Distributed Systems and Data
The rules that change when a system spans machines: network fallacies, CAP, and consistency models. You design service communication, then replication, sharding, sagas, and CQRS for data that no longer fits one database.
This domain teaches the physics of distributed systems: what the network refuses to promise, what CAP actually says, and which consistency model your use case really needs. On that base you design service communication, then the data layer: replication, sharding, sagas for transactions that span services, and CQRS with event sourcing where reads and writes part ways.