williamzujkowski/SRE SLO-SLI Calculator

Purpose & When-To-Use

Trigger conditions:

• New service needs reliability targets and SLO definition
• Existing service requires error budget calculation and tracking
• Stakeholders demand SLA commitments with measurable guarantees
• Incident postmortem reveals need for better reliability metrics
• Service experiences reliability issues; need objective measurement
• DevOps team adopting SRE practices and needs SLO framework
• Monitoring alerts are noisy; need error budget-based alerting

Use this skill when you need to establish measurable reliability targets (SLIs/SLOs), calculate error budgets, and integrate with monitoring platforms to track service reliability against user expectations.

Pre-Checks

Before execution, verify:

1. Time normalization: NOW_ET = 2025-10-25T21:30:36-04:00 (NIST/time.gov semantics, America/New_York)
2. Input schema validation:

• service_type is one of: api, frontend, batch, streaming, storage, database
• user_journey is non-empty array describing critical user workflows
• availability_target (if provided) is between 90.0 and 100.0
• monitoring_platform is one of: prometheus, datadog, cloudwatch, newrelic, generic

1. Source freshness: All cited sources (Google SRE books, monitoring docs) accessed on NOW_ET
2. Baseline data: Historical service metrics available for realistic SLO calibration

Abort conditions:

• Service has no measurable user-facing behavior (cannot define meaningful SLIs)
• No monitoring/observability infrastructure exists (cannot measure SLIs)
• Availability target >99.999% without justification (unrealistic for most services)
• User journey description is vague or non-specific

Procedure

Tier 1 (Fast Path)

Token budget: T1 ≤2k tokens

Scope: Common 80% case - standard service with typical reliability requirements.

Steps:

1. Identify service golden signals based on service_type (accessed 2025-10-25T21:30:36-04:00: https://sre.google/sre-book/monitoring-distributed-systems/):

• API/Frontend: Request latency, error rate, throughput
• Batch: Job completion rate, processing time, data quality
• Streaming: Event processing lag, throughput, data loss rate
• Storage: Read/write latency, availability, durability
• Database: Query latency, connection availability, replication lag

1. Map user journey to SLIs:

• For each critical path in user_journey, identify primary golden signal
• Define measurement window (rolling 30-day default per SRE Workbook, accessed 2025-10-25T21:30:36-04:00: https://sre.google/workbook/implementing-slos/)
• Specify aggregation method (e.g., 99th percentile latency, error ratio)

1. Calculate SLO thresholds:

• Use availability_target or default to 99.9% for user-facing services
• Convert availability to error budget: error_budget = (1 - availability_target) * measurement_window
• Example: 99.9% over 30 days = 43.2 minutes downtime allowed

1. Generate basic monitoring query (Prometheus example):

promql # Availability SLI sum(rate(http_requests_total{status!~"5.."}[5m])) / sum(rate(http_requests_total[5m]))

1. Output: SLI definitions, SLO targets, error budget, and sample query

Tier 2 (Extended Design)

Token budget: T2 ≤6k tokens

Scope: Production services requiring comprehensive SRE implementation with alerting and error budget policies.

Steps:

1. Detailed SLI specification (accessed 2025-10-25T21:30:36-04:00: https://sre.google/workbook/implementing-slos/):

• Availability SLI: Ratio of successful requests to total requests
• Latency SLI: Proportion of requests faster than threshold (e.g., 95% <200ms)
• Freshness SLI: For batch/streaming - data staleness acceptable threshold
• Correctness SLI: Data quality or functional correctness metric
• Throughput SLI: Minimum requests/events processed per time window

1. Multi-window SLO definition:

• 30-day rolling window: Primary SLO for long-term reliability trends
• 7-day rolling window: Short-term health indicator
• 28-day sliding window: Monthly reporting alignment
• Specify different thresholds per window if needed (looser short-term, stricter long-term)

1. Error budget calculation with burn rate (accessed 2025-10-25T21:30:36-04:00: https://landing.google.com/sre/workbook/chapters/alerting-on-slos/):

• Calculate total error budget for each time window
• Define burn rate thresholds:
• Fast burn (2% budget consumed in 1 hour): Page immediately
• Moderate burn (5% budget consumed in 6 hours): Alert on-call
• Slow burn (10% budget consumed in 3 days): Ticket for review
• Generate burn rate queries:

promql # 1-hour burn rate (fast burn) (1 - (sum(rate(http_requests_total{status!~"5.."}[1h])) / sum(rate(http_requests_total[1h])))) > (14.4 * (1 - 0.999)) # 14.4x faster than budget allows

1. Platform-specific query generation:

• Prometheus: PromQL with recording rules for SLI components
• Datadog: Datadog Query Language with monitors
• CloudWatch: CloudWatch Metrics Math expressions
• New Relic: NRQL queries with baseline
• Include both SLI measurement and error budget consumption queries

1. Alerting policy design:

• Define alert severity levels (page, alert, ticket)
• Map burn rate to severity:
• Critical: >2% error budget consumed in 1 hour
• Warning: >5% error budget consumed in 6 hours
• Info: >10% error budget consumed in 3 days
• Include alert content: current error rate, time to budget exhaustion, remediation guidance

1. SLA derivation (if customer-facing):

• SLA = SLO - safety margin (typically 0.1-0.5%)
• Example: Internal SLO 99.95%, external SLA 99.9%
• Include consequences for SLA breach (credits, penalties)

1. Error budget policy:

• Define feature freeze conditions: when error budget <10% remaining in 30-day window
• Escalation path: SRE team → Engineering manager → VP Engineering
• Recovery actions: halt risky deploys, prioritize reliability work

1. Comprehensive output:

• Detailed SLI definitions with measurement methodology
• Multi-window SLO targets
• Error budget calculations with burn rates
• Complete monitoring queries for chosen platform
• Alerting policy with severity mapping
• Error budget policy document
• (Optional) SLA terms if customer-facing

Sources cited (accessed 2025-10-25T21:30:36-04:00):

• Google SRE Book Ch. 4: https://sre.google/sre-book/service-level-objectives/
• Google SRE Workbook Ch. 2: https://sre.google/workbook/implementing-slos/
• Google SRE Workbook Ch. 5: https://landing.google.com/sre/workbook/chapters/alerting-on-slos/
• Prometheus Best Practices: https://prometheus.io/docs/practices/alerting/

Tier 3 (Deep Dive, not implemented)

Token budget: T3 not implemented for this skill

Rationale: T2 tier provides comprehensive SRE implementation with multi-window SLOs, error budget policies, and platform-specific monitoring integration. T3 would potentially cover highly specialized scenarios (custom SLI aggregation algorithms, multi-cluster federation, advanced anomaly detection) that are better handled through expert SRE consultation or custom tooling development.

Decision Rules

SLI selection criteria:

• Choose availability SLI if user journey depends on service being reachable (most common)
• Choose latency SLI if user experience degrades with slow responses (interactive services)
• Choose freshness SLI if data staleness impacts user decisions (analytics, dashboards)
• Choose correctness SLI if wrong data is worse than no data (financial, healthcare)

SLO threshold selection:

• 99.9% (three nines): Standard for most user-facing services; 43.2 min downtime/month
• 99.95% (three nines five): High-value services; 21.6 min downtime/month
• 99.99% (four nines): Mission-critical services; 4.32 min downtime/month
• <99.9%: Internal tools, non-critical batch jobs
• >99.99%: Rarely justified; requires significant investment and architectural complexity

Ambiguity thresholds:

• If user_journey contains >10 critical paths → ask user to prioritize top 3-5
• If availability_target not specified → default to 99.9% for user-facing, 99.5% for internal
• If service has multiple user types (free vs paid) → ask if different SLOs needed per tier
• If no historical data available → start with conservative SLO (99.0%), iterate after 1 month

Abort/stop conditions:

• User cannot articulate what "service working" means → SLI definition impossible
• Requested SLO (e.g., 99.999%) exceeds architectural capabilities → warn and recommend realistic target
• Monitoring platform cannot measure proposed SLI → suggest alternative SLI or platform upgrade

Output Contract

Required fields:

json
{
  "sli_definitions": [
    {
      "name": "string (e.g., 'API Availability')",
      "type": "availability | latency | freshness | correctness | throughput",
      "description": "string (what this SLI measures)",
      "measurement_method": "string (how to calculate)",
      "data_source": "string (metrics endpoint, logs, traces)",
      "good_events": "string (numerator definition)",
      "total_events": "string (denominator definition)",
      "threshold": "number (for latency/freshness SLIs)",
      "unit": "string (ms, %, count)"
    }
  ],
  "slo_targets": [
    {
      "sli_name": "string (references SLI above)",
      "target": "number (e.g., 99.9)",
      "window": "string (30d, 7d, 28d)",
      "window_type": "rolling | calendar",
      "budget_threshold": "number (alert threshold %)"
    }
  ],
  "error_budget": {
    "total_minutes_30d": "number (allowable downtime)",
    "burn_rate_thresholds": {
      "critical_1h": "number (14.4x for 99.9%)",
      "warning_6h": "number (6x for 99.9%)",
      "info_3d": "number (1x for 99.9%)"
    },
    "current_consumption": "number | null (if historical data available)"
  },
  "monitoring_queries": {
    "platform": "string (prometheus, datadog, etc.)",
    "sli_queries": ["array of query strings"],
    "error_budget_queries": ["array of query strings"]
  },
  "alerting_policy": [
    {
      "severity": "critical | warning | info",
      "condition": "string (burn rate threshold)",
      "notification": "string (page, email, ticket)",
      "duration": "string (alert evaluation window)"
    }
  ]
}

Optional fields:

• sla_terms: Customer-facing SLA commitments (if applicable)
• error_budget_policy: Freeze/escalation procedures
• dashboard_config: Monitoring dashboard layout recommendations

Validation:

• All SLI names must be unique
• SLO targets must reference defined SLIs
• Error budget calculations must align with SLO targets
• Monitoring queries must be syntactically valid for target platform

Examples

Input:

json
{
  "service_type": "api",
  "user_journey": [
    "User searches products",
    "User adds item to cart",
    "User completes checkout"
  ],
  "availability_target": 99.9,
  "monitoring_platform": "prometheus"
}

Output (abbreviated):

json
{
  "sli_definitions": [
    {
      "name": "API Availability",
      "type": "availability",
      "measurement_method": "ratio of successful HTTP responses (2xx/3xx) to total requests",
      "good_events": "http_requests_total{status=~\"2..|3..\"}",
      "total_events": "http_requests_total"
    }
  ],
  "slo_targets": [{"sli_name": "API Availability", "target": 99.9, "window": "30d"}],
  "error_budget": {"total_minutes_30d": 43.2},
  "monitoring_queries": {
    "platform": "prometheus",
    "sli_queries": [
      "sum(rate(http_requests_total{status=~\"2..|3..\"}[5m])) / sum(rate(http_requests_total[5m]))"
    ]
  }
}

Quality Gates

Token budgets:

• T1: ≤2k tokens for simple SLI/SLO definition with basic monitoring
• T2: ≤6k tokens for comprehensive SRE implementation with error budget policies

Safety requirements:

• Validate SLO targets are achievable given current architecture
• Warn if SLO >99.99% (requires significant investment)
• Ensure error budget policies prevent excessive risk-taking

Auditability:

• All SLI definitions include explicit measurement methodology
• Monitoring queries are reproducible and version-controlled
• Error budget calculations show math and assumptions

Determinism:

• Same inputs produce same SLI definitions
• Error budget formulas are consistent with SRE literature
• Burn rate thresholds use standard multipliers (1x, 6x, 14.4x)

Resources

Primary sources:

• Google SRE Book: https://sre.google/sre-book/table-of-contents/
• Google SRE Workbook: https://sre.google/workbook/table-of-contents/
• Prometheus Documentation: https://prometheus.io/docs/
• Datadog SLO Monitoring: https://docs.datadoghq.com/monitors/service_level_objectives/

Reference implementations:

• Prometheus SLO Recording Rules (accessed 2025-10-25T21:30:36-04:00)
• Sloth SLO Generator (accessed 2025-10-25T21:30:36-04:00)
• OpenSLO Specification (accessed 2025-10-25T21:30:36-04:00)

Additional reading:

• The Art of SLOs (book): https://www.alex-hidalgo.com/the-art-of-slos
• SLO Workshop: https://www.usenix.org/conference/srecon19americas/presentation/fong-jones