SLA
A Service Level Agreement is the contractual commitment a service provider makes to its customers about expected service behaviour, with financial or contractual consequences when the commitment is missed. SLAs are typically looser than internal SLOs — a 99.9% uptime SLA is usually backed by a 99.95% internal SLO so the operations team has headroom before customers are entitled to credits.
SLAs are external-facing and lawyer-reviewed; SLOs are internal-facing and engineer-set. The relationship matters: if your SLA promises 99.9% and your SLO is also 99.9%, the operations team has zero buffer before the legal department gets involved. Healthy practice maintains at least one 9 of headroom between SLO and SLA. SLAs are most useful for B2B services where customers can negotiate; for consumer services, the SLA is usually a goodwill statement rather than a binding contract.
Related terms
- SLO
A Service-Level Objective is a target reliability metric for a service — typically expressed as a percentage over a time window.
- SLI
A Service Level Indicator is a numerical measurement of one specific dimension of a service's behaviour — request latency, error rate, throughput, availability — expressed over a defined window.
- Error budget
An error budget is the allowable reliability gap between the SLA (customer contract) and the SLO (operational target).