Measuring And Managing Service Reliability In SRE: SLI, SLO, SLA, And Error Budget
Published: 2024-01-19 12:18:43

**_SLI, SLO, SLA, and Error Budget: how do they interact with each other? How to apply these concepts from the SRE discipline in your IT operation? How can this measurement directly influence the company’s results?_**


Continuing our series of content on Site Reliability Engineering, in this article, we will address the concepts of SLI, SLO, SLA, and Error Budget, topics previously explored in the [introductory article](https://kwan.com/blog/site-reliability-engineering-fundamental-concepts-and-how-to-put-them-in-practice/) on the subject, which you can read about on our blog.

When it comes to today’s technology landscape, where online systems and digital services play a crucial role in our lives, the reliability of these services is essential. Imagine depending on a transportation app or an e-commerce website… and they keep failing?






## SLI, SLO, SLA, and Error Budget

To ensure that these services work reliably, the concepts of SLI, SLO, SLA, and Error Budget are applied, aiming to play a vital role. In this article, we will explore these concepts and their importance in creating robust and resilient systems.



### Service-Level Indicator (SLI)

Service Level Indicators (SLIs) are quantitative metrics that measure some specific aspect of the quality of a service. They are used to evaluate the performance of a system or service against certain criteria. For example, in a video streaming service, an SLI might be the content availability rate. In other words, what percentage of time users can watch content without interruptions. Measuring SLIs is the first step to understanding the quality of the service offered.


![SLI formula](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/7pk9433zlgxhgi3mhpsa.png)


### Service-Level Objective (SLO)

Service Level Objectives (SLOs) are targets or limits that are defined according to the SLIs, mentioned in the previous paragraph, and that represent the desired performance values ​​that a service must maintain. For example, an SLO for the streaming service mentioned above could be: “Content must be available to users 99.9% of the time within a month.” Establishing SLOs allows you to set clear performance and quality expectations for both internal teams and end users.



### Service-Level Agreement (SLA)

Service Level Agreements (SLAs) are formal agreements between service providers and customers, which specify the minimum levels of performance that will be provided. They are based on SLOs and establish the obligations and responsibilities of the parties involved. An SLA can define penalties if the service does not meet agreed levels. For example, a cloud provider may guarantee 99.99% uptime in its SLA. If downtime exceeds this limit, customers may receive compensation due to breach of contract established by the SLA between both parties.



### Error Budget

The concept of Error Budget is based on the idea that perfect reliability is unattainable. Rather than that, it is proposed that a margin should be reserved for unforeseen circumstances. This budget establishes the tolerance limit for non-compliance with SLOs. For example, if the SLO corresponds to 99.9% availability, the Error Budget could be 0.1%. This means that the development team can make changes or updates that can result in up to 0.1% of downtime, without violating the established agreement, the SLA.


![Erros Budget](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/5pyq15a2olwulxfqlvbl.png)


## Integrating the Concepts

The effective integration of these concepts – SLI, SLO, SLA, and Error Budget – is essential to ensure that a service is reliable and meets user expectations. Collaboration between development, operations, and business teams plays a crucial role in this process.

By collaboratively defining SLIs, development and operations teams gain a shared understanding of the critical metrics that affect the user experience. This establishes a solid foundation for defining SLOs, which, in turn, guide teams toward achieving tangible performance goals. They let everyone know exactly what the goal is and what standards must be maintained.

From SLOs, SLAs can be established to formalize commitments between service providers and customers. This is particularly crucial in business scenarios where service reliability is directly linked to customer satisfaction and company reputation. When SLAs are clear and realistic, the creation of unrealistic expectations is avoided, which can lead to financial loss if they are not met.

However, it is important to recognize that the pursuit of absolute reliability can result in an excessive allocation of resources. This is where the concept of the Error Budget comes in. The Error Budget recognizes that the pursuit of total perfection can be costly and instead allows some room for inactivity or errors. This does not mean that quality must be compromised, but rather that resources can also be allocated to innovation and continuous development, thus balancing stability with evolution.


![SRE cycle](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/80ki8a1zqiyftnfnp2ov.png)


## Application of Concepts

Considering a practical scenario to illustrate how these concepts apply, imagine an e-commerce platform during a major promotion (Black Friday, for example), where user demand increases significantly. The SRE (Site Reliability Engineering) team defined an SLI to measure the success rate of transactions. If this rate drops below 95%, it may be considered a problem.

Based on the defined SLI, the SRE team has defined an SLO of 98% for the success rate of transactions during the promotion, this means that the team is aiming to maintain the success rate at 98% or higher, with this SLO definition, the team established an SLA with the development team, where it was agreed that if the success rate falls below 97%, the development team commits to investigate and fix the problem within a specific time frame.

However, during the campaign, an unexpected issue occurred that led to a temporary drop in the transaction success rate to 96%. This means that the 2% Error Budget was used. The development team, knowing that they still have a margin for error, can act in a more balanced way. They can fix the problem responsibly without the pressure of directly violating the SLA.





## The Journey to Reliability

The concepts of SLI, SLO, SLA, and Error Budget play a fundamental role in the journey to create and maintain reliable services in a constantly evolving digital environment. They offer a structured set of guidelines that enable technology teams to measure, set goals, establish commitments, and deal with the realities of occasional failures.

Collaboration is the backbone of this approach. As teams from different domains work together to define SLIs, establish SLOs, and craft SLAs, a shared understanding of expectations and responsibilities emerges. The Error Budget recognizes that complete perfection may not be achievable, but it provides scope for continuous improvement and innovation, after all, we’re all human and mistakes happen.

In practice, applying these concepts can be the difference between a service that leaves users frustrated due to frequent failures… or one that inspires trust and loyalty. Leading technology companies like Google have successfully embraced these concepts, demonstrating that reliability is achievable when a disciplined approach is adopted.





## Measuring and Managing Service Reliability in SRE – Final Considerations

Ultimately, the goal of the Site Reliability Engineering discipline is to create an environment where users can trust that services will be available and functional when they need them the most. The journey to reliability is ongoing and requires adapting to changing user demands and technologies. By understanding and applying the principles of SLI, SLO, SLA, and Error Budget, organizations are prepared to face the challenges and opportunities that the digital world presents.

So may we continue to strive for excellence in reliability, while also embracing the need for continued innovation and growth.

In the next article, we will discuss in more detail the concepts of Automation in Site Reliability Engineering, aiming to reduce repetitive tasks (Toil) and how to apply it effectively during the implementation of the discipline.

Article written by Elton Padilha and originally published at https://kwan.com/blog/measuring-and-managing-service-reliability-in-sre-sli-slo-sla-and-error-budget/ on January 9, 2024.