URL shortener - one write, a million reads

The tension

Uniqueness is bought with a single global counter: every write in the world waits on one INCR, that counter is the only node here with no replica, and base62 of a counter makes every link on the service enumerable by walking the range. Read scale is bought with the opposite mistake - the CDN caches the 301, so after the first visitor a deleted or re-pointed link keeps redirecting until the TTL expires, and the clicks that never reach the service are never counted.