A numeric metric describing service health. Examples: percentage of successful requests, p99 latency, uptime. "What we measure"
An internal target for an SLI. "The bar we set for ourselves". Example: 99.9% of requests completed in < 500 ms. The SLO is stricter than the SLA — a "buffer" for the team.
A legal contract with the customer. "What we promise externally". Violating the SLA triggers financial penalties. Hence the SLA is always softer than the SLO.
SLO 99.9% = 0.1% of allowed errors per month ≈ 43 minutes of downtime. The error budget shows how much "reliability balance" remains. If the budget is exhausted — new deploys stop.