Composition Structure Part One

Concept and Basic Notation

An entity model is a system of entity types and relationships and for a reader of a model the most important relationships - those to understand first - are those that represent associations between entities and their parts. These relationships are called composition relationships and the term composition structure is used to refer to the subsystem consisting solely of entity types and compositions relationships. 
In the sections which follow we describe notations for describing the properties of entities and composition relationships including cardinality and optionality (the crows foot and dashing of lines), exclusion (the exclusion arc), recursion (looping structures), abstraction (nested boxes), the absolute (diagram root). Almost all this notation applies equally to more general types of relationship but the approach is to introduce the notation in this more restricted setting. All of the notation is exactly as used in the SSADM method as described, for example, in Richard Barker's book, but with the significant exception of the special treatment of composition relationships, and thereby their foregrounding, and the notation for the absolute.

Composition relationships are shown top-down, which is to say that they are drawn leaving the lower edge of the box representing type of the whole and entering the upper edge of the type representing the part, as here:

Figure 1

  • an entity of the whole type contains one or more of the part type

The fragment in Figure 1 signifies that there are one or more entities of type part type within the whole. Looking at composition relationships the other way around - bottom up - then they are seen to relate entities with the contexts in which they exist and it is because of this that these are the most important relationships in an entity model - they provide context to entities. The presence of the crows foot is representative of multiplicity - if the crows foot is present the notation asserts that there may be many parts of type part type within each entity of type whole type. If the crows foot is absent, as in figure 2 then the assertion is that there is exactly one entity of type part type within the whole.

Figure 2

  • an entity of the whole type contains exactly one of the part type

A further distinction is made by use of a half-dashed line to represent the possibility of zero. The two possibilities are shown in figures 3 and 4.

Figure 3

  • an entity of the whole type contains zero, one or more of the part type

Figure 4

  • an entity of the whole type contains zero or one of the part type

If there are parts of different types, then the structure is shown branching as for example here

or here: 

Figures 5 and 6 show examples of this. More examples follow as more features of the notation are introduced. If you find yourself disagreeing with these examples - thinking that what they express is contrary to your understanding then it is reasonable to suppose that I will have achieved my aim of showing how the notation works - how it can be used to express precise models and that it is then possible to consider these models, to disagree with them or, perhaps, to
refine them.

Figure 5 - the atomic nucleus:

  • a nucleus is composed of one or more protons and zero, one or more neutrons

Figure 6  - from grammar:

  • a complex sentence is composed of exactly one main clause and one or more subordinate clauses

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