You tap like. How does your friend's phone know a second later?
From your tap to your friend's buzz: events, a queue, a worker, live connections, Apple and Google push, and grouping 1,000 likes into one alert.
System Design
How big apps really work inside.
How do apps handle a million likes at once without crashing?
When millions tap like at once, one database row becomes a traffic jam. How big apps survive: a queue, split counters and a cache.