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Figure 12-5   Multiple server configuration
12.3  High availability
The concept of high availability roughly equates to a system and its data being available 
(accessible by users) almost all of the time, 24 hours a day, 7 days a week, and 365 days a 
year. In actuality, 100% availability is not a cost-effective reality today for most 
implementations; rather, it is a goal. The goal is to design and build systems that are highly 
available by minimizing both planned and unplanned outages that can be caused by SPOFs.
12.3.1  Redundant systems: All platforms
Various techniques are employed on all platforms to achieve near high availability. These 
techniques are based on creating as much redundancy as possible within the system and the 
data that the systems include:
/SM590000Preventing data loss: Employing various levels of RAID to store the data on disk.
/SM590000Duplicating the data: Creating near real-time copies of the data on backup devices that 
replace the online devices if the online devices fail.
/SM590000Duplicate systems: A duplicate system (hardware, software, and data) is maintained 
(either locally or remotely), and when the main system fails, users are automatically 
directed to the duplicate system.
/SM590000Network redundancy: Creating multiple paths through the network so that if one path (or 
router) fails, the network continues to function.
All of these techniques work well and provide various levels of near real-time high availability 
based on the degree to which the redundant systems are created and are kept in 
active-standby mode.
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