Site Reliability Engineering

Chapter 4. Service Level Objectives

Written by Chris Jones, John Wilkes, and Niall Murphy with Cody Smith

Edited by Betsy Beyer

It’s impossible to manage a service correctly, let alone well, without understanding which behaviors really matter for that service and how to measure and evaluate those behaviors. To this end, we would like to define and deliver a given level of service to our users, whether they use an internal API or a public product.

We use intuition, experience, and an understanding of what users want to define service level indicators (SLIs), objectives (SLOs), and agreements (SLAs). These measurements describe basic properties of metrics that matter, what values we want those metrics to have, and how we’ll react if we can’t provide the expected service. Ultimately, choosing appropriate metrics helps to drive the right action if something goes wrong, and also gives an SRE team confidence that a service is healthy.

This chapter describes the framework we use to wrestle with the problems of metric modeling, metric selection, and metric analysis. Much of this explanation would be quite abstract without an example, so we’ll use the Shakespeare service outlined in “Shakespeare: A Sample Service” to illustrate our main points.

Service Level Terminology

Many readers are likely familiar with the concept of an SLA, but the terms SLI and SLO are also worth careful definition, because in common use, the term SLA is overloaded and has taken on a number of meanings depending ...