Site Reliability Engineering (SRE) Basics Cheat Sheet

Foundational SRE concepts including SLIs, SLOs, error budgets, and toil reduction for running reliable production systems.

2 PagesIntermediateJan 28, 2026

Core SRE Vocabulary

The key definitions underpinning SRE practice.

SLI (Service Level Indicator)- A quantitative measure of a service's behavior, e.g. request latency, error rate
SLO (Service Level Objective)- A target value or range for an SLI over a time window, e.g. 99.9% of requests < 300ms
SLA (Service Level Agreement)- A contractual promise to a customer, usually with consequences if the SLO is missed
Error Budget- The allowed amount of unreliability (1 - SLO) within a window, used to balance velocity vs stability
Toil- Manual, repetitive, automatable operational work that scales linearly with service growth

Error Budget Calculation

Compute remaining error budget for a 99.9% SLO over 30 days.

python

slo_target = 0.999total_minutes = 30 * 24 * 60  # 43200allowed_downtime_minutes = total_minutes * (1 - slo_target)print(allowed_downtime_minutes)  # 43.2 minutes/month# If 12 minutes of downtime already occurred this month:remaining_budget = allowed_downtime_minutes - 12print(remaining_budget)  # 31.2 minutes remaining

Multi-Window Burn Rate Alert

Example Prometheus alert firing on fast error-budget burn.

yaml

- alert: HighErrorBudgetBurn  expr: |    (      sum(rate(http_requests_total{status=~"5.."}[1h]))      / sum(rate(http_requests_total[1h]))    ) > (14.4 * 0.001)  for: 2m  labels:    severity: page  annotations:    summary: "Burning error budget 14.4x faster than sustainable"

Key SRE Practices

Recurring activities that operationalize reliability.

Blameless postmortems- Root-cause analysis focused on systems and process, not individuals
Capacity planning- Forecasting resource needs ahead of demand growth
Release engineering- Standardized, repeatable build and deployment processes
On-call rotation- Shared, time-boxed responsibility for responding to production alerts

Pro Tip

When the error budget is exhausted, freeze risky feature launches and redirect the team to reliability work — this is the mechanism that actually enforces the SLO, not the number itself.

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