Information About Application Level HA in the Cisco DCNM Open Virtual Appliance
To achieve HA for applications that are run on the Cisco DCNM Open Virtual Appliance, you can run two virtual appliances. You can run one in Active mode and the other in Standby mode.
This document refers to these appliances as OVA-A and OVA-B, respectively.
In this scenario:
All applications run on both appliances.
The application data is either constantly synchronized or applications share a common database as applicable.
Only one of the applications running on the two appliances serves the client requests. Initially this would be the applications running on OVA-A. The application continues to do so until one of the following happens:
The application on OVA-A crashes.
The operating system on OVA-A crashes.
OVA-A is powered off for some reason.
At this point, the application running on the other appliance (OVA-B) takes over.
For DHCP, when the first node fails, the second node starts serving the IP addresses.
The existing connections to OVA-A are dropped and the new connections are routed to OVA-B.
This scenario demonstrates why one of the nodes (OVA-A) is initially referred to as the Active node and OVA-B is referred as the Standby node.
The application-level and virtual machine (VM)-level and switchover process is as follows.
If any of the applications managed by the load-balancing software (DCNM/AMQP) goes down on OVA-A, the Active node that handles the client requests detects the failure and redirects subsequent requests to the Standby node (OVA-B). This process provides an application-level switchover.
If the Active node (OVA-A) fails or is powered-off for some reason, the Standby node (OVA-B) detects the failure and enables the VIP address for Cisco DCNM/AMQP on OVA-B. It also sends a gratuitous ARP to the local switch to indicate the new MAC address that is associated with the IP address. For applications not using VIP, the DHCPD running on OVA-B detects the failure of DHCPD on OVA-A and activates itself; whereas LDAP running on OVA-B continues running as LDAP is deployed Active-Active. Consequently, a VM-level failover is accomplished for all four applications (DCNM/AMQP/DHCP/LDAP).
Manually Triggered Failovers
An application-level failover can also be triggered manually. For instance, you might want to run AMQP on OVA-B and the rest of the applications on OVA-A. In that case, you can log in to the SSH terminal of OVA-A and stop AMQP by using the appmgr stop amqp command.
This failover triggers the same process that is described in the Automatic Failover; subsequent requests to the AMQP Virtual IP address are redirected to OVA-B.