Tech_Interview_Prep

SLIs, SLOs & Error Budgets

The vocabulary reliability is measured in, and how an error budget turns 'be reliable' into a concrete number.

OverviewQ&AMCQ

What it is

SLIs, SLOs, and error budgets give reliability a shared, quantitative vocabulary instead of a vague goal like "be reliable."

Key points

  • SLI (Service Level Indicator): a specific measured metric, e.g. "percentage of requests served in under 200ms."
  • SLO (Service Level Objective): the target for an SLI, e.g. "99.9% of requests under 200ms, measured over 30 days."
  • Error budget: 100% minus the SLO — the amount of unreliability allowed before the target is breached. A 99.9% SLO gives roughly 43 minutes of budget per month.
  • The error budget is what makes SLOs actionable: if it's being burned too fast, that's a trigger to slow down feature releases and prioritize reliability work instead — a concrete decision rule, not just a dashboard number.