From Theory to Running SLOs#

Every SRE resource explains what SLOs are. Few explain how to actually implement them from scratch – the Prometheus queries, the error budget math, the alerting rules, and the conversations with product managers when the budget runs out. This guide covers all of it.

Step 1: Choose Your SLIs#

SLIs must measure what users experience. Internal metrics like CPU usage or queue depth are useful for debugging but are not SLIs because users do not care about your CPU – they care whether the page loaded.

Why Regular Reviews Matter#

Reliability does not improve by accident. Without a structured review cadence, teams operate on vibes – “things feel okay” or “we’ve been having a lot of incidents lately.” Reliability reviews replace gut feelings with data. They surface slow-burning problems before they become outages, hold teams accountable for improvement actions, and create a shared understanding of system health across engineering and leadership.

Weekly Reliability Review#

The weekly review is a 30-minute tactical meeting focused on what happened this week and what needs attention next week. Attendees: on-call engineers, team leads, SRE. Keep it tight.

The SRE Model#

Site Reliability Engineering treats operations as a software engineering problem. Instead of a wall between developers who ship features and operators who keep things running, SRE defines reliability as a feature – one that can be measured, budgeted, and traded against velocity. The core insight is that 100% reliability is the wrong target. Users cannot tell the difference between 99.99% and 100%, but the engineering cost to close that gap is enormous. SRE makes this tradeoff explicit through service level objectives.

SLI, SLO, and SLA – What They Actually Mean#

An SLI (Service Level Indicator) is a quantitative measurement of service quality – a number computed from your metrics. Examples: the proportion of successful HTTP requests, the proportion of requests faster than 500ms, the proportion of jobs completing within their deadline.

An SLO (Service Level Objective) is a target value for an SLI. It is an internal engineering commitment: “99.9% of requests will succeed over a 30-day rolling window.”