Breaking down the SLO, SLA, and SLI metrics

👉 SLO (Service Level Objective) is a measure that defines the target level of availability or performance of a service provided to a user. It's part of a service quality agreement (SLA), where the SLO acts as a specific metric that needs to be achieved. For example, an SLO might state that a web service must be available 99.9% of the time over a month. SLOs help development and operations teams stay focused on maintaining a certain level of service quality.

👉 SLA (Service Level Agreement) is an agreement between a service provider and a client that defines the expected level of service quality and availability. An SLA describes specific metrics, such as response time, availability, and performance, as well as the actions to take in case of non-compliance with these requirements. For example, if some service goes down and its availability falls outside the agreed-upon SLO, the moment comes when the service provider has to compensate for the downtime according to the terms of the SLA.

👉 SLI (Service Level Indicator) is a specific metric used to measure the level of quality or performance of a service. An SLI shows how well a service performs its functions with respect to a particular aspect, for example response time or availability. It's the actual measurement that helps determine whether the service meets the established objectives (SLO) and terms (SLA).

📎 Examples for better understanding:

Imagine you have a web service.

1️⃣ SLI (Service Level Indicator): — This is the actual metric, for example the average server response time, which comes to 200 milliseconds.

2️⃣ SLO (Service Level Objective): — This is the goal you want to achieve, for example that 95% of requests are handled in 300 milliseconds or faster.

3️⃣ SLA (Service Level Agreement): — This is the contract with the client, which includes the SLO and other terms. For example, you commit to keeping the average response time at no more than 300 milliseconds in 95% of cases and guarantee compensation if this condition is violated.

📎 Thus, the SLI is what you measure, the SLO is your goal, and the SLA is your commitment to the client. These parameters are key in the work of an SRE.