Subsystem Relationship Model Notation

This is the second in a series of technical notes outlining the Shlaer-Mellor and Executable UML notation supported by OOA Tool. This technical note covers Subsystem Relationship Models (SRM) which are used to show the subsystems within a domain and identify any spanning relationships between those subsystems. The model itself is automatically derived. However, OOA Tool currently requires someone to layout the model since automatic layout of diagrams where all nodes are connected in many ways is incredibly difficult to perform.

Executable UML [xtUML02] does not name SRMs explicitly. However, diagrams in UML are generally called diagrams, not models. Thus, the name Subsystem Relationship Diagram is used here when referring to an SRM in Executable UML notation. The other Executable UML variant [xUML04] doesn't seem to use SRMs at all.

Below is an example Subsystem Relationship Model in Shlaer-Mellor notation taken from [OOA91] (see page 153):

Subsystems are represented using rectangle shapes containing the subsystem name, optional prefix letters and an optional number range along with prominent object names (prefixed by square bullets). The relative position of the prefix letters and number range was reversed in OOA08 compared to OOA91. OOA08 adopts a format consistent with that used for Objects within Object Information Models. Showing prominent object names in subsystem shapes is also a new feature of OOA08.

All key letters for objects assigned to a subsystem begin with the subsystem's specified prefix letters. Prefix letters are automatically added to key letters in OOA Tool. All objects and relationships assigned to a subsystem should have numbers within the subsystem's specified number range. However, automatic numbering of objects and/or relationships can be disabled in OOA Tool which may lead to objects and relationships having numbers outside the specified number range (see Manual and Automatic Numbering).

All spanning relationships between a particular pair of subsystems are listed on a single rectilinear link. These links only exist if one or more spanning relationships exist. However, since subsystems within a domain are peer-to-peer, there may be many such links in a typical domain.

Below is an example Subsystem Relationship Diagram in Executable UML notation taken from [xtUML02] (see page 271):

Subsystems are represented using folder shapes containing the subsystem name. Prominent objects are not currently shown. Domains and subsystems use the same folder shapes since both are a type of UML package. However, Executable UML (xtUML) uses a smaller width folder for domains compared to subsystems and subsystem shapes show the subsystem name vertically centred (as packages should in UML).

Subsystem dependencies listing spanning relationships are represented using short dashed rectilinear links. They don't have an arrow since they are bidirectional dependencies. The spanning relationships are shown as a UML property string on the link. All UML property strings within OOA Tool use brace spacing in a consistent way, i.e. no space after '{' or before '}'.

Finally, the Executable UML example above is shown below in Executable UML2 notation:

Subsystems are still represented using folder shapes containing the subsystem name. However, the subsystem name is now shown in bold. Furthermore, the subsystem's optional prefix letters and number range is now shown as a right aligned UML property string below the subsystem name. This is consistent with how key letters and numbers are shown on class shapes within Class Diagrams.

Subsystem dependencies are still represented using short dashed rectilinear links. However, a consistent dash length is used across all package diagrams in Executable UML2. See page 43 of [UMLManual05] for an example UML2 package diagram.