01 / 2·Storage & redirectsSystem Design
URL Shortener Design — When One Link Goes Viral
One tracking link gets shared a million times. Design the two APIs — and the cache — so the redirect holds when a single code is on fire.
02 / 2·EmailSystem Design
The Shipping Email Went Out Twice.
order-9912 shipped once. The customer got two emails. The second one was the same fact, delivered again.