Communication Diagrams PDF

Communication Diagrams Massimo Felici  1 Realizing Use cases in the Design Model  Slide 1: Realizing Use cases in the Design Model Use-case driven design is a key theme in a variety of software processes based on the UML UML supports specific...

Communication Diagrams Massimo Felici  1 Realizing Use cases in the Design Model  Slide 1: Realizing Use cases in the Design Model Use-case driven design is a key theme in a variety of software processes based on the UML UML supports specific modelling constructs that realize use cases in the implementation Collaborations (Communications) enhance the systematic and aggregate behavioural aspects of the system Collaborations support traceability from requirements expressed in use cases into the design Rationale 2 Communication Diagrams Model collaborations between objects or roles that deliver the functionalities of use cases and operations Model mechanisms within the architectural design of the system Capture interactions that show the passed messages between objects and roles within the collaboration Model alternative scenarios within use cases or operations that involve the collaboration of different objects and interactions Support the identification of objects (hence classes) that participate in use cases  3 Communication Diagrams The communication is implicit in a Sequence Diagram, rather than explicitly represented as in a Communication Diagram There is some redundancy between Communication and Sequence Diagrams – They differently show how elements interact over time – They document in detail how classes realize user cases – Communication Diagrams show relationship between objects – Sequence Diagrams focus on the time in which events occur  Slide 3: Communication Diagrams Communication Diagrams, formerly called Collaboration Diagrams. UML Interaction Diagrams refine the kind of activity undertaken in checking with CRC cards. Example 4 Sequence and Communication Diagrams What is a Collaboration? 5 Communication Diagrams A Collaboration is a collection of named objects and actors with links connecting them A Collaboration defines a set of participants and relationships that are meaningful for a given set of purposes A Collaboration between objects working together provides emergent desirable functionalities in Object-Oriented systems Objects collaborate by communicating (passing messages) with one another in order to work together  Slide 5: Communication Diagrams Objects and actors collaborate in performing some task. Each object (responsibility) partially supports emergent functionalities. Objects are able to produce (usable) high-level functionalities by working together.  6 Collaborations Actors Each Actor is named and has a role One actor will be the initiator of the use case Objects Each object in the collaboration is named and has its class specified Not all classes need to appear There may be more than one object of a class Links Links connect objects and actors and are instances of associations Each link corresponds to an association in the class diagram  7 Interactions Use Cases and Class Diagrams constrain interactions Associations and Links in a Collaboration Diagram show the paths along which messages can be sent from one instance to another A message is the specification of a stimulus A stimulus represents a specific instance of sending the message, with particular arguments Specification and Instance Levels 8 Communication Diagrams Specification level shows generic cases of collaborations (communications) Generic form captures a collaboration among class roles and association roles and their interactions Instance level shows a specific instance of an interaction taking place and involving specific object instances Instance form captures a scenario among objects conforming to class roles and links conforming to association roles Simple Example 10 Communication Diagrams  11 Messages A message on a communication diagram is shown using an arrow from the message sender to the message receiver Message Signature: return-value, message-name, argument-list Each message in a collaboration diagram has a sequence number. The top-level message is numbered 1. Messages sent during the same call have the same decimal prefix but suffixes of 1, 2, etc. according to when they occur.  12 Messages Synchronous Asynchronous Creation Reply Synchronous Messages Synchronous messaging involves a client that waits for the server to respond to a message. Messages are able to flow in both directions, to and from. Essentially it means that synchronous messaging is a two way communication. i.e. Sender sends a message to receiver and receiver receives this message and gives reply to the sender. Sender will not send another message until get reply from receiver. Asynchronous Messages Asynchronous messaging involves a client that does not wait for a message from the server. An event is used to trigger a message from a server. So even if the client is down , the messaging will complete successfully. Asynchronous Messaging means that, it is a one way communication and the flow of communication is one way only.  13 Messages Procedural or Synchronous: A message is sent by one object to another and the first object waits until the resulting action has completed. Asynchronous: A message is sent by one object to another, but the first object does not wait until the resulting action has completed. Flat: Each arrow shows a progression from one step to the next in a sequence. Normally the message is asynchronous. Return: the explicit return of control from the object to which the message was sent.  14 Messages Messages occurring at the same time: Adding a number-and-letter notation to indicate that a message happens at the same time as another message Invoking a message multiple times: Looping constraint, e.g., *[i=0..9] Sending a message based on a condition: A guardian condition is made up of a logical boolean statement, e.g., [condition=true] When a participant sends a message to itself Where should messages go? 15 Messages The message is directed from sender to receiver The receiver must understand the message The association must be navigable in that direction Law of Demeter Dealing with a message m an Object O can send messages to: – Itself – Objects sent as argument in the message m – Objects O creates in responding to m – Objects that are directly accessible from O, using attribute values Activations 16 Flow of Control – Procedural interactions At most one object is computing at any time – Activation An object has a live activation from when it receives a message until it responds to the message – Waiting for response Synchronous messages on sending a message to another object, an object will wait until it receives a response – Activation task Activations are stacked and the top activation has control. When the top action responds the next to top regains control and so on...  17 Creation and Deletion In Sequence Diagrams, It is possible to use the lifelines – New objects have their icon inserted when they are created – Destroyed objects have their lifeline terminated with × In Communication Diagrams the objects are labelled: – New for objects created in the collaboration – Destroyed for objects destroyed during the collaboration  Slide 17: Example 1: n := getName() :Lecturer {destroyed} 3:destroy() :UTO 2: new DirectorOfStudies (n) :DirectorOfStudies {new} :Lecturer :UTO 1: n := getName() 2: new DirectorOfStudies (n) :DirectorOfStudies 3:destroy()  18 Communication vs. Sequence Diagrams Communication Sequence Diagrams Diagrams Participants ✦ ✦ Links ✦ Message Signature ✦ ✦ Parallel Messages ✦ ✦ Asynchronous messages ✦ Message Ordering ✦ Create & Maintain ✦  Slide 18: Communication vs. Sequence Diagrams Shows participants effectively: Both Communication and Sequence diagrams show participants effectively Showing the links between participants: Communication diagrams explicitly and clearly show the links between participants Showing message signatures: Both Communication and Sequence diagrams show messages effectively Support parallel messages: Both Communication and Sequence diagrams show parallel messages effectively Support asynchronous messages: Sequence diagrams explicitly and clearly show the links between participants Easy to read message ordering: Sequence diagrams explicitly and clearly show message ordering Easy to create and maintain: Communication diagrams do have the edge on the ease-of-maintenance  19 Constructing Communication Diagrams 1. Identify behaviour 2. Identify the structural elements 3. Model structural relationships 4. Consider the alternative scenarios  Slide 19: Constructing Communication Diagrams 1. Identify behaviour whose realization and implementation is specified 2. Identify the structural elements (class roles, objects, subsystems) necessary to carry out the functionality of the collaboration; Decide on the context of interaction: system, subsystem, use case and operation 3. Model structural relationships between those elements to produce a diagram showing the context of the interaction 4. Consider the alternative scenarios that may be required; Draw instance level collaboration diagrams, if required; Optionally, draw a specification level collaboration diagram to summarise the alternative scenarios in the instance level sequence diagrams  20 Readings Required Readings UML course textbook, Chapter 10 on More on Interaction Diagrams Suggested Readings K.J. Lieberherr, I.M. Holland. Assuring good style for object-oriented programs. IEEE Software 6(5):38-48, 1989.  21 Summary Interaction Diagrams – Sequence Diagrams – Communication Diagrams Communication Diagrams Rationale Communication Diagrams – Collaborations – Interactions – Messages Constructing Communication Diagrams