Geo-redundancy/Redundancy And DR Synopsis
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High Availability (HA) is an approach to IT system design and 
configuration that ensures |VOSS-4-UC| is operational
and accessible during a specified time frame. This is achieved 
using redundant hardware and resources. If there
is a failure, an automatic failover will occur to a second node.

Web server proxy nodes perform load-balancing between application roles, so that load
is distributed. During provisioning, the web server proxy is provided with all 
the IP addresses of the application nodes. The web server software then does
load balancing among these nodes, according to its configuration. If a node 
fails to respond in a set time, the proxy will send the transaction to 
another node. This means that in the event that an Application role is lost, 
the WebProxy will transparently bypass the faulty Application role.

The proxy web server that is configured to be located in the primary site normally
load balances to the two unified nodes in the primary site. The proxy web server
falls back to the two nodes in the Disaster Recovery site if the nodes in the primary
site are down. The web proxy nodes in the secondary site defaults load balancing
to the two unified nodes configured for the secondary site.

Data is replicated between Database roles, and role failure is recoverable.
This is done using the database replication facility.
Automatic failover between Database roles occurs while there is greater than
50% Database role availability. Once there is insufficient role availability,
the system needs to be manually re-provisioned.

HA can be increased by adding nodes to the cluster.  Application performance
and availability can be increased by adding additional application role servers.

Backups can be scheduled to run automatically across the cluster. Backups
include application data, configuration and software. Backups can take place
to both local disk and remote network location. Every node upgrade is
preceded by a snapshot backup which allows any upgrade to be rolled back.  
Refer to the |Platform Guide| for details.
 
.. |VOSS-4-UC| replace:: VOSS-4-UC
.. |Unified CM| replace:: Unified CM
.. |Platform Guide| replace:: Platform Guide
.. |Operations Guide| replace:: Operations Guide