System Design Foundations

Make a contract someone can falsify

Specify operation, eligible requests, success boundary, load envelope and time window. Latency, freshness, availability and durability need separate evidence.

Calculate with units

1M users × 12 events/day = 12M/day ≈139 writes/s average. At 500 bytes each: 6 GB/day; 30 days =180 GB raw. 50% indexes ×3 replicas →810 GB, before backups and headroom.

Challenge assumptions

Double payload and retention →4× raw storage. Peak factor, tenant skew and maximum payload are separate dimensions. 2,000 requests/s ×0.040 s =80 mean in-service requests, not a p99 capacity guarantee.

Trace the path, then measure the percentile

15 ms ingress +10 auth +max(25,70) parallel calls +5 encoding =100 ms for this modeled request. Per-hop p99 values do not generally add to end-to-end p99. A timeout must fit the remaining request deadline.

Submit evidence

Document acknowledgment durability and failure domain. Reconstruction: 600K×10/day, 400 bytes,14 days,1.5 indexes,3 copies →151.2 GB. Deliver contracts, units, a path trace and a failure test before choosing components.