Architectural and design patterns help software architects to break systems in to smaller, more maintainable sections organised by their functionality and usage. [...] There are three major layers in software architecture: presentation, domain and data source. [...] Now that we know what types of layers we can find in software architecture, let's have a look at how we can organize domain logic and data sources layers.

He looks at three patterns that can help organize your domain logic - a transactional script (one file that does one thing), a table module (logic lives in the tables) and his focus, the domain model. He shows (via a graph) how the domain logic approach can cut through some of the effort it could take to improve on the other two. He gives a two suggestions of how to access the data in your domain layer - a Gateway or a Data Mapper. To reinforce the ideas he's presented, he includes some code snippets of a basic domain model for a Store, Customer and Product and uses them in both Gateway and Data Mapper examples.