Service Level Indicators (SLIs) and Service Level Objectives (SLOs) are essential concepts for maintaining reliable systems in DevOps.

SLIs are precise measurements that show how well a service is performing from a user's perspective. Common SLIs include:

• Request success rate;
• Response time;
• System availability;
• Error rate.

SLOs are the specific targets you set for your SLIs. An SLO defines the level of performance you promise to maintain. For example, you might set an SLO that 99.9% of user requests must succeed each month.

By using SLIs and SLOs, you can:

• Monitor the health and reliability of your systems;
• Identify issues before they affect users;
• Set clear expectations for service quality;
• Make informed decisions about when to prioritize reliability improvements.

SLIs and SLOs help you focus on what matters most to users and ensure your services remain dependable over time.

Difference Between SLIs and SLOs

Service Level Indicators (SLIs) are specific, quantitative measures that reflect how a service is performing. An SLI could be the percentage of successful HTTP requests, average response time, or system uptime. These indicators give you clear, objective data about your system’s health.

Service Level Objectives (SLOs) are explicit targets for those indicators. An SLO defines what level of performance you expect from your service, such as “99.9% of requests must succeed each month” or “average response time must be below 200 milliseconds.” SLOs set the standard you want to meet based on your SLIs.

Why Measurable Goals Matter

Setting measurable goals using SLIs and SLOs helps you:

• Track service health with clear, objective data;
• Identify and address issues before they affect users;
• Align your team around shared expectations and priorities;
• Build trust with stakeholders by showing commitment to reliability.

When you set and monitor measurable goals, you improve both performance and accountability. Your team knows what success looks like and can focus on delivering consistent, high-quality service.

Example: Defining an SLI and Setting an SLO for a Login Service

Suppose your team manages a login service that users rely on to access your application. You want to measure and guarantee the reliability of this service.

Step 1: Define the SLI (Service Level Indicator)

• Choose a metric that reflects user experience;
• For a login service, a common SLI is successful login rate;
• Example SLI: the percentage of login attempts that succeed within 2 seconds.

Step 2: Set the SLO (Service Level Objective)

• Decide the target you want to meet for this SLI;
• Example SLO: "99.9% of login attempts must succeed within 2 seconds over a 30-day period."

By tracking this SLI and SLO, you can quickly detect if users are having trouble logging in and take action to maintain high reliability.