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Key takeaway

Error budget is a concept in site reliability engineering that sets a limit on the acceptable level of errors or incidents that can occur within a given time frame. This article delves into how error budgets are used to balance innovation and reliability, allowing teams to prioritize improvements and allocate resources effectively based on their error budget consumption.


An error budget is a concept used in software development and operations to measure and manage the acceptable level of errors or incidents that can occur within a system. It is a way to balance the need for innovation and rapid deployment with the need for reliability and stability.

The basic idea behind an error budget is that no system is perfect, and some level of errors or incidents is expected to occur. By defining an acceptable level of errors, teams can prioritize their efforts and resources accordingly. The error budget represents the maximum allowable number or severity of errors within a given time period.

The error budget is typically measured using key performance indicators (KPIs) such as uptime, response time, or error rate. These KPIs are monitored and tracked over time to determine if the system is within the defined error budget or if corrective actions need to be taken.

When the error budget is exhausted, it indicates that the system is becoming less reliable and stable. At this point, teams may need to slow down or pause new feature development to focus on improving the system's reliability and reducing errors.

Error budgets are often used in conjunction with service level objectives (SLOs) and service level agreements (SLAs). SLOs define the desired level of reliability and performance, while SLAs specify the guarantees provided to customers. The error budget helps ensure that the system meets the agreed-upon levels of reliability and performance.

By using an error budget, teams can strike a balance between innovation and reliability, allowing them to continuously improve and evolve their systems while maintaining a high level of customer satisfaction.

How to use an error budget

Using an error budget effectively is crucial for managing the trade-off between innovation and reliability in software development and operations. Here are some key steps to consider when using an error budget:

Define acceptable error levels: Start by establishing clear and measurable criteria for what constitutes an acceptable level of errors or incidents within a given time period. This could be based on key performance indicators (KPIs) such as uptime, response time, or error rate. Ensure that these criteria align with the expectations of stakeholders and customers.

Monitor and track performance: Continuously monitor and track the relevant KPIs to measure the system's performance against the defined error budget. Use monitoring tools and techniques to collect data and generate insights about the system's reliability and stability. Regularly analyze this data to identify trends, patterns, and areas for improvement.

Set thresholds and triggers: Determine thresholds or triggers that indicate when the error budget is being depleted or nearing exhaustion. These thresholds can be based on predefined percentages or specific values of the KPIs. When the thresholds are crossed, it signals the need for action and decision-making to address the issues impacting system reliability.

Prioritize actions: When the error budget is being consumed rapidly or has been exhausted, prioritize actions to improve system reliability. Allocate resources and efforts to address the most critical issues that contribute to errors or incidents. Consider factors such as the severity of the impact, the frequency of occurrence, and the potential for mitigating risks.

Balance innovation and stability: Use the error budget as a guide to strike a balance between innovation and stability. While it is important to deliver new features and enhancements, ensure that sufficient resources and attention are dedicated to maintaining and improving system reliability. Make informed decisions about feature development, considering the impact on the error budget.

Foster a culture of learning and improvement: Encourage a culture of continuous learning and improvement within the team. Regularly review and reflect on the data and insights derived from monitoring the error budget. Share learnings, best practices, and success stories to drive collective improvement efforts and enhance system reliability over time.

Communicate and align: Maintain open and transparent communication with stakeholders about the status of the error budget. Clearly communicate the goals, progress, and challenges related to system reliability. Engage in regular discussions with stakeholders to ensure alignment on expectations and priorities.

What is a maintenance window?

Maintenance windows are carefully chosen to minimize the impact on service availability and performance. They are typically scheduled during periods of low user traffic or when the service experiences minimal usage. By selecting these specific timeframes, SRE teams aim to ensure that users and customers are least affected by any potential disruptions or downtime.

Within the SRE framework, maintenance windows are an integral part of the change management process. They involve thorough planning, coordination, and communication with stakeholders. SRE teams assess the risks associated with the planned activities and evaluate their potential impact on service reliability and performance.

Throughout the maintenance window, SRE teams closely monitor the service to detect any issues or anomalies. They perform testing and verification activities to ensure that the service is functioning correctly after the changes have been implemented. This monitoring helps identify and address any potential problems promptly.

By utilizing maintenance windows effectively, SRE teams can balance the need for system updates and improvements with the goal of maintaining a reliable and high-performing service. It allows them to minimize disruptions, adhere to service commitments, and provide a positive user experience while ensuring the overall stability and availability of the system.

How to choose your maintenance windows

Firstly, it is essential to schedule maintenance windows during periods of low user traffic or minimal service usage. By selecting these specific timeframes, SRE teams aim to minimize the impact on users and customers. This involves analyzing historical usage patterns and understanding peak and off-peak hours to identify suitable windows.

Secondly, effective communication is vital when planning and executing maintenance windows. SRE teams must inform users, customers, and relevant stakeholders well in advance about the upcoming maintenance activities. Clear and concise communication should include details about the purpose of the maintenance, expected impact on service availability, and any necessary actions users may need to take.

Coordination with other teams within the organization is also crucial. SRE teams collaborate with development, operations, and other relevant teams to ensure smooth execution of the maintenance activities. This coordination helps address dependencies, align priorities, and minimize potential conflicts that could impact the success of the maintenance window.

Risk assessment plays a significant role in managing maintenance windows. SRE teams evaluate the potential risks associated with planned activities and changes. They consider factors such as the complexity of the changes, potential dependencies, and the likelihood of service disruptions. This assessment helps identify mitigation strategies and contingency plans to handle unforeseen issues or complications that may arise during the maintenance window.

Lastly, documentation and post-mortem analysis are essential for continuous improvement. SRE teams maintain detailed records of maintenance activities, including the changes made, any issues encountered, and the resolutions applied. Post-mortem analysis allows teams to learn from past experiences, identify areas for improvement, and refine their processes for future maintenance windows.

By following these practices, SRE teams can effectively manage maintenance windows, ensuring minimal disruption to users and customers while maintaining the reliability and availability of services. It enables organizations to deliver a high-quality user experience and meet their service level objectives.

How Harness can help manage reliability

The obvious decisions start when you don’t meet your SLO target and the remaining error budget falls below a threshold (you can define the threshold). The most common path includes stopping feature launches until the service is within SLO again, or working on reliability-related bugs. Reviewing your SLOs and error budgets on a periodic basis makes sure that you’re meeting expectations and reliability requirements. Error budgets act as an incident trigger to initiate a blameless postmortem (root cause analysis) on the impacted business services.

Calculation of the error budget according to the Google SRE book’s appendix:

Error Budget = 1 – Availability SLO

For example, if the SLO is 99.9%, then to calculate the error budget:

Error Budget = 1 – 99.9% = 0.1% = ~10 minutes of SLO violation per week or ~131 minutes/quarter

This 0.1% is the unavailability window. After exhausting the error budget, previously agreed-upon error budget policies help prevent any further customer impact. New releases are kept on hold while the team performs more testing. Having a healthy and mature SLO and error budget culture lets you refine how you measure and discuss the reliability requirements of your service.

In a distributed environment, offering 100% availability is technically complex and costly. Establishing SLOs and creating an error budget can be a long journey, but the results are well worth the investment. You will become equipped with the needed ammunition to detect potential customer-impacting issues before they become customer-facing. You must continually monitor key paths within your service that are frequently visited by your users. Aggregating this data helps define alerts and other actions in the event of a breach or near-breach.

Harness has created a solution to make it easier for newcomers to start using SLIs, SLOs, and Error Budgets. The solution also helps teams advance to a point of implementing SLO policies to automate guardrails within CI/CD pipelines. Learn more about Harness SRM, part of the Harness Software Delivery Platform, and request a demo today.

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