Failover Server Candidates

If a server crashes suddenly, VOV has the capability to start a replacement server on a pre-selected host. This capability requires that the pre-selected host is configured as a failover server.

The configuration instructions follow.

Note: The vovserverdir command only works from a VOV-enabled shell when the project server is running.

Edit or create the file servercandidates.tcl in the server configuration directory. Use the vovserverdir command with the -p option to find the pathname to this file.
```
% vovserverdir -p servercandidates.tcl
/home/john/vov/myProject.swd/servercandidates.tcl
```
The servercandidates.tcl file should set the Tcl variable ServerCandidates to a list of possible failover hosts. This list may include the original host on which the server was started.
```
set ServerCandidates {
    host1
    host2
    host3
}
```

Install the autostart/failover.csh script as follows:

% cd `vovserverdir -p .`
% mkdir autostart
% cp $VOVDIR/etc/autostart/failover.csh autostart/failover.csh
% chmod a+x autostart/failover.csh

Activate the failover facility by running vovautostart.
```
% vovautostart
```
For example:
```
% vovtaskermgr show -taskergroups
ID         taskername        hostname         taskergroup
000404374  localhost-2      titanus          g1
000404375  localhost-1      titanus          g1
000404376  localhost-5      titanus          g1
000404377  localhost-3      titanus          g1
000404378  localhost-4      titanus          g1
000404391  failover         titanus          failover
```
Note: Each machine listed as a server candidate must be a vovtasker machine; the vovtasker running on that machine acts as its agent in selecting a new server host. Taskers can be configured as dedicated failover candidates that are not allowed to run jobs by using the -failover option in the taskers definition.

Preventing jobs from running on the candidate machine eliminates the risks of machine stability being affected by demanding jobs. The -failover option also enables some failover configuration validation checks. Finally, failover taskers are started before the regular queue taskers, which helps ensure a failover tasker is available as soon as possible for future failover events.

Refer to the tasker definition documentation for details on the -failover option.