Active-active HA subordinate units sessions can resume after a failover
In an active-active cluster, subordinate units process sessions. After a failover, all cluster units that are still operating may be able to continue processing the sessions that they were processing before the failover. These sessions are maintained because after the failover the new primary unit uses the HA session table to continue to send session packets to the cluster units that were processing the sessions before the failover. Cluster units maintain their own information about the sessions that they are processing and this information is not affected by the failover. In this way, the cluster units that are still operating can continue processing their own sessions without loss of data.
The cluster keeps processing as many sessions as it can. But some sessions can be lost. Depending on what caused the failover, sessions can be lost in the following ways:
• A cluster unit fails (the primary unit or a subordinate unit). All sessions that were being processed by that cluster unit are lost.
• A link failure occurs. All sessions that were being processed through the network interface that failed are lost.
This mechanism for continuing sessions is not the same as session failover because:
• Only the sessions that can be are maintained.
• The sessions are maintained on the same cluster units and not re-distributed.
• Sessions that cannot be maintained are lost.