HA and scaling are not the same thing. High availability represents architectural decisions to maintain operation in the face of subsystem failures. With respec

HA and scaling are not the same thing. High availability represents architectural decisions to maintain operation in the face of subsystem failures. With respect to high availability within a data center, this can be achieved with a passive secondary system and a mechanism to failover to this secondary in the face of a primary failure. Such an HA strategy is valid, but provides no horizontal scalability. Scalability is the ability to serve more traffic. Scaling horizontally implies adding hardware. Though the HA example given above includes additional hardware compared to a single primary node, it does not support any additional traffic. Scaling would imply that the addition of the secondary server allows the system to serve more traffic.