Reliability
With the advent of networks and particularly the Internet, reliability has taken on new ramifications. For example a web server needs to respond to queries from anywhere in the world where people may be on different time zones. For applications such as this, it has become common to talk about 24/7 availability (24 hour, 7 days a week). Another term used is 5 nines availability (99.999% availability). This means that, on average the service is down only 0.001% of the year.
Self Test Question
Work out the down time per year if a system is available 99.999% of the time.
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Answer
5 minutes (per year)
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Exercise
Suppose that you were commissioning the following systems. Suggest appropriate figures for their availability and assess the impact of the systems not meeting your requirements.