A server is an actual physical computer that participates in the cluster. Each server has a host name and an IP address assigned when it is installed. Understudy uses this host name and IP address to identify the server and for internal communication between members of the cluster. Because server host names and IP addresses are not protected by failover, they should not be published in URLs.
This version of Understudy supports two cluster servers.
A failover-protected virtual host is an virtual host managed by the Understudy software to provide failover protection. The host names of protected virtual hosts are the ones published to the world in URLs.
For each configured virtual host, there is a primary cluster server and a backup server. The primary server is the physical computer to which internet requests are routed when no failures occur, while the backup server is the physical computer to which these requests are routed when a failure has occurred in the primary server, until this failure has been fixed. Note that in order to use Understudy to protect a web site, an instance of a web server needs to run on each of the corresponding cluster server computers, and the contents of the web site needs to be replicated on both servers, or be located on a separate file system which is accessible from both servers.
This version of Understudy supports up to 128 failover-protected virtual hosts.
A service monitor monitors an internet service, such as a web server, FTP server, or SMTP mail server. Each failover-protected virtual host may have one or more service monitors configured. Periodically, the Understudy management daemon attempts to connect to each service on the configured port number, and makes a simple request. If there is no response for a configured service, Understudy will move the affected IP address of the virtual host to the backup server. If both the primary and the backup servers are suffering from service failures, the affected virtual host will be configured on the primary server.
Note: Understudy does not copy changes made to web pages on the primary server to the backup server, or maintain shared session variables. This can be done by using external software.