Buying and accepting
A promise with a number and a consequence attached. Without the consequence it is a wish with a percentage in it.
Also called Service level agreement · Соглашение об уровне сервиса
Formula
Availability = Uptime ÷ (Uptime + Downtime), over a stated window
- Window
- monthly, quarterly or annual — and it decides everything, because a longer window absorbs a single long outage
- Downtime
- measured by whom, from where, and whether partial degradation counts
- Consequence
- what happens when it is missed. Not in the formula, and the only part that makes the rest binding
The window is the term most often left vague and most often decisive. The same eight hours of outage breaches a monthly 99.9% comfortably and passes an annual 99.9% with room left over.
Three things decide whether an SLA means anything, and the percentage is not among them. Who measures it — a supplier reporting on its own availability is marking its own homework, and an independent check from outside the network is a different number. What counts as down — a site returning 200 with a stale build or a broken checkout is available by most definitions and useless by every business one. And what happens when it is missed.
That last one is where most agreements collapse. A remedy of a service credit against next month's fee is not compensation for a day of lost trading; it is a discount on the thing that failed. It is still worth having, because a supplier who will not attach any consequence at all has told you what the promise is worth.
- 99%
- 7 hours 18 minutes down
- 99.5%
- 3 hours 39 minutes
- 99.9%
- 43 minutes 50 seconds
- 99.95%
- 21 minutes 55 seconds
- 99.99%
- 4 minutes 23 seconds
A 30-day month is 43,200 minutes, so each figure is arithmetic anybody can check. The useful question is not which number to ask for but which one the business would actually notice: for most operations the difference between 99.9% and 99.99% is invisible, and the difference in price is not.
Buyers push from 99.5% to 99.9% and leave the measurement, the window, the exclusions and the remedy untouched — and those four decide whether the number is ever breached. Ask what was excluded: scheduled maintenance, third-party failures, and “events outside our control” routinely cover the majority of real downtime.
Knowing the definition is not the same as being able to check the figure. These are the procedures that do the second thing.
- Writing a specification you can accept work against
- “Here is the brief. Redesign the site, improve the funnel, modernise the stack.” · 60 minutes, 5 questions.
The definitions are the easy part. Whether the figure on your dashboard was computed this way is a different question, and usually the more expensive one.