Onion model


The onion model is a graph-based diagram and conceptual model for describing relationships among levels of a hierarchy, evoking a metaphor of the layered "shells" exposed when an onion is bisected by a plane that intersects the center or the innermost shell. The outer layers in the model typically add size and/or complexity, incrementally, around the inner layers they enclose.
An onion diagram can be represented as an Euler or Venn diagram composed of a hierarchy of sets, A1...Ak where each set An+1 is a strict subset of An.
Such formats supported by Microsoft PowerPoint's SmartArt wizard invoke the term "stacked Venn".

In computing

The onion model in computing is used as a metaphor for the complex structure of information systems. The system is split into layers to make it easier to understand. A simple example is to start with the program, operating system and hardware layers. Each of these layers can then be subdivided.