Tuesday, 22 July 2008

Manual and Automatic Numbering

Shlaer-Mellor OOA uses numbers to help analysts identify various model elements. Some model elements use numbers as primary identifiers, e.g. relationships. While other model elements use numbers as secondary identifiers, e.g. objects, states, events, processes and synchronous services. Note that, OOA08 defines operations as a process and synchronous service supertype, generalizing process and synchronous service numbers as operation numbers.

If an object or relationship has been assigned to a subsystem then its number is restricted to the number range associated with that subsystem. These number ranges are typically assigned at intervals of 100, e.g. 1-99, 101-199, 201-299 etc. This helps analysts identify which subsystem an object or relationship is assigned to simply from looking at its number.

These numbering conventions were introduced in OOA91 at a time when there was minimal CASE tool support. They were designed to help analysts handle a mostly paper driven modelling process. OOA08 assumes a CASE tool is being used to capture and maintain models since OOA Tool has been implemented hand-in-hand with OOA08 as a cross platform reference implementation. The standard edition of this CASE tool is distributed freely to help promote the use of Shlaer-Mellor OOA/RD in a UML dominated industry.

OOA08 supports all of the numbering conventions used in OOA91. However, all of these numbers should by default be automatically allocated by your CASE tool. The CASE tool should also allow an analyst to override the automatic numbering so that previously defined models can still be captured and maintained. The CASE tool should also support and enforce subsystem number ranges.

OOA08 supports manual and automatic numbering by defining a naming attribute called Order of type Ordinal ID, a naming attribute called Manual Number and a mathematically dependent attribute called Number for each of the numbered model elements. The order attribute is automatically updated as a model element is reordered within the context of its parent model element. Ordinal ID attributes will be discussed along with Arbitrary ID attributes in a future post. The manual number attribute is maintained by the analyst (after manual numbering is enabled). The number attribute is derived from either the order attribute or the manual number attribute dependent on whether manual numbering is enabled. Deriving object and relationship numbers is more complicated since objects and relationships may be assigned to a subsystem. If that is the case then automatic numbers are derived from the subsystems lowest number offset by the assigned object or relationship order attribute which is distinct from the object or relationship order attribute. OOA Tool does not allow an analyst to define non-unique manual numbers.

The attributes in OOA08 which control each model element number are given below:

  • object numbers are controlled by the Manual Object Numbering attribute defined on an object's parent information model,
  • relationship numbers are controlled by the Manual Relationship Numbering attribute defined on a relationship's parent information model,
  • state numbers are controlled by the Manual State Numbering attribute defined on a state's parent state model,
  • event numbers are controlled by the Manual Event Numbering attribute defined on an event's parent event destination,
  • and operation numbers are controlled by the Manual Operation Numbering attribute defined on an operation's parent event destination.
The fact that all operations have a parent event destination will be discussed in a future post.

OOA08 supports manual and automatic subsystem number ranges by defining the optional descriptive attributes Manual Lowest Number and Manual Highest Number along with the mathematically dependent attributes Lowest Number and Highest Number. The manual lowest and highest number attributes are maintained by the analyst. The lowest and highest number attributes are derived from the manual numbers dependent on whether the manual number range allows all objects and relationships to be allocated unique numbers within the specified number range. OOA Tool will automatically override the manual number range with a valid range if necessary to ensure all object and relationship numbers are unique.

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