SLA vs SLO vs SLI: Whats the Difference?

SLA vs SLO vs SLI: Whats the Difference?

In this video, we cover the key differences between SLA, SLO, and SLI defining each term and giving real world examples of how they differ.

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SLA vs SLO vs SLI Whats the difference?
In this video, we will define these terms, compare them to one another and give real-world examples of how they work.

SLAs, SLOs, and SLIs might sound like a jumble of acronyms, but they are crucial in setting clear expectations for service delivery, ensuring reliability, satisfaction, and continuous improvement. But what exactly are they, and how do they differ?

Service Level Agreements, or SLAs, are essentially a promise or guarantee from the service provider to the customer. They outline the expected level of service, detailing the products or services to be delivered as well as the consequences for missing these service levels. SLAs are typically drafted by legal departments with insights from product managers and are designed to be customer-facing. It sets the stage for accountability and sets clear expectations right from the start. For instance, PagerTree, an OnCall incident management software solution, promises a 99.9% monthly availability to its customers. This is part of their SLA.

Service Level Objectives, or SLOs, on the other hand, are the internal goals set to achieve or exceed the “Promise or guarantee” made in the SLA. These are specific, measurable deliverables that internal teams aim to meet. They are typically drafted by product managers and are crucial for internal teams to ensure they meet or exceed the performance outlined in the SLA. PagerTree’s SLO, or internal goal, is 99.99% uptime. That's a difference of almost 8 hours yearly between their “Customer Promise” and “Internal Goal” This is also known as error budgeting. If you like to know more about error budgeting and other common metrics, there will be a link in the description below.

Service Level Indicators, or SLIs, are the actual measured performance metrics. They are specific, quantifiable, and crucial for evaluating the service quality provided. Monitoring SLIs can be done through various tools like Prometheus and Datadog. SLIs are for both internal teams and customers to determine if the “promise” made in the SLA is being met. SLIs help in identifying issues, understanding user experience, and making data-driven decisions by internal teams before there is a breach of SLA.

In the real world, the relationship between SLAs, SLOs, and SLIs can be illustrated through a simple graph.

The SLA line in yellow indicates a “promise” made to the customer that response times will meet or exceed 300ms.
The SLO line in purple indicates the “Internal Goal” for performance set by the project manager to meet or exceed the goal of 250ms
The SLI line in blue shows the actual measured performance of the service being provided.

In example A the Performance is exceeding both the SLA and SLO with response times between 180ms and 250ms.

In example B, the performance is exceeding the promise made in the SLA but missing the goal set in the SLO. These response times range from 250ms to 300ms performing inside of the error budget. This example illustrates an opportunity for internal improvement. Luckily, because of the teams error budgeting, they are not failing the customers expectations.

In example C the performance is not meeting the “Promise” made in the SLA or the “internal goal” set in the SLO. This means performance is in breach of contract and the consequences outlined in the SLA can come into effect.

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