Site Reliability Engineering
Dependency Reliability Calculator
Enter dependency availabilities to compute composite system availability, showing how series dependencies multiply failure probability.
No data is transmitted — everything runs locallyTool
About this tool
Dependency Reliability Calculator
The Dependency Reliability Calculator computes composite system availability from series dependencies, with redundancy modeling and SLO headroom calculation.
• Calculate achievable SLO given dependency availabilities before writing an SLA
• Show stakeholders why a 99.9% SLO requires more than three 99.9% dependencies
• Model availability improvement from adding redundancy to a single dependency
• Identify the lowest-availability dependency that caps total system availability
Affiliate disclosure
Uptime, incident, and on-call management. Better Stack provides status pages, incident management, and on-call scheduling for engineering teams.
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FAQ
What does this tool tell you?
The Dependency Reliability Calculator computes composite system availability from series dependencies, with redundancy modeling and SLO headroom calculation.
What affects the result most?
System availability = product of all dependency availabilities (in series). 3 services at 99.9% each = 99.9% × 99.9% × 99.9% = 99.7% — not 99.9%. Redundancy (parallel): availability = 1 - (1 - availability)^n — two 99% services = 99.99%.
How should I use the result?
The calculation is deterministic — the same inputs always produce the same output — so the most useful workflow is to vary one input at a time and see which factor moves the result most. That tells you where to focus your attention before committing to a decision.
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