servername systemd[1]: Starting The Apache HTTP Server...
  Nov 27 08:13:59 servername systemd[1]: Started The Apache HTTP Server.

• Websites are unavailable with a 503 error after for several seconds after applying web-server related settings in Plesk.

Cause

Plesk restarts Apache service to apply the new configuration when:

• a domain is created/removed,
• an SSL certificate has been issued/renewed/assigned (as manually, as well during keep-secured scheduled task),
• PHP Settings or Hosting settings for a domain have been changed.

Resolution

To avoid frequent Apache restarts, consider one of the following options:

• Enable Apache graceful restart

• Increase Apache restart interval in Plesk at Tools & Settings > Apache Web Server > Set the Apache restart interval value to "300" or more (value in seconds):