Availability Continuum
The continuum in Figure 1.5 shows a general classification of availability based on the amount of downtime an application will tolerate without impacting the business. You would write your service level agreements (SLAs) to support and try to achieve one of these continuum categories.
Topping the chart is a category named ”Extreme Availability,” so named to indicate that this is the least tolerant category and is essentially a zero (or near zero) downtime requirement (sustained 99.5% to 100% availability). Next is the ”High Availability” category, which depicts a minimal tolerance for downtime (sustained 95% to 99.4% availability). I would think that most “critical” applications would fit into this category of availability need. Then comes the ”Standard Availability” category with a more normal type of operation (sustained 83% to 94% availability). The ”Acceptable Availability” category is for those applications that are deemed “noncritical” to the business such as online Employee Benefit Package self-service applications. These can tolerate much lower availability ranges (sustained 70% to 82% availability). And lastly, the ”Marginal Availability” category are for those “nonproduction” custom applications such as marketing mailing label applications that can tolerate significant downtime (sustained 0% to 69% availability). Again remember, availability is measured by the planned operation times of the application.
Achieving the mythical five 9s (a sustained 99.999% availability) falls directly into the “Extreme Availability” category. In general the computer industry calls this “high availability,” but this author pushes this type of near zero downtime requirement into its own “extreme” category all by itself. Most applications can only dream about this level of availability because of the costs involved, the high level of operational support required, the specialized hardware that must be in place, and many other extreme factors.
As you recall, downtime can be either unplanned or planned. Figure 1.6 shows how the availability continuum categories lay out over these aspects.
As you can see, having “Extreme Availability” places you in the far upper right quadrant of both near zero planned and near zero unplanned downtime.
Figure 1.7 shows the same unplanned/planned availability axis and availability categories, but now includes several common types of applications in the industry and places them in their approximate area of availability need.
Leading the way might be a financial institution's automated teller machine system (ATMs). Having this type of system available 100% of the time can be crucial to a company's customer service perception in this highly competitive industry. A 911 emergency tracking system is another slightly different application that again demands some extreme availability need. In the next plateau, you will find systems such as eCommerce applications (online order systems). These types of systems will tolerate some limited downtime, but clearly need to be very highly available (minimal downtime) because of the potentially high dollar loss if the ordering system is not available to take orders. Other types of applications such as email systems and inventory management tend to only require a standard level of availability, while many human resources and accounting systems can operate just fine in a lower (Acceptable Availability) mode. Marginally available applications are typically those applications such as marketing mailing label runs or other one-off, nonproduction applications that can be scheduled to run whenever it is convenient or within some predefined range of availability.