Hexagonal Architecture

The Core Decision Rule

To decide if something belongs inside or outside the hexagon, ask:

"Does it do I/O or run out-of-process?"

• No → Inside the hexagon (domain or application layer)
• Yes → Outside (adapter)

Critical: Consider ALL dependencies. A component's dependencies disqualify it even if the component itself doesn't do I/O. If it depends on Spring, a database driver, or any framework—it's outside.

Layer Responsibilities

Common misconception: The hexagon is NOT just the domain. The hexagon contains both domain AND application layers. Adapters sit outside.