# Composite SLA calculator

Work out the real availability of a service that needs several dependencies, with replicas for the ones that have them.

## How it works

When a request needs every dependency, their availabilities multiply, so the whole is always less available than its weakest part.

Replicas that can stand in for each other fail only when all fail at once, which raises that dependency’s availability.

```
serial = a₁ × a₂ × … × aₙ
with n replicas = 1 − (1 − a)ⁿ
```

## Questions

### Why is my composite lower than every dependency?

Failures add up: three 99.9% dependencies in a row give about 99.7%.

### Do replicas really fail independently?

Often not. A shared region, network or deploy can take all copies down, so treat replica numbers as a best case.

## Related tools

- [SLO & error budget calculator](https://powertraces.com/tools/slo-error-budget)
- [Burn-rate alert planner](https://powertraces.com/tools/burn-rate-alerts)
- [Uptime nines converter](https://powertraces.com/tools/uptime-nines)
- [Latency percentile calculator](https://powertraces.com/tools/latency-percentiles)

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