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Maintaining High Availability

A highly available site provides resiliency from errors of the underlying platform and single points of failure. Availability encompasses reliability, recovery, and failure. One of the most common measures of availability is the percentage of time that a given system is active and working. The following table correlates the percentage of availability to calendar time equivalents.

Acceptable Uptime (%) Downtime Per day Downtime Per month Downtime Per year
95 72.00 minutes 36 hours 18.26 days
99 14.40 minutes 7 hours 3.65 days
99.9 86.40 seconds 43 minutes 8.77 hours
99.99 8.64 seconds 4 minutes 52.60 minutes
99.999 0.86 seconds 26 seconds 5.26 minutes

This table shows that a system with 99.9 percent acceptable uptime can only be inoperable for 86.4 seconds per day or 4 minutes per month. This table should help the business decision makers plan their solutions.

To increase availability, the system deployment, software, and management practices for the solution must be carefully engineered. Since it is very difficult to predict when or even how often a system can fail, the way to plan for better reliability is to shorten the recovery time. If your system can recover from failures within 86.4 seconds then you can have a failure every day and still achieve 99.9 percent availability.

In contrast to the above concept of availability as a function of uptime is the view of availability as a function of successful transactions completed with the solution. In other words, if the Web site handles 100,000 requests per day, then 99.9 percent availability implies 100 failed requests per day. If you consider this concept as the measure of availability then the requirements for availability in business planning might vary. However, you might consider varying request traffic in this solution.

The following table defines typical availability measurements. Use this table to help you decide what level of availability you need for your site.

Level of availability Availability target Downtime per year
Commercial 99.5% 43.8 hours
Highly available 99.9% 8.8 hours
Fault resilient 99.99% 53 minutes
Fault tolerant 99.999% 5 minutes
Continuous 100% 0

This section describes the Commerce Server features that support high availability, identifies single point of failures of each Commerce Server system, and describes techniques you can use to support high availability.

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