Artifact: Signal

Signal	A signal is an asynchronous communication entity which may cause a state transition in the state machine of an object that receives it.
UML representation:	Class, stereotyped «signal».
Role:	Software Architect
Optionality:	Signals are a suitable model for communication in distributed, concurrent systems.
More information:	Checkpoints:Signal Concepts: Events and Signals Concepts: Concurrency
Purpose Properties Timing Responsibility Tailoring

Input to Activities:

Capsule Design

Class Design

Output from Activities:

Identify Design Elements

Purpose

The purpose of a signal is to provide one-way asynchronous communication from one object to another.

Properties

Property Name	Brief Description	UML Representation
Name	The name of the signal.	attribute
Brief Description	A brief description of the role and purpose of the signal.	Tagged value, of type "short text".
Responsibilities	The responsibilities defined by the class.	tagged value
Relationships	The generalizations in which the signal participates.	generalization relationship
Operations	The operations defined by the signal.	operation
Attributes	The attributes defined by the signal.	attributes

Timing

Some signals, specifically those representing the external events and the significant internal events to which the system must respond, are identified early in the elaboration phase. Other signals needed to communicate asynchronously within the system are identified in the latter part of the elaboration phase. All signals and events are architecturally significant and should be completely identified by the end of the elaboration phase.

Responsibility

The software architect is responsible for all signals, ensuring that signals are being used appropriately.

Tailoring

Signals are used to define entities for asynchronous messaging. They are suitable for communication in distributed and concurrent systems. Avoid naming signals in a 'receiver-specific' manner - this limits their generality and usefulness, bearing in mind that signals may be broadcasted to a set of objects. The important thing is to communicate the interesting occurrence the sender has detected (which prompted it to send the signal). A signal may also have a list of parameters which are represented as its attributes. Operations may be provided on a signal to access these attributes.