Service Level Objective (SLO)

A service level objective (SLO) is a specific, measurable reliability target for your service — for example, "99.9% of API requests succeed in under 300 ms, measured over 30 days." It's built from a service level indicator (SLI), the raw metric you actually measure, and it's the internal goal that sits behind any external service level agreement (SLA) you promise customers. The clever part is the error budget: a 99.9% target means you're allowed 0.1% failures, roughly 43 minutes of downtime a month. That budget becomes a shared decision-making tool — if you've burned it, you freeze risky launches and fix reliability; if you have budget to spare, you can ship faster. For SaaS builders, SLOs replace vague "the site should be up" arguments with a number everyone agrees on, and they stop you from over-engineering reliability nobody needs. Start with one or two user-facing SLOs (availability, latency) rather than instrumenting everything at once.