text: **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: 
  
  (cid:2) Preventing data loss: Employing various levels of RAID to store the data on disk. 
  
  (cid:2) Duplicating the data: Creating near real-time copies of the data on backup devices that 
  replace the online devices if the online devices fail. 
  
  (cid:2) Duplicate 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. 
  
  (cid:2) Network redundancy: Creating multiple paths through the network so that if one path (or 
  router) fails, the network continues to function. 