Business logic layer

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A Business logic layer (BLL) is a software engineering practice of compartmentalizing the rules and calculations of a software application from its other design elements.

This Business logic layer is usually one of the tiers in a multitier architecture. It separates the business logic from other modules, such as the Data access layer and User interface.

By doing this, the business logic of an application can often withstand modifications or replacements of other tiers. For example, in an application with a properly separated Business logic layer and Data access layer, the Data access layer could be rewritten to retrieve data from a different database, without affecting any of the business logic. This practice allows software application development to be more effectively split into teams, with each team working on a different tier simultaneously.

Within a BLL objects can further be partitioned into Business Processes (aka Business Activities) and Business Entities. Business Process objects typically implement the Controller Pattern, ie: they contain no data elements but have methods that orchestrate interaction among Business Entities. Business Entities typically correspond to entities in the logical domain model (rather than the physical database model). Domain Entities are a super-set of Data Layer Entities or Data Transfer Objects, and may aggregate zero or more DLEs/DTOs.