Service Level Objectives (SLOs)

What is an SLO?

A service-level objective (SLO) is a key element of a service-level agreement (SLA) between the service provider and the customer. SLOs are agreed upon as a means of measuring the performance of the Service Provider. The SLA is the entire agreement that specifies what service is to be provided, how it is supported, times, locations, costs, performance, and responsibilities of the parties involved. SLOs are specific measurable characteristics of the SLA.

Example: Availability, Throughput, Frequency, Response time

SLO and SLA are quite similar, however, SLA is usually a weaker target than SLO. SLO is about time and usually answers the following questions: What percentage of time X was able to meet the Y threshold of the Z indicator? Short-term SLOs are important for developers, and SRE teams, while long-term SLOs are important for managing organizations, reviewing goals, and more.

Why SLO?

To develop an effective SLO, understand users' interactions with the service, which are called critical user journeys (CUJs). A CUJ considers the goals of users, and how users use services to accomplish those goals. The CUJ is defined from the perspective of the customer without consideration for service boundaries. 

Examples: 

• Reliability is the most critical feature of any service. A common metric for reliability is uptime, which conventionally means the amount of time a system has been up. 

• More helpful and precise metric is availability. Availability answers the question of whether a system is up but in a more precise way than by measuring the time since a system was down. Availability is often described in terms of nines—such as 99.9% available (three nines), or 99.99% available (four nines). 

Measuring an availability SLO is one of the best ways to measure your system's reliability.