High availability overview

Business continuity is the capability of a business to withstand outages and to operate important services normally and without interruption in accordance with predefined service-level agreements. To achieve a given level of business continuity that you want, a collection of services, software, hardware, and procedures must be selected, described in a documented plan, implemented, and practiced regularly. The business continuity solution must address the data, the operational environment, the applications, the application hosting environment, and the end-user interface. All must be available to deliver a good, complete business continuity solution.

Business continuity includes disaster recovery (DR) and high availability (HA), and can be defined as the ability to withstand all outages (planned, unplanned, and disasters) and to provide continuous processing for all important applications. The ultimate goal is for the outage time to be less than .001% of total service time. A high availability environment typically includes more demanding recovery time objectives (seconds to minutes) and more demanding recovery point objectives (zero user disruption) than a disaster recovery scenario.

High availability solutions provide fully automated failover to a backup system so that users and applications can continue working without disruption. HA solutions must have the ability to provide an immediate recovery point. At the same time, they must provide a recovery time capability that is significantly better than the recovery time that you experience in a non-HA solution topology.