SLOs and Error Budgets

Define what "reliable enough" means for users, then agree in advance what the organization does when the service falls short.

SLO Design

Design service level indicators (SLIs) and objectives (SLOs) that track what users experience, and that the organization will actually use to make decisions.

Core ideas

• An SLO is a target for "reliable enough". Above it, nearly all users are satisfied; below it, they start to complain or leave.
• 100% is never the target. Every extra nine costs more and is worth less to users, dependencies between you and the user add their own failures, and a 100% goal leaves no room for change, which is the main source of outages.
• Express every SLI as good events / valid events, a ratio between 0% and 100%. Then error budget = 100% − SLO, and one set of tooling (numerator, denominator, threshold) works for every SLI.
• Separate the SLI specification (what matters to the user, e.g. "home page loads in < 100 ms") from the SLI implementation (where and how it is measured). One spec can have several implementations that trade off fidelity, coverage and cost.
• SLOs need an owner empowered to trade velocity for reliability — usually the product owner.

Procedure