slo-error-budget

SLO and Error Budget Skill

Produce a complete, implementable SLO document for a service — covering what to measure, what target to set, how to calculate the error budget, and what to do when it burns.

A good SLO is not a target to hit. It is an agreement about what reliability means for your users — and a framework for making principled trade-offs between reliability and velocity.

Required Inputs

Ask for these if not already provided:

• Service name and brief description of what it does
• Primary users — who depends on this service and how
• User-facing interactions to protect — e.g. API calls, page loads, transactions
• Current reliability data — error rate, latency, uptime (last 30–90 days if available)
• Existing on-call setup — who responds to alerts?
• Deployment frequency — how often does the team ship?
• Any existing SLAs with customers — these constrain SLO targets

Key Definitions

Always establish these before writing the SLO:

| Term | Definition | |---|---| | SLI (Service Level Indicator) | The metric being measured — e.g. "% of requests completing successfully in <500ms" | | SLO (Service Level Objective) | The target for that metric — e.g. "99.5% of requests" | | SLA (Service Level Agreement) | The contractual commitment to customers — must be looser than the SLO | | Error budget | The allowed headroom below 100% — the budget for planned and unplanned downtime | | Burn rate | How fast the error budget is being consumed |

Output Format

SLO Document: [Service Name]

Service: [Name] | Team: [Team name] Owner: [Name / role] | Approved by: [Name] Effective date: [Date] | Review date: [Date + 3 months] Version: [1.0]

Why This SLO Exists

[2–3 sentences. What reliability problem are we solving? What was happening before this SLO that made us need it? What decision-making does this SLO enable?]

Service Overview

What this service does: [One sentence] Who depends on it: [Internal teams / external customers / both — describe] Critical user journeys protected by this SLO:

1. [Journey 1 — e.g. "User completes a payment"]
2. [Journey 2]
3. [Journey 3]

SLIs — What We Measure

Define one SLI per user journey or reliability dimension. Keep it to 3–5 SLIs maximum.

SLI 1: [Name — e.g. Request Success Rate]

| Field | Detail | |---|---| | What it measures | [e.g. "% of API requests that return a non-5xx response"] | | Good event definition | [e.g. "HTTP response with status 2xx or 4xx, completed within 500ms"] | | Bad event definition | [e.g. "HTTP response with status 5xx, or any response taking >500ms"] | | Measurement source | [e.g. "Application load balancer access logs / Datadog APM / Prometheus"] | | Measured over | Rolling 28-day window | | Exclusions | [e.g. "Health check endpoints excluded / Requests during planned maintenance excluded"] |

SLI 2: [Name — e.g. Latency]

| Field | Detail | |---|---| | What it measures | [e.g. "P99 response time for the /checkout endpoint"] | | Good event definition | [e.g. "Request completes in ≤500ms at P99"] | | Bad event definition | [e.g. "Request takes >500ms at P99"] | | Measurement source | [Source] | | Measured over | Rolling 28-day window | | Exclusions | [Any exclusions] |

SLI 3: [Name — e.g. Data Freshness / Queue Depth / etc.]

[Same structure]

SLO Targets

| SLI | Target | Window | Error Budget | |---|---|---|---| | [SLI 1 name] | [X]% | 28-day rolling | [100 - X]% = [Y minutes/month] | | [SLI 2 name] | [X]% | 28-day rolling | [100 - X]% = [Y minutes/month] | | [SLI 3 name] | [X]% | 28-day rolling | [100 - X]% = [Y minutes/month] |

How targets were set:

• Historical baseline (last 90 days): [X]%
• Target is set [above / at] historical baseline to [improve reliability / reflect current reality while formalising the commitment]
• Rationale: [1–2 sentences]

What 100% is NOT the target: [Brief explanation of why targeting 100% is counterproductive — it discourages feature development and doesn't reflect user reality]

Error Budget Calculation

For SLI 1 ([Name]), at [X]% target:

Error budget = (100% - SLO target) × measurement window
             = (100% - [X]%) × 28 days × 24 hours × 60 minutes
             = [Y]% × [Z total minutes]
             = [N] minutes of allowed failure per 28-day window

In plain terms: We can afford [N] minutes of [bad events] in any rolling 28-day window before we breach the SLO.

Burn Rate Alerts

Burn rate = how fast the error budget is being consumed relative to the budget window. A burn rate of 1 = consuming the budget at exactly the rate that would exhaust it over 28 days.

| Alert | Burn rate | Window | Severity | Response | |---|---|---|---|---| | Page (critical) | >14× | 1 hour | P1 | Page on-call immediately — budget exhausted in <2 hours | | Page (high) | >6× | 6 hours | P2 | Page on-call — budget exhausted in <5 days | | Ticket (warning) | >3× | 3 days | P3 | Create ticket — review at next team meeting | | Info | >1× | 28 days | Info | Log only — budget on track to exhaust by end of window |

Alert implementation: [Link to alert config in monitoring tool — e.g. Datadog, Prometheus/Alertmanager, Grafana]

Error Budget Policy

This policy defines what to do with the error budget — both when it's healthy and when it's burning.

When budget is healthy (>50% remaining)

• Feature development and deployments proceed at normal pace
• The team may take on riskier experiments
• Reliability improvements are scheduled but not urgent

When budget is at risk (25–50% remaining)

• Deployment frequency reduced — team ships only well-tested changes
• One reliability improvement added to current sprint
• Weekly error budget review added to team standup

When budget is nearly exhausted (<25% remaining)

• Feature work paused in favour of reliability improvements
• No new deployments without explicit on-call approval
• Daily review of error budget burn rate
• CSM / support notified to manage customer expectations

When budget is exhausted (0% remaining — SLO breached)

• All feature work stops
• On-call engineer and engineering manager notified immediately
• Post-incident review (PIR) required within 5 business days
• SLO target may be temporarily relaxed (with stakeholder approval) while root cause is addressed

Dashboard and Reporting

SLO dashboard: [Link to Datadog / Grafana / etc. dashboard]

Metrics exposed:

• Current SLO compliance (rolling 28-day)
• Error budget remaining (% and minutes)
• Burn rate (current and trend)
• Incident count and MTTR this window

Reporting cadence:

| Audience | Frequency | Format | |---|---|---| | Engineering team | Weekly | Slack summary — #[service]-slo | | Engineering manager | Monthly | SLO review meeting | | Stakeholders / customers | Quarterly | SLO compliance summary |

Exclusions and Edge Cases

Planned maintenance: Error budget is not consumed during pre-announced maintenance windows. Maintenance must be communicated [X hours] in advance via [channel].

Dependency failures: If SLO breach is caused by an upstream dependency outside our control, document it — but it still counts against our error budget (our users don't distinguish between our failures and our dependencies' failures).

Force majeure: [Policy for cloud provider outages, major infrastructure events]

SLO Review Cadence

| Review | When | Who | Output | |---|---|---|---| | Error budget review | Weekly | Team | Budget health check — adjust if burning fast | | SLO target review | Quarterly | Team + EM | Adjust targets if baseline has shifted significantly | | Annual SLO audit | Annually | Team + Stakeholders | Review SLIs — are we measuring the right things? |

When to change the SLO target:

• Historical baseline has improved significantly and ta