The representation of a physical part from the system specification.

The Unified Modeling Language (UML) specifies a component as a set of constructs that can be used to define software systems of arbitrary size and complexity. In particular, it specifies a Component as a modular unit with well-defined Interfaces that is replaceable within its environment. The Component concept addresses the area of component-based development and component-based system structuring, where a Component is modeled throughout the development life cycle and successively refined into deployment and run-time.

An important aspect of component-based development is the reuse of previously constructed Components. A Component can always be considered an autonomous unit within a system or subsystem. It has one or more provided and/or required Interfaces (potentially exposed via Ports), and its internals are hidden and inaccessible other than as provided by its Interfaces. Although it may be dependent on other elements in terms of Interfaces that are required, a Component is encapsulated and its Dependencies are designed such that it can be treated as independently as possible. As a result, Components and subsystems can be flexibly reused and replaced by connecting (‘wiring’) them together.

The aspects of autonomy and reuse also extend to Components at deployment time. The artifacts that implement Component are intended to be capable of being deployed and re-deployed independently, for instance to update an existing system.

The Components package supports the specification of both logical Components (e.g., business components, process components) and physical Components (e.g., EJB components, CORBA components, COM+ and .NET components, WSDL components, etc.), along with the artifacts that implement them and the nodes on which they are deployed and executed. It is anticipated that profiles based around Components will be developed for specific component technologies and associated hardware and software environments.

A Component represents a modular part of a system that encapsulates its contents and whose manifestation is replaceable within its environment.

A Component is a self-contained unit that encapsulates the state and behavior of a number of Classifiers. A Component specifies a formal contract of the services that it provides to its clients and those that it requires from other Components or services in the system in terms of its provided and required Interfaces.

A Component is a substitutable unit that can be replaced at design time or run-time by a Component that offers equivalent functionality based on compatibility of its Interfaces. As long as the environment is fully compatible with the provided and required Interfaces of a Component, it will be able to interact with this environment. Similarly, a system can be extended by adding new Component types that add new functionality. Larger pieces of a system’s functionality may be assembled by reusing Components as parts in an encompassing Component or assembly of Components, and wiring them together.

A Component is modeled throughout the development life cycle and successively refined into deployment and run-time.

A Component may be manifested by one or more Artifacts, and in turn, that Artifact may be deployed to its execution environment. A DeploymentSpecification may define values that parameterize the Component’s execution.

The required and provided Interfaces of a Component allow for the specification of StructuralFeatures such as attributes and Association ends, as well as BehavioralFeatures such as Operations and Receptions. A Component may implement a provided Interface directly, or its realizing Classifiers may do so, or they may be inherited. The required and provided Interfaces may optionally be organized through Ports; these enable the definition of named sets of provided and required Interfaces that are typically (but not always) addressed at run-time.

A Component has an external view (or ‘black-box’ view) by means of its publicly visible Properties and Operations.

Optionally, a Behavior such as a ProtocolStateMachine may be attached to an Interface, Port, and to the Component itself, to define the external view more precisely by making dynamic constraints in the sequence of Operation calls explicit.

The wiring between Components in a system or other context can be structurally defined by using Dependencies between compatible simple Ports, or between Usages and matching InterfaceRealizations that are represented by sockets and lollipops on Components on Component diagrams. Creating a wiring Dependency between a Usage and a matching InterfaceRealization, or between compatible simple Ports, means that there may be some additional information, such as performance requirements, transport bindings, or other policies that determine that the Interface is realized in a way that is suitable for consumption by the depending Component. Such additional information could be captured in a profile by means of stereotypes.

A Component also has an internal view (or “white-box” view) by means of its private Properties and realizing Classifiers. This view shows how the external Behavior is realized internally. Dependencies on the external view provide a convenient overview of what may happen in the internal view; they do not prescribe what must happen. More detailed behavior specifications such as Interactions and Activities may be used to detail the mapping from external to internal behavior.

The execution time semantics for an assembly Connector in a Component are that requests (signals and operation invocations) travel along an instance of a Connector. The execution semantics for multiple Connectors directed to and from different roles, or n-ary Connectors where n> 2, indicates that the instance that will originate or handle the request will be determined at execution time.

A number of UML standard stereotypes exist that apply to Component. For example, Subsystem to model large-scale Components, and Specification and Realization to model Components with distinct specification and realization definitions, where one specification may have multiple realizations (see the Standard Profiles).

A Component may be realized (or implemented) by a number of Classifiers. In that case, a Component owns a set of ComponentRealizations to these Classifiers.

A component acts like a Package for all model elements that are involved in or related to its definition, which should be either owned or imported explicitly. Typically the Classifiers that realize a Component are owned by it.

The isDirectlyInstantiated property specifies the kind of instantiation that applies to a Component. If false, the Component is instantiated as an addressable object. If true, the Component is defined at design-time, but at run-time (or executiontime) an object specified by the Component does not exist, that is, the Component is instantiated indirectly, through the instantiation of its realizing Classifiers or parts.

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