Planning for High Availability
Availability is a measure of fault tolerance for a computer, cluster, or system, and its programs. This measure takes into account both the mean time between failures (MTBF) and the mean time to repair (MTTR), and includes downtime for both planned and unplanned events. Many e-commerce sites are mission-critical, so they should be designed to be highly available, meaning they are up and operating at an acceptable service level at least 99.9 percent of the time.
Planning for high availability includes planning hardware and software configurations, as well as defining and testing operational procedures.
The following table lists some of the questions that you need to answer in planning for availability.
| Planning question | Recommendation |
| What level of availability do you require? | Weigh the cost of availability against the business cost of downtime and establish an appropriate availability level. |
| How will you replicate data across your servers? | Use Microsoft SQL Server to replicate databases. |
| How will you monitor availability? | Create a process to determine whether you are meeting your availability goals. |
| How will you respond to disasters? | Develop a plan for recovering data in the event of a catastrophic failure of all or part of your system. Test this plan by simulating a disaster. |