Understanding Error Budgets, SLOs, and SLAs
A comprehensive guide to reliability engineering concepts
What is an Error Budget?
An error budget is a concept in Site Reliability Engineering (SRE) that quantifies the acceptable level of failure or downtime for a service. It's based on the understanding that 100% reliability is neither practical nor necessary for most systems.
Error budgets provide a concrete way to balance reliability with innovation. They create a shared understanding between development and operations teams about how reliable a service needs to be, and when it's acceptable to take risks.
Key Benefits of Error Budgets:
- Provides an objective metric for service reliability
- Creates a balance between reliability and feature development
- Enables data-driven decisions about when to focus on stability vs. new features
- Aligns incentives between development and operations teams
When a service depletes its error budget, teams typically shift focus from developing new features to improving reliability until the error budget is replenished.
Service Level Objectives (SLOs)
A Service Level Objective (SLO) is a target reliability goal for a service. SLOs define what "good enough" reliability looks like for a particular service, and they form the basis for error budgets.
SLOs are typically expressed as a percentage of successful operations over a specific time window. For example, "99.9% of requests will be successful over a 30-day rolling window."
Components of an SLO:
- Service Level Indicator (SLI): A quantitative measure of service level (e.g., request latency, error rate)
- Target value: The desired level of reliability (e.g., 99.9%)
- Time window: The period over which the SLO is measured (e.g., 30 days)
The difference between your SLO and 100% is your error budget. For example, with a 99.9% availability SLO, your error budget is 0.1% of the time window.
Service Level Agreements (SLAs)
A Service Level Agreement (SLA) is a contract between a service provider and its customers that defines the expected level of service. Unlike SLOs, which are internal goals, SLAs have financial or legal consequences if they're not met.
SLAs typically include:
- Specific metrics and thresholds for service performance
- Methods for measuring and reporting on those metrics
- Penalties for failing to meet the agreed-upon service levels
- Exclusions or conditions where the SLA doesn't apply
Relationship between SLAs and SLOs:
SLOs should be stricter than SLAs. A common practice is to set SLOs at least one order of magnitude more reliable than SLAs. For example, if your SLA promises 99.9% availability, your internal SLO might be 99.95% or 99.99%. This creates a buffer that helps prevent SLA violations.
Calculating Error Budgets
Error budgets are calculated based on your SLO and the time window you're measuring against. The basic formula is:
For example, with a 99.9% availability SLO over a 30-day period (43,200 minutes):
This means you can have up to 43.2 minutes of downtime in a 30-day period before violating your SLO.
When an incident occurs, you calculate its impact on your error budget using:
× (1 - desired uptime))
Best Practices
- Start with realistic SLOs: Base your initial SLOs on historical performance data rather than aspirational targets.
- Choose meaningful SLIs: Focus on metrics that directly impact user experience.
- Set different SLOs for different service tiers: Not all services need the same level of reliability.
- Review and adjust regularly: SLOs should evolve as your service and user expectations change.
- Communicate clearly: Make sure all stakeholders understand what the SLOs mean and how error budgets work.
- Automate monitoring and alerting: Track error budget consumption in real-time to catch issues early.