In the context of Site Reliability Engineering (SRE), a “service” generally refers to a specific software application or a set of tightly related functionalities that deliver value to users. Services can be as broad as an entire web application or as specific as a microservice handling a particular aspect of that application, such as payment processing or user authentication.

When defining Service Level Objectives (SLOs) for a service, you are essentially setting targets for the desired level of performance and reliability of that service. Here are a few key points to consider when defining SLOs for a service:

  • Service Identification: Before setting objectives, clearly identify what constitutes the service. This might include its components, dependencies, and the functionalities it provides.

  • User Expectations: Understand what users expect from the service. This could involve performance (e.g., response times), availability (e.g., uptime percentages), or correctness (e.g., error rates).

  • Measurement: Determine how these expectations can be quantitatively measured using specific metrics. Common metrics include latency, throughput, error rates, and availability.

  • Thresholds: Establish acceptable thresholds for these metrics, which form the basis of the SLOs. These thresholds are typically derived from user expectations and business needs.

  • Timeframes: Define the timeframes over which measurements will be taken and evaluated against the SLOs. This could be over a rolling window (e.g., 30 days) or a fixed period (e.g., per month).

  • Consequences: Consider the consequences of not meeting these objectives, which can help in prioritizing engineering efforts and resource allocation. This may involve setting up alerts or automating responses when SLOs are at risk of being breached.

In essence, the “objectives” in SLOs are quantifiable goals set for the reliability and performance metrics of the service, ensuring it meets the expected standards necessary for a good user experience and operational continuity.