Composition (UML)

A Composition is an Association and designates a special form of an Aggregation. It is used in cases where the abstraction of a model requires a strong emphasis on the whole/part-relationship, approximately corresponding to denoting a parent/child-relationship of the elements within the Composition.

A Composition between entities can be illustrated in UML with a filled diamond-shaped adornment starting at the root of the Composition (the composite), then connecting to its part(s) with a solid line.

^{Figure 2 A composite and its parts.}

In a Composition, the owning element (the Composite) is responsible for the lifetime of its parts. In a system, Compositions do not share instances of their parts: Each part always belongs to only one composite at a time. As such, the parts of a composite share the lifetime of the owning composite: If the composite is destroyed, its parts are destroyed, too. However, there is an important detail in this statement (see Lifecycle).

Lifecycle

The following applies to the relationship between a composite A and its parts :

• the owning object A is solely responsible for its parts
• the parts may not belong to another composite, for as long as they belong to A.

This implies that the parts exclusively belong to A, for as long as A exists, and A is responsible for the deposition of its parts. That must not necessarily mean that the parts are destroyed when A is destroyed. Rather, A sets up the constraints for what happens to its parts once it gets destroyed. It is entirely possible that existing parts might become the parts of another composite. However, the deposition must be controlled through A. Rumbaugh et al. state that once the owning element is destroyed, the owning element may either destroy its parts, too, or permit other elements to take ownership:

"If the composite is destroyed, it must either destroy all its parts or else give responsibility for them to other objects." [📖UREF, p. 227]

The "one-composite-at-a-time"-rule

A Composition serves as an abstraction to parent/child-relationships. It is defined through the sum of its parts, i.e. the owning object and its parts with the prerequisite that a Composition - as a stronger form of an Aggregation - maintains and controls references to its parts, and no other object must do this for as long as the owning composite exists. Other elements may reference the parts, but they may not control their lifecycle.

In the Toolbox-entry Association, the Company's and Employee's relationship with Address is modelled as plain Association.

We understand that Company and Employee must not share the same Address-objects, since changing an address $A$ for Company $C$ would also change the data for Employee $E$. The formal definition for a Composition in UML rules the simultaneously existence of more than one owning object out.

How should a model depict the relationship between those elements? An Address is unique, so wouldn't it make sense to share addresses to prevent redundancy? The Association example shows that it would be good practice to use Value Objects: For as long as shared addresses do not change their values, owning objects may reference the same instances which will help with keeping memory footprint low. However, designing the model as a Composition forbids such an implementation:

^{Figure 3 Each Company and Employee denote a Composite with an Address being one of its parts, but implementing the model so that instances are shared violates the definition of a Composition in UML.}

If Address is navigable through Employee and Company, and there is an instance that is shared by both Employee and Company, which of those elements is then considered to be the owning object of Address? Not using arrows as indicators for the navigability doesn't help in this case: If a Composition is used in a design, then a part of it "may be part of only one composite object at a time." [📖UREF, p. 228] and further "during the life of the composite, no other object may have responsibility for it." [📖UREF, p. 227]

Modeling the relationship as an Aggregation provides a looser contract on the relationship between the element, and permits the developers to choose implementation details like applying the Value Object-design to Address.

^{Figure 4 The relationship modelled as Aggregation. Each Company and Employee may share instances of Addresses now.}

Example

The following model hold Compositions and Aggregations:

^{Figure 4 A Department is a Composite wit one to n Employees. An Employee can only belong to one Department.}

• Company is the composite for Employees
• Company is the composite for Departments
• Departmentis an aggregate for Employees
• Employee is the composite for Account

A Company has at least one Employee. One Employee has one Account. An Account knows the Employee it belongs to.

A Department is associated with multiple Employees.

Lifecycle impacts

If an Employee leaves the Company, the link to the Employee's Account is also destroyed: Without an Employee, an Account has no meaning in this context and model.

If a Department closes, it has no impact to the Departments Employees: They remain in the Company.

If the Company closes, the Departments shut down. Closing the Company also removes all its Employees in the given model.

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