System Metrics: Latency, QPS, Availability Nines & Capacity Math
Calculate Back-of-the-Envelope QPS math, storage sizing, latency p99 metrics, and evaluate horizontal vs vertical scaling.
Learning Objectives & Back-of-the-Envelope Estimation
### Learning Objectives
- Convert Daily Active Users (DAU) into Queries Per Second (QPS) average and peak workloads.
- Evaluate SLA availability thresholds (99.9% vs 99.99% "four nines").
- Distinguish between Vertical Scaling (Scale-Up) and Horizontal Scaling (Scale-Out).
- Understand Latency percentiles (p50, p95, p99 tail latency).
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### Back-of-the-Envelope Estimation Rules
- $1 ext{ Day} = 86,400 ext{ seconds} approx 10^5 ext{ seconds}$.
- **QPS Equation**: $ ext{QPS} = rac{ ext{Daily Active Users} imes ext{Requests per User}}{86,400}$.
- **Peak QPS**: $ ext{Average QPS} imes 2$.
