Definition

service level indicator

Contributor(s): Alex Gillis

A service level indicator (SLI) is a metric that indicates what measure of performance a customer is receiving at a given time. The term is used by DevOps engineers when discussing quality of service (QoS). SLIs are a direct measurement of a service’s behavior and are typically documented in service level agreements (SLAs), as well as in service level objectives (SLOs). 

Potential service level indicators that providers may want to document include latency, error rate, throughput, availability and response time. Storage systems, for example, will typically measure latency, durability and availability for SLIs. Most metrics are gathered server-side with monitoring software such as Nagios.

It is important that organizations decide which measurements in their provided system should be used as SLIs. Having too many SLIs can be distracting and prevent engineers from paying enough attention to the more critical performance indicators. When choosing what metrics to review, engineers should have a point of view that focuses on what functions are most important for the user.

SLIs vs. SLOs vs. SLAs

SLIs are a part of what make up both service level agreements  and service level objectives (SLOs). An SLA is an agreement between a service provider and an end-user; including the service provided, support, cost and performance.

The SLO is a part of the SLA. The SLO gives a method of evaluation in the performance of a service. Where an SLI is a measurement the service provider uses, the SLO defines internal goals for consistent service. For example, the SLI may be the latency in a service , while the SLO is the acceptable error rate in latency measured in milliseconds.

Together, SLAs, SLOs and SLIs describe the properties of important metrics, the acceptable ranges of those metrics and communicates that to the end-user.

This was last updated in May 2019

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