Layered architecture has one structural flaw: the business layer depends on the persistence layer, which depends on a specific database technology. This means the most valuable code in your system — your domain logic — is coupled to an infrastructure decision. Onion Architecture (Jeffrey Palermo, 2008) and Hexagonal Architecture (Alistair Cockburn, 2005) independently arrived at the same solution: invert the dependency. Make all infrastructure depend on the domain — never the reverse. This lesson covers both models, explains how they relate, and shows what the dependency inversion looks like in real Java code.
ℹ️Onion and Hexagonal architectures are not competing ideas — they are two ways of describing the same core principle: the domain is the center of the universe, and everything else — databases, HTTP, message queues, UI — is a plug-in that the domain does not know about.
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