Coming from a NetScaler background I was used to the concept of a failover server. As in a virtual server would have a pool of servers it would load balance amongst and if all of them are down I can define a failover server that could be used. You would define the failover server as a virtual server with no IP, and tell the primary virtual server to failover to this virtual server in case of issues.
Looking around for a similar option with NSX I discovered it’s possible using application rules. Instead of defining two virtual servers though, here you define two pools. One pool for the primary servers you want to load balance, the other pool for the failover server(s).
Then you create an application rule this:
acl adfs_pri_down nbsrv(pool-adfs-https-443) eq 0
use_backend pool-bkpadfs-https-443 if adfs_pri_down
Once again, the syntax is that of HAProxy. You define an ACL –
adfs_pri_down is what I am defining for my purposes as this is for load balancing some ADFS servers – and the criterion is
nbsrv(pool-adfs-https-443) eq 0. The
nbsrv criterion checks the pool you pas on to it (
pool-adfs-https-443 in my case) and returns the number of servers that are up. So the ACL basically is a boolean one that is true if the number of usable servers is 0.
use_backend rule switches to using the backup pool I have defined (
pool-bkpadfs-https-443 in this case) if the ACL is true.
That’s all. Pretty straightforward!