8.4 Service level agreements, read critically
You can read a provider's SLA and say what it does and does not promise you.
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A provider's SLA promises less than it appears to, and this topic reads one critically: availability percentages converted into minutes per month, which is the only honest form; payouts as service credits rather than your losses; and the exclusions and definitions that decide whether an outage even counts. The composition arithmetic matters most — four services at 99.9 percent in series is not 99.9 percent. It sits beside the SLO topic on purpose: the provider's SLA is a floor to build on, and your own internal target must fit underneath it.
Work through these
Availability percentages as minutes per month, which is the only form that means anything
An availability percentage means nothing until converted into minutes of downtime per month, which is the only form a person can reason about. Doing that conversion is the first move when reading any agreement.
What an SLA actually pays out — service credits, not your losses
What an agreement pays when it is breached is usually a credit against future usage, not compensation for what the outage cost you. Reading this clause changes how much comfort the number provides.
Exclusions, maintenance windows and the definition of 'unavailable'
The exclusions, the scheduled maintenance, and the precise definition of unavailable decide what actually counts as a breach. That definition is usually narrower than it first appears.
Composing SLAs: four services at 99.9% in series is not 99.9%
Chaining services multiplies their availabilities, so four dependencies at a high figure produce a lower figure overall. This arithmetic is what makes architecture diagrams into reliability estimates.
Writing an internal SLO you can actually meet, underneath the provider's SLA
Underneath the provider's promise sits your own target, which has to account for your code and your mistakes as well as theirs. Setting it honestly is more useful than adopting the provider's number.
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