Guides
Short, plain-English explanations behind the numbers.
What is an SLA?
A Service Level Agreement (SLA) is a formal commitment — often contractual — that a service will meet a specific reliability target, most commonly expressed as an uptime percentage over a billing period. Missing an SLA usually triggers service credits or penalties for the provider.
SLA vs SLO vs SLI
SLI (Service Level Indicator)
The actual measurement — e.g. the percentage of successful requests, or measured uptime over the last 30 days.
SLO (Service Level Objective)
The internal target a team aims for, usually stricter than the external SLA, so there's margin before customers are affected.
SLA (Service Level Agreement)
The external, often contractual promise — typically looser than the internal SLO, with real consequences (credits, refunds) if missed.
How many nines do you need?
Each additional "nine" cuts allowed downtime by roughly 10x, but the engineering cost to get there grows non-linearly. Most SaaS products target 99.9% (three nines) — it's achievable without heavy multi-region infrastructure. Reserve 99.99%+ for services where outages carry serious financial, safety, or contractual consequences, since the redundancy and on-call maturity required rises sharply at that tier.