UML Composition vs Aggregation vs Association

UML Association Aggregation Composition notation

What makes a UML Composition different from an Aggregation or a regular 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 leads 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 2.5) meta model?

UML 2.5 Associations Meta Model

This is a the part of the UML meta model 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 UML specifications 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).

But a bit further, in the semantics section of Properties we find the same explanation except for a small addendum

  • shared
    […] 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.

memberEnd->exists(aggregation <> AggregationKind::none) implies (memberEnd->size() = 2 and memberEnd->exists(aggregation = AggregationKind::none))

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.

And the second part of the OCL constraint tells us that only one of the two ends can play the whole part, so the other end must play the part part.

Looking further in the specs we find in the semantics section of the Property the following

Composite aggregation is a strong form of aggregation that requires a part object be included in at most one composite object at a time. If a composite object is deleted, all of its part instances that are objects are deleted with it.

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

  • that requires a part object be included in at most one composite
    object at a time
    So a part cannot play the role of part in two compositions at the same time. This implies that the multiplicity of a composite association can only be [0..1] or [1..1] on the composite end.
  • If a composite object is deleted, all of its part instances that are objects are deleted with it.
    This is one of the parts where v 2.5 is different from previous versions. Previous versions of the UML specifications had the phrase “are normally deleted with it”. By leaving the “normally” out there no more ambiguity. Deleting the whole will always result in deleting the part in a composition.

But there still a loophole for the delete story.

NOTE. A part object may (where otherwise allowed) be removed from a composite object before the composite object is deleted, and thus not be deleted as part of the composite object.

So before the whole is deleted we can remove the part to avoid having to delete the part as well.

And then there a last paragraph that deals with Compositions

Compositions may be linked in a directed acyclic graph with transitive deletion characteristics; that is, deleting an object in one part of the graph will also result in the deletion of all objects of the  subgraph below that object. The precise lifecycle semantics of composite aggregation is intentionally not specified. The order and way in which composed objects are created is intentionally not defined. The semantics of composite aggregation when the container or part is typed by a DataType are intentionally not specified.

  • 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.
  • […]are intentionally not specified
    This is simply sad… These few sentences basically tell us again nothing at all, except that we shouldn’t look for a specification of these aspects in the UML specifications.

So 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” should be deleted when the “whole” is deleted (except when we move them to another “whole” first)

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 should not be deleted when a Group is deleted.

To summarize

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.

37 thoughts on “UML Composition vs Aggregation vs Association

  1. UML did not created the concepts of aggregation and composition. UML is just agnostic to its interpretation/hard use – as it is for a lot of other things, since it is just a notation and not a method. Go see the old papers of the 70’s that first defined what were aggregation/composition relationships (The Smiths’s paper “Database abstractions- aggregation and generalization” for a good start). UML specification is not THE BIBLE, the source of truth. There was object orientation before UML, and sometimes I find that UML lost connection with those roots (roots= “An Object Modeling Technique for Conceptual Design” by Rumbaugh). If a book writer gives throws some light over the cumbersome UML specification, it is something good, not bad.

    • Alex,

      I guess you are probably right, and that indeed concepts with the same name existed before UML. But I’m talking about these concepts as defined in UML, not elsewhere.
      In fact the UML specification IS the Bible with regards to UML. Unfortunately there is no standard specification of OO. So the UML specification is the only official specification to go by.
      I’m all for shedding light on UML,but in the end the only authority remains the UML specification as published by the OMG.

  2. I’m refering to your excellent post in my StackOverflow answer, where I try to destroy the widespread false belief that the UML composition concept is defined via the lifecycle dependency of the components on the composite. My fight against this false belief provides a story in the sociology of opinion dynamics because mayn people seem to think that I must be wrong because most others do hold this (lifecycle dependency) belief. I’m glad that there are a few experts like you who arrive at their own conclusion based on their own analysis, and do not parot what others have said.

    • Hi Gerd. I’m glad you like the article.
      One of the reasons I wrote is was because I found different opinions all over the web that didn’t necessarily comply with what I found in the UML specifications.
      The problem is that few people actually read the UML specs (can you really blame them) and thus have to get their info elsewhere. And a lot of UML books actually provide a practical implementation view of UML instead of the theoretical.
      There’s nothing wrong with practical implementation of course. Each implementation of UML in a concrete modelling method should have these, but one should know when this is the case and where you are diverting from the UML standard.

      • Geert:

        I went through the entire chain in which I too participated actively.

        You are right that “a lot of UML books actually provide a practical implementation view of UML”. As you say “There is nothing wrong with practical implementation of course” but I would add “THEY MUST BE CONSISTENT WITH the principles and recommendations of the standard”.

        We can insist on it only if the “standard is well-drafted, verified and validated and worthy of compliance”. Unfortunately UML specification which technically is NOT a standard has fallen short of such respect and adherence. In the first place there is NO glossary and the same term is defined in different parts of the UML spec with emphasis on different aspects without any coherence and consistency. There is no clarity on what is essential (mandatory) and what is optional. The UML spec continues to bloat without consolidating around some “core” or “kernel” (suggested by Ivar Jacobson and Steve Cook in 2009. Ivar Jacobson is pushing his kernel proposal through SEMAT (having given up on UML of OMG, it seems) but I do not know if those efforts are successful.

        What you say “few people actually read the UML specs” is true but the problem is that UML is spec is NOT readable—particularly by busy professionals who seek some clarity and help without having to wade through 800+ pages of bloated mass of text and sophisticated graphics.


    • Your rephrasing just reflects your own private understanding of these terms, but it is not compatible with the official UML definitions (in particular, the UML definition of composition.does not imply any lifecycle dependencies). While you are free to choose using your own private language, this choice is certainly not a good one if you want to communicate with other developers who use/accept UML.

  3. Hi.

    Your post explains the differences quire well, but I am struggling with one of your statements regarding the final diagram. You state:

    “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.”

    I cant see how the diagram makes this clear. Can you please elaborate. I cant see how the constraint ‘they cannot be part of two sections at the same time’ is portrayed.


    • Mark,

      The fact that you cannot share an “part” is what makes the aggregation different from the composition. The definition states: “Composite aggregation is a strong form of aggregation that requires a part object be included in at most one composite object at a time.” So if yo model the relation using a composition this is a given constraint that you do not have make explicit otherwise.

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