On the  previous post we explained to configure Reverse Proxy on Apache in Linux. If you may get an error 503 Service Unavailable on browser, 

The problem is not in your Apache configuration, you have received  still get a 503 page and check in apache error log using "tail" command,

# tail -f /var/log/apache2/error.log

[Thu May 12 12:17:30.476714 2016] [proxy:error] [pid 9293:tid 139882185815808] AH00940: HTTP: disabled connection for (test.thelinuxfaq.com)
[Thu May 12 12:17:35.314103 2016] [proxy:error] [pid 9293:tid 139882177423104] (111)Connection refused: AH00957: HTTP: attempt to connect to (test.thelinuxfaq.com) failed

The problem is with the application running on port number 8081, you have set the reverse proxying  to   ProxyPass / http://test.thelinuxfaq.com:8081/.

Now, when the Apache is trying to proxy the request it's not getting back the response from the   8081, so that Apache throw the message "Service Unavailable".

Just start the service which configured in port 8081 the problem will be solved.