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SysML Distilled: Understanding Block Definition Diagrams

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The Block Definition Diagram is the primary kind of diagram you create to communicate structural information about a system. This powerful design technique creates extensibility in your system design by decoupling the clients of services from any specific implementation of a provider of those services. As your stakeholders’ requirements evolve over time, you can modify existing providers or add new ones with minimal impact on the rest of the system design.
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The most common kind of SysML diagram is the block definition diagram. You can display various kinds of model elements and relationships on a BDD to express information about a system’s structure. You can also adopt design techniques for creating extensible system structures, a practice that reduces the time and cost to change your design as your stakeholders’ needs evolve.

3.1. Purpose

The model elements that you display on BDDs—blocks, actors, value types, constraint blocks, flow specifications, and interfaces—serve as types for the other model elements that appear on the other eight kinds of SysML diagrams. We refer to elements that appear on BDDs as elements of definition. Elements of definition, in a real sense, form the foundation for everything else in your system model. That’s why I’m covering BDDs first.

Elements of definition are important; the structural relationships among them—associations, generalizations, and dependencies—are arguably more important. You display these relationships on BDDs, too. With these relationships, you often create BDDs that convey system decomposition and type classification.

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