Understanding SLA, SLO, SLI

A Service Level Agreement, Service Level Objective, and Service Level Indicator form the foundation of reliability planning in modern operations. Each term plays a distinct role in how teams define, measure, and commit to service performance targets. This article breaks down their definitions, relationships, and practical implementation steps.

Service Level Indicator (SLI)

It represents the raw measurement of some aspect of system performance. An SLI is numerical value derived from monitoring data and reflects service health over time. Find some example below:

• Request latency measured in milliseconds

• Success rate defined as the ratio of successful to total requests

• Error rate calculated as percentage of failed transactions

Service Level Objective (SLO)

It is a target value or range for a specific SLI over a defined period. This target serves as an internal reliability benchmark that teams aims to meet consistently. Some example below:

• A percentage goal such as 99.9 percent availability

• A defined measurement window

• Alignment with user expectations

Service Level Agreement (SLA)

An SLA is a formal, often contractual, commitment between a service provider and its customers. It outlines the established SLOs, the measures to address breaches, and any corresponding penalties or credits. Components of an SLA include:

• The exact SLOs that the provider guarantees

• Definitions of service credits or penalties if targets are missed

• Exclusions and limitations such as scheduled maintenance windows

Relationship among SLI, SLO and SLA

SLI, SLO, SLA build on each other in a hierarchy of measurement, targets and commitment. An SLI provides the data, an SLO set the performance goal, and an SLA formalizes that goal at the customer level.

Implementing SRE

Integrating SLIs, SLOs, and SLAs into your reliability strategy follows a structured process. Each step ensures alignment between technical capabilities and customer expectations.

1. Identify critical user journey and select relevant SLIs

2. Defined realistic SLO targets based on historical data

3. Set up alerting and dashboard to monitor SLI performance against SLOs

4. Draft SLAs that reflect SLOs and outlines remedies for breaches

5. Continuously review and adjust targets as the service and user needs evolve

Conclusion

Understanding and applying SLIs, SLOs, and SLAs empowers teams to deliver reliable services while managing risk and expectations. By measuring what matters, setting clear goals, and formalizing commitments, Site Reliability Engineers can foster trust and drive continuous improvement.