Definition (canonical)
An SLA (Service Level Agreement) is the contractual promise of measurable service quality — most often availability and support response — with defined remedies when the promise breaks. Without metrics, measurement method and remedies, an "SLA" is a marketing paragraph, not an agreement.
The nines — what availability numbers really mean
Uptime percentages translate brutally: 99% = ~7.3 hours down per month; 99.9% ("three nines") = ~44 minutes; 99.99% = ~4.4 minutes. Each nine costs the provider roughly an order of magnitude in engineering — which is why 99.99%+ commitments carry enterprise price tags. Reading discipline: check the measurement window (monthly resets favor providers — one bad day burns a month's budget, then the meter resets), the measurement source (provider's own monitoring vs agreed third-party), and what counts as "down" (full outage only, or degraded performance too?). A 99.9% promise measured yearly, self-reported, excluding degradation is materially weaker than the same number measured monthly by an external monitor.
Beyond uptime: support tiers and the fine print
Support SLAs split response time (someone acknowledges) from resolution targets (it gets fixed) — vendors happily promise the first and rarely commit to the second; know which you're buying. Severity tiers (P1 outage: 15-min response 24/7; P3 question: next business day) are standard and sane. Then the hollowing clauses to hunt in every draft: scheduled maintenance exclusions (unlimited windows = uptime theater), force majeure stretched to cover ordinary subcontractor failures, and remedies capped at trivial levels. Credits are the standard remedy — typically 5–30% of monthly fees, claimed actively (auto-credit clauses are worth negotiating) — and their real function is skin-in-the-game signaling, not compensation: for business-critical dependencies, negotiate termination rights after repeated breaches; a 10% credit doesn't cover a lost weekend of sales.
SLAs as an operating tool — both directions
Selling side: publish SLAs you can measurably beat (99.9% promised, 99.95% delivered builds trust; the reverse burns it), wire monitoring and status pages BEFORE promising, and treat every breach as an incident review with a customer-facing postmortem — handled well, a breach plus honest RCA often deepens trust. Internally, the same math runs as SLO/SLI practice (objectives and indicators without contractual teeth) — error budgets that balance shipping speed against stability. Buying side: map your vendors' SLAs against YOUR customer promises — you cannot honestly sell 99.9% while standing on a best-effort dependency chain (B2B diligence 101).
FAQ
Is an SLA legally required? No — it's a commercial term; but in B2B software procurement its absence reads as immaturity.
SLA vs SLO vs SLI? SLA = external contract with remedies; SLO = internal objective; SLI = the measured indicator feeding both.
Do credits ever matter financially? Rarely — their value is the incentive alignment and the audit trail; the termination trigger is the clause with real teeth.
Related: SaaS · B2B · Customer Success Reviewed: 2026-07-16 · Sources: ITSM canon