SQL, NoSQL, indexing, sharding, replication.
View All Tags"ACID" gets thrown around as a synonym for "a real database," but each letter is a distinct, separately-breakable guarantee — and knowing which one a given failure violates is what actually matters in practice.
A single database server has a hard ceiling — one machine's disk, memory, and CPU. Sharding is how you break past it: split the data across many independent databases instead of scaling one bigger and bigger.
"SQL or NoSQL?" sounds like a technology choice. It's actually a question about how much structure and consistency you're willing to trade for flexibility and scale — and the honest answer for most systems is "some of each," not one winner.