What is the error budget used for?

Error Budget is a predefined acceptable level of unreliability or failures for a service over a certain period of time. It is calculated as 100% minus the target Service Level Objective (SLO). The error budget is used for:

• Prioritizing work: If the error budget is close to being exhausted, the team should temporarily delay developing new features and focus on reliability tasks (bug fixing, technical debt, improving monitoring). If the budget is not being used up, it is safe to work on new features.
• Decision making: The error budget provides a quantitative measure on which informed decisions about releasing new versions, conducting experiments, or making large-scale changes can be based.
• Communication: It serves as a common language between development and business teams to discuss the balance between innovation (new features) and stability (reliability). A high error budget may indicate an overly conservative SLO, while a constantly exhausted budget suggests quality issues or an unrealistic SLO.

Calculation example: If the target reliability (SLO) for the service availability is 99.9% per month, then the error budget for that month is 100% - 99.9% = 0.1% of the total service uptime. This is approximately 43.2 minutes of downtime per month.