The protocols and primitives that make multiple machines behave like one — consensus, sharding, replication, and the theorems that bound what's possible. Every concept comes with an interactive visualization.
How to shard data across servers so adding or removing a node only moves a small fraction of keys.
You can have at most two of consistency, availability, and partition tolerance — and partition isn't optional.
How a cluster of nodes elects a leader and replicates a log so they stay in lockstep — without a coordinator.