In recent years, site reliability engineering (SRE ) has emerged as one of the most sought-after approaches to business operations. As per Google, “Site reliability engineering is what you get when you treat operations as a software problem.” Error budgeting is one of the crucial elements of an SRE approach, and even though all organizations have unique challenges, it is essential to be aware of it.

For engineers tasked with site reliability engineering, one of the primary responsibilities is to ensure that systems are reliable and available to users. Despite best efforts, unexpected issues can inevitably arise, resulting in downtime or degradation of the systems.

To help you navigate these challenges, the concept of an error budget has evolved over time as a key part of the SRE model. An error budget is essentially a budget of downtime or degradation that is acceptable within a given time frame. By setting and tracking an error budget, site reliability engineering managers can prioritize their efforts and make informed decisions about when and how to invest in reliability improvements.

The question is, how do you determine the size of your error budget, and how do you allocate it to your SRE model over time?

In this blog post, we will compare different approaches to managing and allocating an error budget in an SRE model, including threshold-based budgets time-based budgets and rolling budgets. We will also discuss the pros and cons of each approach and offer some best practices for implementing an SRE approach to the error budget in your organization.

What is an error budget?

An error budget is a way of quantifying the amount of downtime or degradation acceptable in a given time frame. It is expressed as a percentage of total uptime or as a number of minutes or hours of downtime. For example, you might have an error budget of 99.9% uptime requirement. This means, if a service receives 1,000,000 requests in one month, with 99.9% SLO, the margin for error budget would be 1,000 errors for the entire month.

The idea behind an error budget is to allow you to balance the trade-offs between reliability and velocity. You want to ensure that your systems are as reliable as possible, but you also want to be able to move quickly and deliver new features and improvements to your users. An error budget can help you to strike a balance between these two requirements.

Threshold-Based Budgets

Using this approach, you set specific thresholds for different metrics or indicators of system health and allocate your error budget accordingly.

For example, you might set a threshold for the number of errors returned by your servers and allocate a certain percentage of your error budget to handling these errors. You might also set a threshold for the response time of your servers and allocate a different percentage of your error budget to improving performance.

The advantage of threshold-based budgeting is that it allows you to focus your efforts on specific areas of your system that need improvement. By setting clear targets and allocating your error budget accordingly, you can prioritize your reliability efforts and make the most impact.

In addition to its benefits, threshold-based budgeting also has some potential drawbacks. One challenge is that it can be difficult to determine the appropriate thresholds to set, as different systems and workloads will have different requirements. Additionally, setting fixed thresholds can be inflexible, as it may not allow for changes in workload or usage patterns over time.

Time-based Budgets

Time-based budgeting is another approach to managing and allocating an error budget. With this approach, you allocate your error budget over a set period such as a week, month or quarter. This allows you to spread out your error budget over time, rather than using it all at once.

For example, you might allocate your error budget monthly, which will allow you a certain amount of downtime or degradation you can use each month. This approach allows you to be more flexible and adapt to changes in workload or usage patterns, as you can adjust your error budget allocation each month.

One advantage of time-based budgeting is that it allows you to prioritize reliability improvements over the long term, rather than focusing solely on short-term goals. It also gives you more control over how you allocate your error budget, as you can distribute it across different parts of your system as needed.

However, there are some potential drawbacks to time-based budgeting. One challenge is that it can be difficult to accurately forecast your error budget needs over time, as a variety of factors such as changes in workload, usage patterns and the complexity of your system can affect it. Additionally, time-based budgeting may not be very effective at identifying and addressing specific bottlenecks in your system.

Rolling Budgets

A third SRE approach to managing and allocating an error budget is rolling budgets. With this approach, you allocate your error budget on a rolling basis, such as over the past week, month or quarter. This allows you to continually reassess and adjust your error budget as needed, based on your actual reliability performance.

For example, you might allocate your error budget monthly, but continually reassess and adjust it based on your reliability performance over the past month. If you have used up your error budget too quickly, you might need to allocate more funds towards reliability improvements. If your error budget has not yet been consumed, you may be able to allocate additional funds towards new features or improvements.

Rolling budgets allow you to be more responsive and adaptive to changes in your system. By continually reassessing and adjusting your error budget, you can identify and address issues or bottlenecks faster, and as they arise. It also makes it simpler for you to monitor the impact of your reliability efforts over time.

However, there are some potential drawbacks to rolling budgets. For example, a rolling budget can be more complex to implement and manage, as you need to continually reassess and adjust it. Additionally, rolling budgets may not be very effective at setting long-term reliability goals, as they focus more on short-term performance.

Conclusion

In site reliability engineering, there are different approaches to managing and allocating an error budget, including threshold-based budgeting, time-based budgeting and rolling budgets. Each approach has its strengths and weaknesses, and the right approach for your organization depends on your specific needs and goals.

Regardless of the approach you choose, it's important to regularly review and assess your error budget, and to communicate it to your team and stakeholders. You can clearly define when it is acceptable to take risks and when it is necessary to prioritize reliability by setting and tracking an error budget. This will help you make informed decisions about how to invest in reliability improvements.

HCLTech’s Cloud Application Reliability Engineering (CARE) is an all-encompassing complete solution for site reliability and platform reliability engineering requirements. The solution addresses the needs of your customer right from the setting up of site reliability engineeringPRE to operating them in their customized environment.

Our expert site reliability engineering consultants can help you set up error budgets, SLIs, SLOs, observability and several other tenets that are key to reliable and resilient operations.

For more information on CARE, please write to contact.hyc@hcltech.com