A Service Level Indicator is a measurable and relevant indicator of service performance.
An SLI Requirement is a statement about a particular value of that indicator.
The most common indicators are listed below.
The minimum percentage of requests that must work (also called the ‘percent OK’).
Example SLI Requirement: “99.99% of requests must complete successfully”.
The maximum amount of time that requests are allowed to take. This is measured in terms of percentiles.
Example SLI Requirement: “95% of requests must be handled in under 500ms”.
The percentiles measured depend on the purpose of the service. There is no ‘one size fits all’ set of percentiles to measure. That said, most services resemble one of the following archetypes, which you can use as a starting point. For more detail, see Gil Tene’s excellent presentation ‘How not to measure latency’.
Goal: Win some gold medals.
The Athlete needs to be faster than everyone else in N% of races. Where N = desired percentage of races to win.
Example: There are 5 races. The Athlete wants to win 3 gold medals. So they must be faster than all their competitors in 60% of races. N = 60.
Key percentiles:
Goal: Keep the heart beating.
The worst case is all that matters.
Key percentiles:
Goals: Be fast enough to make some good trades. Contain risk while you do it.
The Trader must ‘typically’ react quickly to open a position. But their ‘max’ must be reasonable too, to avoid getting stuck with a bad open position.
Key percentiles:
Goal: Keep users happy (don’t make them leave).
The App must be ‘typically’ snappy. It’s OK to take longer sometimes - but not too long, and not too often.
Key percentiles: