Choosing the Right SLIs

Start with what your users care about. For a checkout service, users care about three things: can I complete my purchase (availability), how long does it take (latency), and does it charge me the right amount (correctness). I'd pick SLIs from the four golden signals: latency, error rate, throughput, and saturation. But not all of them become SLOs. For a checkout service, I'd focus on: 1. Availability -- the ratio of successful checkout requests (non-5xx) to total requests. This is your most important SLI because a broken checkout directly costs money. 2. Latency -- the 95th percentile response time for checkout completion. Not the average, because averages hide tail latency. A p95 of 2 seconds means 5% of your users are waiting longer than that, and those are often paying customers mid-purchase. 3. Correctness -- the percentage of checkout transactions where the charged amount matches the cart total. This is easy to overlook but a wrong charge is worse than a slow page. I'd skip throughput as an SLO target because it varies naturally with traffic patterns. It's good to monitor but bad as an objective. I'd also look at the SLIs from the user's perspective, not from the server's. Measure at the load balancer or edge, not inside your application, because that's closer to what the user actually experiences.