High Availability (HA) is a solution that uses the Session Clustering infrastructure to provide availability and continuity of mission critical business applications.
Session Clustering HA is an additional safety layer for maintaining session information integrity in Web cluster environments. HA ensures that sessions will be serviced in case of a single failure.
Session Clustering HA provides all the current Session Clustering functionality and is an optional feature for environments that require High Availability.
As an additional functionality layer, running HA may have a slight impact on performance in comparison to regular session clustering response time.
How does HA operate?
The High Availability layer preserves session information when a server fails. Each session is saved twice, once on the master (originating) Session Clustering Daemon and one on the master's backup Session Clustering Daemon. This means that in the event that a master server fails, requests are re-routed to the backup Session Clustering Daemon. Once a request is re-routed to the backup Session Clustering Daemon, the Backup Session Clustering Daemon turns into a master Session Clustering Daemon and creates a new backup Session Clustering Daemon. A backup Session Clustering Daemon is chosen based on it activity locating the Session Clustering Daemon with the least amount of sessions and open sockets. In the event that two servers fail session information will be preserved in the backup and can be replicated if requested in its lifetime.
Regarding response time, the HA layer will not impose more than a 10% performance degradation over the existing session clustering solution (number of session requests per second).
As mentioned earlier, all servers that need to share session information have to be associated to a cluster in Session Clustering Daemon. The HA layer is also capable of identifying when a fallen server has been recovered and will automatically return the fallen server into service.