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Also known as resiliency, availability is the description of the system's ability to respond to requests no matter the circumstances. Availability requires that the topology provide some degree of process redundancy in order to eliminate single points of failure. Whereas vertical scalability (multiple appservers on one system) can provide this by creating multiple processes, the physical machine then becomes a single point of failure. For this reason, a high availability topology typically involves horizontal scaling across multiple machines or LPARs.
