How to integrate with multiple external services without letting any of them leak into your business logic: one port, multiple adapters, and two ways to choose which adapter should run.
Getting rid of the arrow anti-pattern and writing business rules as a flat flow: guard clauses, early return, and how to test them with unit tests.
An email queue that accepts work with a single INSERT and lets workers handle it reliably: the data model, the race conditions, why exactly-once does not exist, and how to choose between DB polling and a broker.
Splitting the seat, no-fly, passport, amount and promo checks of an airline booking into independent links of a chain, with Spring Boot.