99.999 Per Cent. Or Something Else?

Everyone wants 99.999 per cent service availability, or course. Until they can see the price tag.

The availability targets shall have a special focus when planning for service quality according to the fit-for-purpose strategy. There are 720 hours in a 30-day month. The target availability of 99.999 per cent allows 25 seconds of downtime. The key question is: Does a 30-second service break ruin your business? Really?

Then: 99.999 per cent of what? Measuring the server CPU uptime gives rather different figures from what the end-use sees.
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Value Chain for Availability

We are still wondering what 99.999 per cent or any uptime figure means. Does it have some correlation to the company business? Does the availability figure mean anything? If not, is calculating such a thing a waste of money?

The history of the availability metrics is based on the mainframe architecture from decades ago: The display terminals had fixed cabling attached directly to the mainframe and the application were simple and straightforward. In such a world, the users' perception of the availability was pretty much the same as the mainframe's technical availability.

However, the world has changed, and it changes constantly. Servers are almost immortal in terms of their hardware, and the uptime of the hardware tells you almost nothing about anything. Availability has become a more unclear concept year by year as value chains become more complex.
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End-to-End SLA

Every now and then, the end-to-end SLA comes up as a hot topic. It is about a service agreement where the availability is measured from the end users' workstations, and where a single service provider takes the responsibility for the entire value chain. This would be nice to have, but there are few service providers willing to take on such a responsibility. The volunteers will add a big risk premium into the price tag.

In addition, unrealistic expectations are often placed on target level of the end-to-end SLA. No, the end-to-end SLA is not a means to get 99.999 per cent availability for almost free. By default, there is no such thing as a 99.999 percent end-to-end availability.
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What Is a Service Break?

The service provider and the end-users often disagree on the duration of a service break.

Recovering from a service break is not only restarting the server. It takes seconds or minutes only. However, the end-users may see a much longer outage, even hours.

A service break is not always a simple on-off question. If the performance of the system collapses, the users see the case as an outage even if every component in the value chain is up and running.
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RTO and RPO - Recovery Targets

Traditionally, the IT application development projects do not put much focus on the recovery capabilities of the coming system. Instead, let us finally see if the result is acceptable or not. If yes, go live. If not, go live.

It would be possible to approach the question more systematically, and to set recovery targets in addition to the functional requirements. There are two choices available: RTO and RPO, Recovery Time Objective and Recovery Point Objective.
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Availability Classes

Retelling Orwell: All services are equal, but some services are more equal than others.

The discussion about availability targets gets lost if we think of services as one big mass, where every service is treated in the same way. If we follow the most critical one, the price tag will be astronomical. If we follow the average one, the critical business operations may be compromised.

The criticality of the systems does not always remain constant, but may change with seasonal variations. Again, no space for one-size-fits-all thinking.

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