The Doppler Quarterly Summer 2018 - Page 64

better suited for the specific requirements of each business case. These options include: • Cold restoration, when no alternative failover resources are kept run- ning. This requires a complete rebuild of the application and data envi- ronment (preferably via automation). Because no failover resources are kept running, this design is the cheapest, but takes the longest to execute. • A pilot light system, where a minimal duplicate system is prepared and ready, but only the bare minimum services are up and running until a failure. This approach is the next most expensive, but is quicker to return to service than a cold restoration. • A warm standby system, when a complete running environment is ready to take over, but the resources are downsized, and expanded to full pro- duction capacity only when needed. It is still more expensive than the first two options, but is able to restore a minimal service level quickly. • A hot standby system, where a complete, fully resourced duplicate envi- ronment is ready to take over for the primary at all times. This is the most expensive and most complex system to design, but enables the shortest possible RTO. High Availability This is not actually a backup term, but the concept does overlap significantly with disaster recovery. At its most basic, high availability is concerned with designing and maintaining business solutions that must support very small RTOs (typically measured in minutes or seconds), and RPOs as small as zero (i.e., operational data must be current at all times, even during failures). The line between hot standby designs and high availability implementations gets blurry, as required RTOs and RPOs approach zero. Typically, a high availability solution will include live, redundant synchronization of data, as well as a sepa- rate implementation for longer term backup and archive. Consequently, these implementations can be very expensive. 62 | THE DOPPLER | SUMMER 2018