System Design Fundamentals

Functional requirements state what the system does (features, APIs); non-functional requirements state how well it does them (latency, availability, durability, scale, consistency).

About 11,600 QPS: 10^9 requests per day divided by roughly 86,400 seconds (about 10^5) gives around 10^4 per second; peak is often planned at 2-3x average.

About 1.8 TB: 1 GB per day times about 1,825 days; multiply by the replication factor (often 3) for raw capacity, giving roughly 5.5 TB.

Memory access is about 100 ns; a round trip within one datacenter is about 0.5 ms; a cross-continent round trip is about 100-150 ms. Exact figures vary by hardware, but the orders of magnitude guide design.

About 52.6 minutes per year (0.01% of 525,600 minutes); 99.9% allows about 8.8 hours per year.

Vertical scaling adds resources (CPU, RAM, disk) to one machine; horizontal scaling adds more machines. Vertical hits hardware and cost ceilings and keeps a single point of failure; horizontal needs distribution logic.