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Software Architecture Guide

Provided and Required Interface

Explanation

Section titled “Explanation”

A component is a building block of a system. It usually works with other building blocks. Two ideas describe how it connects to them: what it offers, and what it needs.

Provided interface. This is what a component offers to others. In a UML component diagram (see UML Component Diagram), it appears as a ball, also called a lollipop. The ball carries the name of the interface. A solid line links it to the component that provides it.

Required interface. This is what a component needs from others. It is a usage dependency. UML draws it as a half-circle, called a socket, attached to the component that has the need.

Who defines it. A common view is that the supplier decides what to expose. A required interface turns this around. The client of an interaction defines it.

Why show them. arc42 recommends ball and socket notation. It makes explicit which services, data, or events a providing building block offers.

Ports. A port is a point on a component where interfaces attach (see Port). arc42 suggests using ball and socket together with port symbols. In a modeling tool, the port shows its provided and required interfaces in ball-socket notation. A realization dependency (provided) or a usage dependency (required) is created from the port type. A port can provide or require many interfaces. It therefore groups them, and you can choose which ones to display.

Example

Section titled “Example”

An online shop has an Order component and a Payment component. Payment offers a “Charge” interface, drawn as a ball. Order needs a way to charge customers, so it shows a socket named “Charge”. Reading the diagram, you see that Order depends on Payment through “Charge”. Order’s needs define what “Charge” must contain. Payment provides it.

Trade-offs

Section titled “Trade-offs”

Ports group many interfaces, so a diagram can show only some of them. This keeps pictures readable. It also means a reader may not see every interface unless the hidden ones are documented elsewhere.

Payment provides the Charge interface (realization). Order requires it (usage dependency).

Diagram source 
@startuml
component Order
component Payment
interface Charge
Payment -( Charge
Payment ..|> Charge
Order ..> Charge : use
@enduml

Checks

Section titled “Checks”
  • Provided interfaces are drawn as balls that name the services, data, or events a building block offers.
  • Required interfaces are drawn as sockets on the component that depends on them.
  • Ball and socket notation is combined with port symbols where a component has several interfaces.
Section titled “Related”

Sources

Section titled “Sources”