Uptime, SLOs and SLA Credits

'99.9% uptime' is the most quoted and least understood number in software contracts. Behind it sits a family of measurements — availability, objectives, credits — whose definitions decide whether that promise is a commitment or a rounding exercise. The nines are arithmetic; the meaning is in the fine print.

What It Is

Uptime is the proportion of time a service is operational, conventionally expressed in 'nines': 99% (two nines) permits ~7.2 hours of downtime monthly; 99.9% ~43 minutes; 99.99% ~4.3 minutes. Availability is measured over a window (monthly is standard) against a definition of 'down' — and that definition does the heavy lifting. The SLO (service level objective) is the internal or stated target; the SLA (see separate entry) is the contractual promise with remedies. SLA credits are the standard remedy: a percentage of fees returned or credited when availability falls below threshold, usually tiered by how far it fell and capped. Maintenance windows, measurement method, and exclusions sit in the definitions that decide real coverage.

Why It Matters to You

Two pieces of arithmetic belong in every executive's head. First, nines to minutes: translate any uptime promise into allowable downtime per month, then compare that against what the business process can actually absorb — three-nines on a ward system or a trading platform is a business decision, not a technical one. Second, credits to exposure: a credit capped at 30% of monthly fees means the provider's maximum pain is a discount — check whether chronic-breach termination rights exist, and whether measurement is self-reported or independently verifiable. Also note what uptime does not cover: a system can be perfectly 'up' and unusably degraded, which is why performance SLOs belong beside availability ones.

When You'll Meet It

In every SaaS negotiation, managed-service contract, and availability review. Convert the nines, cap the credits, secure the exit — in that order of importance.