UML Composition vs Aggregation vs Association

The concepts of Association, Aggregation and Composition exist in UML since the first published versions, but the exact  meaning of these concepts, especially the Aggregation still lead to heated debates among UML experts.

But before we go into the details, let’s have a look at how these concepts are defined in UML.

I guess every UML user is familiar with the graphical notation

But how do these concepts look like in the UML (v 2.3) meta model?

This is a the part of the UML metamodel that defines Association. (I’ve hidden the elements not relevant to the subject for clarity)

What we see is that an Association has at least two Properties in the role of memberEnd. A property has an attribute aggregation of type AggregationKind. It’s this AggregationKind that specifies the difference between a regular Assocation, an Aggregation and a Composition.

The three possible values for AggregationKind are defined in the superstructure as follows:

  • none
    Indicates that the property has no aggregation.
  • shared
    Indicates that the property has a shared aggregation.
  • composite
    Indicates that the property is aggregated compositely, i.e., the composite object has responsibility for the existence and storage of the composed objects (parts).

And then below that we find one little sentence that is the main cause for all these discussions:

Precise semantics of shared aggregation varies by application area and modeler.

So basically the OMG is saying: We don’t know what it means, make up your own definition.

Looking for more clues in the definition of Association we find the constraint:

Only binary associations can be aggregations.
self.memberEnd->exists(aggregation <> Aggregation::none) implies self.memberEnd->size() = 2

OK, that doesn’t really help us. All it states is that the Aggregations and Compositions can only exist in Associations that have maximum two members, but that’s like the “normal” Association for most of us. I haven’t seen many Associations with more then two members yet.

Looking further we find in the the semantics section of the Association:

An association may represent a composite aggregation (i.e., a whole/part relationship). Only binary associations can be aggregations. Composite aggregation is a strong form of aggregation that requires a part instance be included in at most one composite at a time. If a composite is deleted, all of its parts are normally deleted with it. Note that a part can (where allowed) be removed from a composite before the composite is deleted, and thus not be deleted as part of the composite.
Compositions may be linked in a directed acyclic graph with transitive deletion characteristics; that is, deleting an element in one part of the graph will also result in the deletion of all elements of the subgraph below that element.
Composition is represented by the isComposite attribute on the part end of the association being set to  true.

So that paragraph already tells us a little bit more about the nature of Aggregation and Composition. Let’s dissect this paragraph and figure out what to remember

  • Aggregations and Compositions are whole/part relations:
    So any member of an Association that has aggregationKind <> none should be considered the whole, where the other end is the part
  • If the whole has aggregationKind = composite then the part can be included in at most one composite at a time:
    So a part cannot play the role of part in two compositions at the same time. This implies that the cardinality of a composite association can only be [0..1] or [1..1] on the composite end.
  • If a composite is deleted, all of its parts are normally deleted with it:
    This part seems clear, although the word normally indicates that this isn’t necessarily the case.
  • Compositions may be linked in a directed acyclic graph with transitive deletion characteristics:
    Now this is a difficult one. The directed acyclic graph part tells us that, when following the links from whole to part, we will not visit the same element twice. Combined with the “at most one composite at a time” constraint this even means that composite relations form a hierarchical tree. The transitive deletion part means that deleting one element from the tree would then delete the whole branch under this element. Unfortunately the word may in the sentence means that this again is no hard constraint, but merely an indication of how it can be used.

And that’s about all  the UML specification has to say about the different types of aggregation. All other constraints you find in books and on the internet are purely interpretations and added semantics of the authors.

Now let’s have a look at some typical examples of Composition and Aggregation (or aggregationKind = shared and aggregationKind = composite as we learned from the specifications)

This example shows the class structure of the popular social networking site LinkedIn

On the right we see the Group structure as a set of Compositions because
a) each “whole” can be considered as a grouping of “parts”
b) each “part” can belong to only one “whole” at a time
c) the “parts” would normally be deleted when the “whole” is deleted

Note that discussions can be moved from one section to another (Usually from Discussions to Jobs or Promotions), but they cannot be part of two sections at the same time.

The relation between Group and User however is an Aggregation because
a) a Group can be considered a “grouping” of users
b) a User can be part of multiple Groups at the same time
c) a User would normally not be deleted when a Group is deleted.

In short, the Composition is a type of Association with real constraints and impact on development, whereas the Aggregation is purely a functional indication of the nature of the Association with no technical impact.

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