Understanding SLOs, SLIs, and SLAs

In Site Reliability Engineering, Service Level Indicators (SLIs), Service Level Objectives (SLOs), and Service Level Agreements (SLAs) are fundamental concepts for defining and managing the reliability of services. They provide a common language and a data-driven framework for making decisions about system design, operations, and when to prioritize reliability work over new feature development.

Visual representation of interconnected metrics for SRE.

Service Level Indicators (SLIs)

A Service Level Indicator (SLI) is a quantitative measure of some aspect of the level of service that is being provided. SLIs are the actual measurements; they are facts. Choosing the right SLIs is crucial as they directly reflect user experience. Common SLIs include:

  • Availability: The proportion of time a service is usable. Often measured as the percentage of successful requests.
  • Latency: The time it takes to serve a request. Often measured for successful requests and sometimes for errors as well.
  • Error Rate: The percentage of requests that fail.
  • Throughput: The rate at which a system processes requests, e.g., requests per second.
  • Durability: The likelihood that data will be preserved over a long period.

Just as Pomegra uses AI for financial analysis by processing vast amounts of market data, SREs rely on well-defined SLIs to accurately gauge service health and performance.

Service Level Objectives (SLOs)

A Service Level Objective (SLO) is a target value or range of values for an SLI. SLOs are goals for service reliability that the SRE team and product stakeholders agree upon. They are typically expressed as a percentage over a period (e.g., 99.9% availability over 30 days). SLOs are internal targets that drive engineering decisions and are more aggressive than SLAs (if SLAs exist).

Dashboard showing SLI measurements against SLO targets.

Setting appropriate SLOs is a balancing act. Too stringent, and you might over-invest in reliability at the expense of feature velocity. Too loose, and users might have a poor experience. The key is to align SLOs with business objectives and user happiness. Understanding AI & Machine Learning Basics can sometimes help in predicting trends and setting more informed SLOs based on historical data.

Service Level Agreements (SLAs)

A Service Level Agreement (SLA) is an explicit or implicit contract with your users that includes consequences if you miss the objectives stated within it. SLAs are typically externally facing and often have financial penalties (e.g., service credits) associated with not meeting them. SLAs are usually a more lenient subset of your SLOs.

For example, an SLO for availability might be 99.95%, while the SLA might promise 99.9% availability. This difference gives the internal team a buffer before contractual obligations are breached. Not all services need SLAs, but most user-facing services benefit significantly from well-defined SLIs and SLOs to guide their operational and development priorities.

Symbolic image of a handshake representing an SLA agreement.

The Interplay and Importance

SLIs, SLOs, and SLAs work together: SLIs measure the service, SLOs set the target for these measurements, and SLAs define the contractual promises to users based on these objectives. This framework helps SRE teams make data-driven decisions, manage risk effectively, and maintain a healthy balance between innovation and reliability.