Wednesday, February 22, 2012

High Availability Basic Concepts-I

For any software application or service, high availability refers to the availability of that application or service to its users without any failure. For simplest example, Google is providing its search capabilities to all the Internet users (virtually) for 24 X 7 via its Google search engine. We assume that as soon as we switch on our PC or laptop (or any other compatible device), and connect to Internet, Google search will be available to us. The use of word virtually compensates for those small time periods, when Google search engine is not available to the users due to server maintenance, or some other reasons. This duration will be called as downtime, and is usually measured over a year. So if any application of service provider claims 99.9% availability, it means that over a year’s time, its services may be down for 0.1% duration of year, i.e. 8 hours and 45 minutes.

The primary goal of any high availability solution is to minimize the impact of downtime. And the Service Level Agreements for any such high availability solution (or service) always covers these clarifications in its terms and conditions. The availability of a solution (application or service, or a group of them) can be expressed as this calculation

Availability = ( Actual Uptime / Expected Uptime ) x 100:

The resulting value is often expressed by industry in terms of the number of 9’s that the solution provides; meant to convey an annual number of minutes of possible uptime, or conversely, minutes of downtime.

Number of 9’s

Availability Percentage

Total Annual Downtime



3 days, 15 hours



8 hours, 45 minutes



52 minutes, 34 seconds



5 minutes, 15 seconds

More details on various high availability applications and services will be covered very soon. (as soon as I'll have free time available for the same :)

Alright, got some time to generate some content as a blog Microsoft's High Availability Solutions.

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