Explain Hexagonal Architecture (Ports and Adapters).

Hexagonal Architecture isolates core business logic from external concerns. Core domain sits at center. Ports define interfaces (incoming: use cases; outgoing: repositories, services). Adapters implement ports for specific technologies (REST adapter, database adapter). Dependencies point inward - domain knows nothing about adapters. Benefits: technology-agnostic core, testable (mock adapters), replaceable infrastructure. Similar: Clean Architecture, Onion Architecture. Structure: application (core), domain, infrastructure (adapters). Prevents framework lock-in.