SPECIFICATION · CONCEPT BRIEFDWG · 15-TWO-CONTINENTS

Two Continents

Light is fast — but not THAT fast. Distance shows up in p95.

§01The speed of light tax

Bits travel at roughly 200,000 km/s through fibre. London to New York and back is ~5,500 km × 2 ≈ 55ms of just-physics latency, before your server does anything. Add TCP handshakes, TLS, and a few hops — you're easily looking at 80–120ms per round trip.

Flow models this as cross-region transit cost: every time a request crosses from one region to another, the simulator adds 80ms (8 ticks). Same-region or unset-region transitions cost nothing extra.

§02Edge caching to the rescue

Place a CDN in the same region as your users. Most reads terminate locally, never crossing the ocean.
Only cache misses pay the cross-region cost.
p95 latency is dominated by the slowest 5% — exactly the requests that crossed the ocean.

⚑ NOTE

This level

Your client lives in EU-West. Your origin server and database are in US-East. The SLA enforces a tight p95. Without an edge CDN in EU-West with a high hit rate, every request pays the Atlantic round-trip and you fail.

⚑ CHEATSHEET · QUICK REFERENCE

If your users are far from your origin, a regional edge cache is not optional — it's required.
p95 latency is what users feel. Cross-region hops blow it up disproportionately.

▸ THE EXERCISE

Your client lives in EU-West; your origin is in US-East. Every cross-Atlantic round-trip costs ~80ms. Place an edge CDN in EU-West so most reads terminate locally and your p95 stays inside SLA.

▸ START EXERCISE BACK TO DRAWING SET