Service Level Agreement
also called SLA
A contractual commitment about service level, with a defined remedy — usually a service credit — when it is missed.
The rule that keeps the three terms coherent: SLA looser than SLO looser than actual performance. If the SLA promises 99.9% and the internal SLO is also 99.9%, then the first breach of the internal target is simultaneously a commercial event, and the team has no room to learn from a miss.
Practical points that get negotiated badly. The measurement window matters enormously — 99.9% monthly permits 43 minutes; the same figure yearly permits nearly nine hours, and a single bad day is absorbed. What is excluded — scheduled maintenance, force majeure, customer-caused issues. And who measures, since a customer measuring from their own network will always see worse numbers than your load balancer does.
Note that a credit is not a remedy for the customer's actual loss; it is a bounded liability for you. Customers whose business genuinely depends on the service should be told to design for your failure rather than to rely on the credit.