How to Draw Class Diagrams

When it comes to system construction, a class diagram is the most widely used diagram. This diagram generally consists of interfacesclassesassociations and collaborations. Such a diagram would illustrate the object-oriented view of a system, which is static in nature. The object orientation of a system is indicated by a class diagram.

Since class diagrams are used for many different purposes, such as making stakeholders aware of requirements to highlighting your detailed design, you need to apply a different style in each circumstance.

The points that are going to be covered are indicated as follows:

  1. General issues
  2. Classes
  3. Interfaces
  4. Relationships
  5. Inheritance
  6. Aggregation and Composition

UML is popular for its diagrammatic notations. We all know that UML is for visualizing, specifying, constructing and documenting the components of software and non-software systems. Hence, visualization is the most important part which needs to be understood and remembered.

UML notations are the most important elements in modeling. Efficient and appropriate use of notations is very important for making a complete and meaningful model. The model is useless, unless its purpose is depicted properly.

Hence, learning notations should be emphasized from the very beginning. Different notations are available for things and relationships. UML diagrams are made using the notations of things and relationships. Extensibility is another important feature which makes UML more powerful and flexible.

The chapter describes basic UML notations in detail. This is just an extension to the UML building block section discussed in Chapter Two.

Structural Things

Graphical notations used in structural things are most widely used in UML. These are considered as the nouns of UML models. Following are the list of structural things.

  • Classes
  • Object
  • Interface
  • Collaboration
  • Use case
  • Active classes
  • Components
  • Nodes

Class Notation

UML class is represented by the following figure. The diagram is divided into four parts.

  • The top section is used to name the class.
  • The second one is used to show the attributes of the class.
  • The third section is used to describe the operations performed by the class.
  • The fourth section is optional to show any additional components.

Class Notation

Classes are used to represent objects. Objects can be anything having properties and responsibility.

Object Notation

The object is represented in the same way as the class. The only difference is the name which is underlined as shown in the following figure.

Object Notation

As the object is an actual implementation of a class, which is known as the instance of a class. Hence, it has the same usage as the class.

Interface Notation

Interface is represented by a circle as shown in the following figure. It has a name which is generally written below the circle.

Interface Notation

Interface is used to describe the functionality without implementation. Interface is just like a template where you define different functions, not the implementation. When a class implements the interface, it also implements the functionality as per requirement.

Collaboration Notation

Collaboration is represented by a dotted eclipse as shown in the following figure. It has a name written inside the eclipse.

Collaboration Notation

Collaboration represents responsibilities. Generally, responsibilities are in a group.

Use Case Notation

Use case is represented as an eclipse with a name inside it. It may contain additional responsibilities.

Use case Notation

Use case is used to capture high level functionalities of a system.

Actor Notation

An actor can be defined as some internal or external entity that interacts with the system.

Actor Notation

An actor is used in a use case diagram to describe the internal or external entities.

Initial State Notation

Initial state is defined to show the start of a process. This notation is used in almost all diagrams.

Initial state Notation

The usage of Initial State Notation is to show the starting point of a process.

Final State Notation

Final state is used to show the end of a process. This notation is also used in almost all diagrams to describe the end.

Final state Notation

The usage of Final State Notation is to show the termination point of a process.

Active Class Notation

Active class looks similar to a class with a solid border. Active class is generally used to describe the concurrent behavior of a system.

Active class Notation

Active class is used to represent the concurrency in a system.

Component Notation

A component in UML is shown in the following figure with a name inside. Additional elements can be added wherever required.

Component Notation

Component is used to represent any part of a system for which UML diagrams are made.

Node Notation

A node in UML is represented by a square box as shown in the following figure with a name. A node represents the physical component of the system.

Node Notation

Node is used to represent the physical part of a system such as the server, network, etc.

Behavioral Things

Dynamic parts are one of the most important elements in UML. UML has a set of powerful features to represent the dynamic part of software and non-software systems. These features include interactions and state machines.

Interactions can be of two types −

  • Sequential (Represented by sequence diagram)
  • Collaborative (Represented by collaboration diagram)

Interaction Notation

Interaction is basically a message exchange between two UML components. The following diagram represents different notations used in an interaction.

Interaction Notation

Interaction is used to represent the communication among the components of a system.

State Machine Notation

State machine describes the different states of a component in its life cycle. The notations are described in the following diagram.

State machine Notation

State machine is used to describe different states of a system component. The state can be active, idle, or any other depending upon the situation.

Grouping Things

Organizing the UML models is one of the most important aspects of the design. In UML, there is only one element available for grouping and that is package.

Package Notation

Package notation is shown in the following figure and is used to wrap the components of a system.

package Notation

Annotational Things

In any diagram, explanation of different elements and their functionalities are very important. Hence, UML has notes notation to support this requirement.

Note Notation

This notation is shown in the following figure. These notations are used to provide necessary information of a system.

Note Notation


A model is not complete unless the relationships between elements are described properly. The Relationship gives a proper meaning to a UML model. Following are the different types of relationships available in UML.

  • Dependency
  • Association
  • Generalization
  • Extensibility

Dependency Notation

Dependency is an important aspect in UML elements. It describes the dependent elements and the direction of dependency.

Dependency is represented by a dotted arrow as shown in the following figure. The arrow head represents the independent element and the other end represents the dependent element.

Dependency Notation

Dependency is used to represent the dependency between two elements of a system

Association Notation

Association describes how the elements in a UML diagram are associated. In simple words, it describes how many elements are taking part in an interaction.

Association is represented by a dotted line with (without) arrows on both sides. The two ends represent two associated elements as shown in the following figure. The multiplicity is also mentioned at the ends (1, *, etc.) to show how many objects are associated.

Association Notation

Association is used to represent the relationship between two elements of a system.

Generalization Notation

Generalization describes the inheritance relationship of the object-oriented world. It is a parent and child relationship.

Generalization is represented by an arrow with a hollow arrow head as shown in the following figure. One end represents the parent element and the other end represents the child element.

Generalization Notation

Generalization is used to describe parent-child relationship of two elements of a system.

Extensibility Notation

All the languages (programming or modeling) have some mechanism to extend its capabilities such as syntax, semantics, etc. UML also has the following mechanisms to provide extensibility features.

  • Stereotypes (Represents new elements)
  • Tagged values (Represents new attributes)
  • Constraints (Represents the boundaries)

Extensibility Notation

Extensibility notations are used to enhance the power of the language. It is basically additional elements used to represent some extra behavior of the system. These extra behaviors are not covered by the standard available notations.


General Issues

This section describes style guidelines that are relevant to various types of class diagrams.

  1. Show visibility only on design models
  2. Assess responsibilities on domain class diagrams
  3. Highlight language-dependent visibility with property strings
  4. Highlight types only on design models
  5. Highlight types on analysis models only when the type is an actual requirement

  • – Design class diagrams should reflect language naming conventions. In the “Analysis and design version of a class” image you see that the design version of the Order class uses names that conform to common Java programming conventions such as placementDateand calculateTaxes().

  • – Model association classes on analysis diagrams. The image that shows the “Modelling association classes” indicates the association classes that are depicted as class attached via a dashed line to an association – the association line, the class, and the dashed line are considered one symbol in the UML.
  • – Do not name associations that have association classes.
  • – Center the dashed line of an association class.

Class Style

A class is basically a template from which objects are created.  Classes define attributes, information that are relevant to their instances, operations, and functionality that the objects support.  Some of the more important guidelines pertinent to classes are listed down below.

  1. Put common terminology for names
  2. Choose complete singular nouns over class names
  3. Name operations with a strong verb
  4. Name attributes with a domain-based noun
  5. Do not model scaffolding code.  Scaffolding code refers to the attributes and operations required to use basic functionality within your classes, such as the code required to implement relationships with other classes.  The image above depicts the difference between the OrderItem class without scaffolding code
  6. Never show classes with just two compartments
  7. Label uncommon class compartments
  8. Include an ellipsis ( … ) at the end of incomplete lists
  9. List static operations/attributes before instance operations/attributes
  10. List operations/attributes in decreasing visibility
  11. For parameters that are objects, only list their type
  12. Develop consistent method signatures
  13. Avoid stereotypes implied by language naming conventions
  14. Indicate exceptions in an operation’s property string.  Exceptions can be indicated with a UML property string, an example of which is shown in the above image.


An interface can be defined as collection of operation signature and/or attribute definitions that ideally defines a cohesive set of behaviors.  Interfaces are implemented, “realized” in UML parlance, by classes and components. In order to realize an interface, a class or component should use the operations and attributes that are defined by the interface.  Any given class or component may use zero or more interfaces and one or more classes or components can use the same interface.

  1. Interface definitions must reflect implementation language constraints.  In the above image, you see that a standard class box has been used to define the interface PersistentObject (note the use of the <<interface>> stereotype).
  2. Name interfaces according to language naming conventions
  3. Apply “Lollipop” notation to indicate that a class realizes an interface
  4. Define interfaces separately from your classes
  5. Do not model the operations and attributes of an interface in your classes. In the above image, you’ll notice that the shipment class does not include the attributes or operations defined by the two interfaces that it realizes
  6. Consider an interface to be a contract

Aggregation and Composition

As it is known an object is made up of other objects. If you were to consider as examples where an airplane consists of wings, a fuselage, engines, flaps, landing gear and so on. A delivery shipment would contain one or more packages and a team consists of two or more employees. These are all examples of the concept of aggregation that illustrates “is part of” relationships. An engine is part of a plane, a package is part of a shipment, and an employee is part of a team. Aggregation is a specialization of association, highlighting an entire-part relationship that exists between two objects. Composition is a much potent form of aggregation where the whole and parts have coincident lifetimes, and it is very common for the whole to manage the lifecycle of its parts. If you were to consider a stylistic point of view, aggregation and composition are both specializations of association where the guidelines for associations do apply.

  1. You should be interested in both the whole and the part
  2. Depict the whole to the left of the part
  3. Apply composition to aggregates of physical items


Inheritance models “is a” and “is like” relationships, enabling you to rather conveniently reuse data and code that already exist. When “A” inherits from “B” we say that “A” is the subclass of “B” and that “B” is the superclass of “A.” In addition to this, we have “pure inheritance” when “A” inherits all of the attributes and methods of “B”. The UML modeling notation for inheritance is usually depicted as a line that has a closed arrowhead, which points from the subclass right down to the superclass.

  1. Plus in the sentence rule for inheritance
  2. Put subclasses below superclasses
  3. Ensure that you are aware of data-based inheritance
  4. A subclass must inherit everything


At this particular juncture, the term “relationships” will encompass all UML concepts such as aggregation, associations, dependencies, composition, realizations, and inheritance. In other words, if it’s a line on a UML class diagram, it can be considered as a relationship. The following guidelines could be considered as “best practices” and and effort should be made to adhere to them at all times.

Figure A

Figure B

Figure C

  1. Ensure that you model relationships horizontally
  2. Collaboration means a need for a relationship
  3. Model a dependency when a relationship is in transition
  4. Depict similar relationships involving a common class as a tree.  In Figure A you see that both Delivery and Order have a dependency on OIDGenerator. Note how the two dependencies are drawn in combination in “tree configuration”, instead of as two separate lines, to reduce clutter in the diagram.
  5. As a rule it is best to always indicate the multiplicity
  6. Avoid a multiplicity of “*” to avoid confusion
  7. Replace relationships by indicating attribute types.  In Figure C you see that the customer has a shippingAddress attribute of type Address – part of the scaffolding code to maintain the association between customer objects and address objects.
  8. Never model implied relationships
  9. Never model every single dependency
  10. Center names on associations
  11. Write concise association names in active voice
  12. Indicate directionality to clarify an association name
  13. Name unidirectional associations in the same direction
  14. Word association names left-to-right
  15. Indicate role names when multiple associations between two classes exist
  16. Indicate role names on recursive associations
  17. Make associations bi-directional only when collaboration occurs in both directions. The lives at association of Figure B is uni-directional.
  18. Redraw inherited associations only when something changes
  19. Question multiplicities involving minimums and maximums

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UML Class Diagram Tutorial

The UML Class diagram is a graphical notation used to construct and visualize object oriented systems. A class diagram in the Unified Modeling Language (UML) is a type of static structure diagram that describes the structure of a system by showing the system’s:

  • classes,
  • their attributes,
  • operations (or methods),
  • and the relationships among objects.

What is a Class?

A Class is a blueprint for an object. Objects and classes go hand in hand. We can’t talk about one without talking about the other. And the entire point of Object-Oriented Design is not about objects, it’s about classes, because we use classes to create objects. So a class describes what an object will be, but it isn’t the object itself.

In fact, classes describe the type of objects, while objects are usable instances of classes. Each Object was built from the same set of blueprints and therefore contains the same components (properties and methods). The standard meaning is that an object is an instance of a class and object – Objects have states and behaviors.


A dog has states – color, name, breed as well as behaviors -wagging, barking, eating. An object is an instance of a class.

What is a class?


UML Class Notation

A class represent a concept which encapsulates state (attributes) and behavior (operations). Each attribute has a type. Each operation has a signatureThe class name is the only mandatory information.

UML Class Notation


Class Name:

  • The name of the class appears in the first partition.

Class Attributes:

  • Attributes are shown in the second partition.
  • The attribute type is shown after the colon.
  • Attributes map onto member variables (data members) in code.

Class Operations (Methods):

  • Operations are shown in the third partition. They are services the class provides.
  • The return type of a method is shown after the colon at the end of the method signature.
  • The return type of method parameters are shown after the colon following the parameter name. Operations map onto class methods in code

Class Operations


Class Visibility

The +, – and # symbols before an attribute and operation name in a class denote the visibility of the attribute and operation.

Class Visibility

  • + denotes public attributes or operations
  • – denotes private attributes or operations
  • # denotes protected attributes or operations

Parameter Directionality

Each parameter in an operation (method) may be denoted as in, out or inout which specifies its direction with respect to the caller. This directionality is shown before the parameter name.

Parameter Directionality


Perspectives of Class Diagram

The choice of perspective depends on how far along you are in the development process. During the formulation of a domain model, for example, you would seldom move past the conceptual perspectiveAnalysis models will typically feature a mix of conceptual and specification perspectivesDesign model development will typically start with heavy emphasis on the specification perspective, and evolve into the implementation perspective.

A diagram can be interpreted from various perspectives:

  • Conceptual: represents the concepts in the domain
  • Specification: focus is on the interfaces of Abstract Data Type (ADTs) in the software
  • Implementation: describes how classes will implement their interfaces

The perspective affects the amount of detail to be supplied and the kinds of relationships worth presenting. As we mentioned above, the class name is the only mandatory information.

Perspectives of Class Diagram


Relationships between classes

UML is not just about pretty pictures. If used correctly, UML precisely conveys how code should be implemented from diagrams. If precisely interpreted, the implemented code will correctly reflect the intent of the designer. Can you describe what each of the relationships mean relative to your target programming language shown in the Figure below?

If you can’t yet recognize them, no problem this section is meant to help you to understand UML class relationships. A class may be involved in one or more relationships with other classes. A relationship can be one of the following types:

Relationships between classes


Inheritance (or Generalization):

A generalization is a taxonomic relationship between a more general classifier and a more specific classifier. Each instance of the specific classifier is also an indirect instance of the general classifier. Thus, the specific classifier inherits the features of the more general classifier.

  • Represents an “is-a” relationship.
  • An abstract class name is shown in italics.
  • SubClass1 and SubClass2 are specializations of SuperClass.

The figure below shows an example of inheritance hierarchy. SubClass1 and SubClass2 are derived from SuperClass. The relationship is displayed as a solid line with a hollow arrowhead that points from the child element to the parent element.

Inheritance (or Generalization)


Inheritance Example – Shapes

The figure below shows an inheritance example with two styles. Although the connectors are drawn differently, they are semantically equivalent.

Inheritance Example - Shapes



Associations are relationships between classes in a UML Class Diagram. They are represented by a solid line between classes. Associations are typically named using a verb or verb phrase which reflects the real world problem domain.


Simple Association

  • A structural link between two peer classes.
  • There is an association between Class1 and Class2

The figure below shows an example of simple association. There is an association that connects the <> class Class1 and <> class Class2. The relationship is displayed as a solid line connecting the two classes.

Simple Association



Cardinality is expressed in terms of:

  • one to one
  • one to many
  • many to many



A special type of association.

  • It represents a “part of” relationship.
  • Class2 is part of Class1.
  • Many instances (denoted by the *) of Class2 can be associated with Class1.
  • Objects of Class1 and Class2 have separate lifetimes.

The figure below shows an example of aggregation. The relationship is displayed as a solid line with a unfilled diamond at the association end, which is connected to the class that represents the aggregate.




  • A special type of aggregation where parts are destroyed when the whole is destroyed.
  • Objects of Class2 live and die with Class1.
  • Class2 cannot stand by itself.

The figure below shows an example of composition. The relationship is displayed as a solid line with a filled diamond at the association end, which is connected to the class that represents the whole or composite.




An object of one class might use an object of another class in the code of a method. If the object is not stored in any field, then this is modeled as a dependency relationship.

  • A special type of association.
  • Exists between two classes if changes to the definition of one may cause changes to the other (but not the other way around).
  • Class1 depends on Class2

The figure below shows an example of dependency. The relationship is displayed as a dashed line with an open arrow.


The figure below shows another example of dependency. The Person class might have a hasRead method with a Book parameter that returns true if the person has read the book (perhaps by checking some database).




Realization is a relationship between the blueprint class and the object containing its respective implementation level details. This object is said to realize the blueprint class. In other words, you can understand this as the relationship between the interface and the implementing class.

For example, the Owner interface might specify methods for acquiring property and disposing of property. The Person and Corporation classes need to implement these methods, possibly in very different ways.



Class Diagram Example: Order System


Class Diagram Example: Order System


Class Diagram Example: GUI

A class diagram may also have notes attached to classes or relationships.

Class Diagram Example: GUI

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Unified Modeling Language

We have concentrated on the kind of information that should be included in architecture documentation. Architecture in some sense expresses what is essential about a software system, and that essence is independent of languages and notations to capture it. Nevertheless, today the Unified Modeling Language (UML) has emerged as the de facto standard notation for documenting a software architecture. However, it must be said that UML makes its main contribution in a view’s primary presentation, and its secondary contribution in the behavior of an element or group of elements. It is up to the architect to augment the UML pictures with the necessary supporting documentation (the element catalog, the rationale, and so forth) that a responsible job requires. UML provides no direct support for components, connectors, layers, interface semantics, or many other aspects of a system that are supremely architectural.

Still, in most cases we can use the constructs that UML does offer to achieve satisfactory effects, at least in crafting the primary presentations of architectural views. We begin by discussing module views.


Recall that a module is a code or implementation unit and a module view is an enumeration of modules together with their interfaces and their relations.


Figure 9.4 shows how module interfaces can be represented in UML. UML uses a “lollipop” to denote an interface, which can be appended to classes and subsystems, among other things.

UML also allows a class symbol (box) to be stereotyped as an interface; the open-headed dashed arrow shows that an element realizes an interface. The bottom of the class symbol can be annotated with the interface’s signature information: method names, arguments, argument types, and so forth. The lollipop notation is normally used to show dependencies from elements to the interface, while the box notation allows a more detailed description of the interface’s syntax, such as the operations it provides.


UML provides a variety of constructs to represent different kinds of modules. Figure 9.5 shows some examples. UML has a class construct, which is the object-oriented specialization of a module. Packages can be used in cases where grouping of functionality is important, such as to represent layers and classes. The subsystem construct can be used if a specification of interface and behavior is required.

Figure 9.5. Examples of module notations in UML


Figure 9.6 shows how the relations native to module views are denoted using UML. Module decomposition relies on the “is-part-of” relation. The module uses view relies on the dependency relation, and the module class view relies on the generalization, or “is-a” relation (also called “inheritance”).

Figure 9.6. Examples of relation notations in UML. Module B is part of module A, module D depends on module C, and module F is a type of module E.



In UML, the subsystem construct can be used to represent modules that contain other modules; the class box is normally used for the leaves of the decomposition. Subsystems are used both as packages and as classifiers. As packages, they can be decomposed and hence are suitable for module aggregation. As classifiers, they encapsulate their contents and can provide an explicit interface. Aggregation is depicted in one of three ways in UML:

  • Modules may be nested (see Figure 9.7 left).

    Figure 9.7. Decomposition in UML with nesting. The aggregate module is shown as a package (left); decomposition in UML with arcs (right).


  • A succession of two diagrams (possibly linked) can be shown, where the second is a depiction of the contents of a module shown in the first.

  • An arc denoting composition is drawn between the parent and the children (see Figure 9.7 right).

In UML, composition is a form of aggregation with implied strong ownership-that is, parts live and die with the whole. If module A is composed of modules B and C, then B or C cannot exist without A, and if A is destroyed at runtime, so are B and C. Thus, UML’s composition relation has implications beyond the structuring of the implementation units; the relation also endows the elements with a runtime property. As an architect, you should make sure you are comfortable with this property before using UML’s composition relation.


Expressing generalization is at the heart of UML in which modules are shown as classes (although they may also be shown as subsystems). Figure 9.8 shows the basic notation available in UML.

Figure 9.8. Documenting generalization in UML with two line styles


The two diagrams in Figure 9.8 are semantically identical. UML allows an ellipsis (…) in place of a submodule, indicating that a module can have more children than shown and that additional ones are likely. Module Shape is the parent of modules Polygon, Circle, and Spline, each of which is a subclass, child, or descendant of Shape. Shape is more general, while its children are specialized versions.


The basic notation for dependency was shown in Figure 9.6. The most architecturally significant manifestation of dependency is found in layers. Sadly, UML has no built-in primitive corresponding to a layer. However, it can represent simple layers using packages, as shown in Figure 9.9. These are general-purpose mechanisms for organizing elements into groups. UML has predefined packages for systems and subsystems. We can introduce an additional package for layers by defining it as a package stereotype. A layer can be shown as a UML package with the constraints that it groups modules together and that the dependency between packages is “allowed to use.” We can designate a layer using the package notation with the stereotype name <<layer>> preceding the layer name, or introduce a new visual form, such as a shaded rectangle.

Figure 9.9. A simple representation of layers in UML



There is no single preferred strategy to document component-and-connector (C&C) views in UML, but a number of alternatives. Each alternative has its advantages and disadvantages. One natural candidate for representing component-and-connector types begins with the UML class concept.

Figure 9.10 illustrates the general idea using a simple pipe-and-filter system. Here, the filter architectural type is represented as the UML class Filter. Instances of filters, such as Splitter, are represented as corresponding objects in an object instance diagram. To provide a namespace boundary, we enclose the descriptions in packages. The representation of MergeAndSort, denoted Details, would be shown as another package elsewhere.

Figure 9.10. Types as classes, and instances as objects, exemplified with a simple pipe and filter


We now take a closer look at this strategy.


The type/instance relationship in architectural descriptions is a close match to the class/object relationship in a UML model. UML classes, like component types in architectural descriptions, are first-class entities and are rich structures for capturing software abstractions. The full set of UML descriptive mechanisms is available to describe the structure, properties, and behavior of a class, making this a good choice for depicting detail and using UML-based analysis tools. Properties of architectural components can be represented as class attributes or with associations; behavior can be described using UML behavioral models; and generalization can be used to relate a set of component types. The semantics of an instance or type can also be elaborated by attaching one of the standard stereotypes; for example, the «process» stereotype can be attached to a component to indicate that it runs as a separate process. Note that the relationship between MergeAndSort and its substructure is indicated using a dependency relation.


Interfaces to components, sometimes called ports, can be shown in five ways, as shown in Figure 9.11, described in increasing order of expressiveness. However, as expressiveness rises so does complexity, so you should pick the first strategy that will serve your purposes.

  • Option 1: No explicit representation. Leaving out interfaces leads to the simplest diagrams but suffers from the obvious problem that there is no way to characterize the names or the properties of the interfaces in the primary presentation. Still, this choice might be reasonable if the components have only one interface, if the interfaces can be inferred from the system topology, or if the diagram is refined elsewhere.

  • Option 2: Interfaces as annotations. Representing interfaces as annotations provides a home for information about them, although annotations have no semantic value in UML so cannot be used as a basis for analysis. Again, if the detailed properties of an interface are not of concern, this approach might be reasonable.

  • Option 3: Interfaces as class/object attributes. Treating interfaces as attributes of a class/object makes them part of the formal structural model, but they can have only a simple representation in a class diagram-essentially, a name and a type. This restriction limits the expressiveness of this option.

  • Option 4: Interfaces as UML interfaces. The UML lollipop notation provides a compact description of an interface in a class diagram depicting a component type. In an instance diagram, a UML association role, corresponding to an interface instance and qualified by the interface type name, provides a compact way to show that a component instance is interacting through a particular interface instance. This approach provides visually distinct depictions of components and interfaces, in which interfaces can clearly be seen as subservient.

    However, this strategy provides no means to depict the services required from a component’s environment, often a key part of an interface. Furthermore, it is meaningful for a component type to have several instances of the same interface type, but it is not meaningful to say that a class realizes several versions of one UML interface. For example, there is no easy way to define a Splitter filter type that has two output ports of the same “type” using this technique. Finally, unlike classes, UML interfaces do not have attributes or substructure.

  • Option 5: Interfaces as classes. Describing interfaces as classes contained by a component type overcomes the lack of expressiveness of the previous alternatives: We can now represent interface substructure and indicate that a component type has several interfaces of the same type. A component instance is modeled as an object containing a set of interface objects. However, by representing interfaces as classes, we not only clutter the diagram but also lose clear visual discrimination between interfaces and components. We could use a notational variation in which the interfaces are contained classes, as shown in the lower part of option 5 in Figure 9.11. Indicating points of interaction is counterintuitive, however, as containment usually indicates that a class owns other classes whose instances may or may not be accessible through instances of the parent class.

Figure 9.11. Five ways to represent interfaces to components (ports)



There are three reasonable options for representing connectors. Again, the choice is between expressiveness and semantic match on the one hand and complexity on the other.

  • Option 1: Connector types as associations and connector instances as links. In an architectural box-and-line diagram of a system, the lines between components are connectors. One tempting way to represent connectors in UML is as associations between classes or links between objects. This approach is visually simple, provides a clear distinction between components and connectors, and uses the most familiar relationship in UML class diagrams: association. Moreover, associations can be labeled, and a direction associated with the connector can be indicated with an arrow. Unfortunately, connectors and associations have different meanings. A system in an architectural description is built up by choosing components with behavior exposed through their interfaces and connecting them with connectors that coordinate their behaviors. A system’s behavior is defined as the collective behavior of a set of components whose interaction is defined and limited by the connections between them.

    In contrast, although an association, or link, in UML represents a potential for interaction between the elements it relates, the association mechanism is primarily a way of describing a conceptual relationship between two elements. In addition, an association is a relationship between UML elements, so it cannot stand on its own in a UML model. Consequently, a connector type cannot be represented in isolation. Instead, you must resort to naming conventions or to stereotypes whose meanings are captured by description in UML’s object constraint language. Further, the approach does not allow you to specify a connector’s interfaces.

  • Option 2: Connector types as association classes. One solution to the lack of expressiveness is to qualify the association with a class that represents the connector type. In this way, the connector type or connector attributes can be captured as attributes of a class or object. Unfortunately, this technique still does not provide any way of explicitly representing connector interfaces.

  • Option 3: Connector types as classes and connector instances as objects. One way to give connectors first-class status in UML is to represent connector types as classes and connector instances as objects. Using classes and objects, we have the same four options for representing roles as we had for interfaces: not at all, as annotations, as interfaces realized by a class, or as child classes contained by a connector class. Given a scheme for representing interfaces, an attachment between a component’s interface and a connector’s interface may be represented as an association or a dependency.


In addition to representing individual components and connectors and their types, we also need to encapsulate graphs of components and connectors: systems. Three options are available.

  • Option 1: Systems as UML subsystems. The primary UML mechanism for grouping related elements is the package. In fact, UML defines a standard package stereotype, called «subsystem», to group UML models that represent a logical part of a system. The choice of subsystems is appropriate for any mapping of components and connectors, and it works particularly well for grouping classes. One of the problems with using subsystems, as defined in UML 1.4, is that, although they are both a classifier and a package, the meaning is not entirely clear. Some have argued that we should be able to treat a subsystem as an atomic class-like entity at certain stages in the development process and later be able to refine it in terms of a more detailed substructure. Having the ability to do this would make the subsystem construct more appropriate for modeling architectural components.

  • Option 2: Systems as contained objects. Object containment can be used to represent systems. Components are represented as instances of contained classes, and connectors are modeled using one of the options outlined earlier. Objects provide a strong encapsulation boundary and carry with them the notion that each instance of the class has the associated “substructure.” However, this approach has problems, the most serious being that associations, used to model connectors, between contained classes are not scoped by the class. That is, it is not possible to say that a pair of classes interacts via a particular connector, modeled as an association, only in the context of a particular system. So, for example, indicating that two contained classes interact via an association is valid for instances of classes used anywhere else in the model.

  • Option 3: Systems as collaborations. A set of communicating objects connected by links is described in UML using a collaboration. If we represent components as objects, we can use collaborations to represent systems. A collaboration defines a set of participants and relationships that are meaningful for a given purpose, which in this case is to describe the runtime structure of the system. The participants define classifier roles that objects play, or conform to, when interacting. Similarly, the relationships define association roles that links must conform to.

    Collaboration diagrams can be used to present collaborations at either the specification or the instance level. A specification-level collaboration diagram shows the roles, defined within the collaboration, arranged in a pattern to describe the system substructure. An instance-level collaboration diagram shows the objects and links conforming to the roles at the specification level and interacting to achieve the purpose. Therefore, a collaboration presented at the instance level is best used to represent the runtime structure of the system.

    Figure 9.12 illustrates this approach. The Filter architectural type is represented as previously. Instances of filters and pipes are represented as corresponding classifier roles-for example, /Splitter indicates the Splitter role-and association roles. The objects and links conforming to those roles are shown in the collaboration diagram at the instance level, indicated by underscored names.

    Figure 9.12. Systems as collaborations


    Although this is a natural way to describe runtime structures, it leaves no way to explicitly represent system-level properties. There is also a semantic mismatch; a collaboration describes a representative interaction between objects and provides a partial description, whereas an architectural configuration is meant to capture a complete description.


In UML, a deployment diagram is a graph of nodes connected by communication associations. Figure 9.13 provides an example. Nodes may contain component instances, which indicates that the component lives or runs on the node. Components may contain objects, which indicates that the object is part of the component. Components are connected to other components by dashed-arrow dependencies (possibly through interfaces). This indicates that one component uses the services of another; a stereotype may be used to indicate the precise dependency if needed. The deployment type diagram may also be used to show which components may run on which nodes, by using dashed arrows with the stereotype «supports».

Figure 9.13. A deployment view in UML


A node is a runtime physical object that represents a processing resource, generally having at least a memory and often processing capability as well. Nodes include computing devices but also human or mechanical processing resources. Nodes may represent types of instances. Runtime computational instances, both objects and components, may reside on node instances.

Nodes may be connected by associations to other nodes. An association indicates a communication path between them. The association may have a stereotype to indicate the nature of the communication path (for example, the kind of channel or network).

The nesting of symbols within the node symbol signifies a composition association between a node class and constituent classes or a composition link between a node object and constituent objects.


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