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A measurable reliability target that tells you when good enough is reached.
Quick Definition
A service level objective (SLO) is a target level of reliability or performance for a service, such as 99.9% availability. SLOs guide engineering priorities and error budgets, balancing the cost and effort of reliability against user expectations.
A Service Level Objective, or SLO, is a precise reliability target for a service: 99.9 percent of requests succeed, 95 percent complete under 300 milliseconds, measured over a rolling window. SLOs turn the vague demand for reliability into a number a team can engineer toward and measure against.
SLOs are also the honest framework for cost decisions, because reliability is purchased: each additional nine multiplies infrastructure through redundancy, multi-zone footprints, and standby capacity. An explicit SLO forces the question of what the business actually needs, and the error budget, the allowed shortfall, gives teams permission to stop gold-plating once the target is met.
Example. A team discovers it runs an internal analytics service with the same multi-zone redundancy as its payment system. Setting an honest 99.5 percent SLO for analytics lets it drop to a single zone, saving 40 percent of that service's cost with no one affected.
SLOs come from SRE practice and pair with monitoring that measures them continuously. The Cloud Cost Management Guide covers aligning infrastructure tiers to objectives instead of habit.
The allowed failure within an SLO, such as 0.1 percent of requests. Spending it on velocity is fine; exhausting it triggers a focus on stability.
They cap reliability investment at what the business needs, exposing services over-engineered far beyond their actual importance.
An SLO is an internal target. An SLA is a contractual promise to customers, usually set looser than the internal SLO behind it.