SLA vs SLO vs SLI Explained

For engineering teams scaling SaaS products, terms like SLA, SLO, and SLI appear constantly in reliability discussions. Understanding the differences between these metrics is essential to building an effective incident response and monitoring plan.

Here is a straightforward, developer-friendly guide to what they mean and how they connect.

1. Service Level Indicator (SLI)

The Measurement: What is the raw metric?

An SLI is a quantitative measure of service performance. It represents a single, verifiable metric that tells you how well your service is behaving in real-time.

• Examples:
  • The percentage of HTTP requests that return in less than 200ms.
  • The ratio of successful API responses (HTTP 200-299) to total requests.
  • The latency of your message queue queue processor.

2. Service Level Objective (SLO)

The Target: What is our target goal?

An SLO is a target reliability goal for your service, defined in terms of an SLI. It tells your engineering team how reliable the service needs to be.

• Examples:
  • Our API uptime SLI must be 99.9% over any rolling 30-day window.
  • 95% of API requests must return response times of < 200ms.

3. Service Level Agreement (SLA)

The Promise: What are the commercial consequences if we fail?

An SLA is a legal agreement between you and your customers. It promises a specific level of service reliability (usually matching or lower than your internal SLO) and outlines what happens if you fail to meet it (e.g. refunds or service credits).

• Examples:
  • We guarantee 99.9% monthly uptime. If we fail, we refund 10% of your monthly subscription fee.

How to Set Up Your Reliability Stack

To manage these metrics without overhead:

1. Define SLIs that directly impact user experience (like login API uptime).
2. Set reasonable SLOs (e.g. 99.9%), allowing your team a clear "error budget" to deploy new code without risking reliability goals.
3. Track and publish status via public Pingzo Status Pages to prove compliance to your enterprise buyers.