Story: The pros and cons of high-availability systems
High Availability and Disaster Recovery Planning/Provisioning
High Availability
Applications which on a day-to-day basis must have little-or-no down time, must be provisioned robustly within the data center to provide a Service Level said to provide “high availability” for the system (e.g., Availability: 99.9%-99.999%, Response Time: 1-4 seconds, Transaction Volume 99.9%-100%).
A robust technical design within a data center for an application which provides high availability, however, does not provide a viable disaster recovery capability for the application, as a disaster recovery solution must be able to recover the system within an acceptable amount of time (as set by the supported Business Units)even in the event of a total data center loss.
Disaster Recovery Vs High Availability
Disaster recovery planning and provisioning for an application, ensures that regardless of what disaster befalls an application, its data center, or its infrastructure, the application can be recovered to acceptable functionality within an acceptable amount of time as defined by its supporting business unit(s). Thus, disaster recovery planning and provisioning must utilize a remote location for recovering the application.
Disaster recovery solutions can be expensive. Generally speaking the quicker the recovery time objective (RTO), the more expensive the disaster recovery provisioning will be. For that reason, an application/system disaster recovery time must be set in terms of the maximum outage that the company can afford before severe, crippling, or long term adverse impacts are incurred by the company. To set the RTO quicker will often result in the cost of provisioning a disaster recovery solution to meet that RTO being prohibitive, or exceeding the risk being mitigated.
Last, where a system is so critical to the survival of the company that virtually no down time under any circumstances is acceptable, lest the company potentially fail, high availability disaster recovery provisioning (split processing, mirroring) becomes a part of the overall high availability solution for the system.
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