1 Interaction Design & UML Sequence Diagram Once we have decomposed the system and designed the individual components (or classes), we need to depict how these pieces collaborate to deliver the services. (of course, you may go back and re-design the components after looking at the collaboration)The interactions among the individual participants (classes) can be captured with different UML notations, but mainly through Sequence DiagramsSequence diagram depicts the “message flow” among the participants and thus depicts the collaboration among the participants.
2 General Sequence Diagram It is composed of a diagram frame:1) with an identifier name2) the individual participants (classes) in the form of “lifeline” composed of:A rectangle depicting the participating objectA dotted line that extends for the time period of the interaction3) messages to communicate among the participantsorder processclientorderinventorycreateLocate item
3 Message Arrows for Communications The message arrows represent the communications between two objects in a sequence diagram. It goes from the lifeline of one object to that of another objectSynchronous message where the sending object suspends action and waits for the response to the messageAsynchronous message where the sending object continues with its operations without waiting for the responseA return of control from the synchronous messageA creation of a new entity(filled head)(open head)
4 Message Specification Every synchronous and asynchronous arrow must be labeled with a message specification on top of the message arrow.The message format :return_variable_name = message_name (param_list)Both the i) return_variable_name and ii) the “=“ sign are suppressed if there is no return valuemessage_name is never suppressed (required field)param_list is a list of arguments separated by commas and is suppressed when there is no argumentSee page 364 for detailsof param list
5 Examples of Sequence Diagram’s message specification age = getAge or age = getAge( )message specifies that the return value from getAge operation is assigned to the variable age. Note that age is a variable accessible by the object that sent the messagecheckStatus (flag = status, machine)message specifies that checkStatus operation passes a parameter and gets back the status information which assigned to a “local” variable, flag. (local meaning the object that sent the message)
6 Execution Occurrence in Sequence Diagram An operation is executing when a process is runningAn operation is suspended when it sends a synchronous message and waiting for a return message.An operation is considered active when it is either executing or suspendedAn object is active if one or more of its operation is active. While an object is active it is shown with an execution occurrence (a thin rectangle covering the dashed line).A synchronous message always initiates a new execution occurrence(e.g. Order in the diagram)sampleclientorderinventorycreate(ord#)locate_item(i)
7 Interaction Fragments Sequence diagram depicts the interactions among the entities. The natural flow of “control” in the diagram is sequential from top to bottom and follows the direction of the message arrows. The natural sequential control can be broadened with “Interaction Fragments”:Optional Fragment ( “if –then” )Alternative Fragment ( “if-then-else-if - - -” or “case” )Break Fragment ( “break” )Loop Fragment ( iterations or “loop” )Note that these fragments are like the control structures that exist ina programming language.
8 Depicting a Fragment Graphically sd sampleclientorderinventoryInteractionFragmentcreateoperatorop1InteractionFragmentOperationName:(e.g. loop)Interaction FragmentOperandLocate item
9 Depicting an Optional Fragment sd sampleclientorderinventoryInteractionFragmentInteractionFragmentOperationName:(optional)Opt[new_cust]cr_custinfo()InteractionFragmentguardcreate_orderlocate_itemInteraction FragmentOperandThis Optional Fragment has only 1 operand and guard in brackets. A guardis a Boolean expression. The Optional Fragment is performed if the guard is true atthat point of the interaction. It is like the “if” structure of programming language
10 Depicting an Alternative Fragment sd sampleclientorderinventoryInteractionFragmentInteractionFragmentOperationName:(Alternative)Alt[new_cust=yes]cr_custinfo()InteractionFragmentguard[new_cust= no]get_custinfo()Cr_order( )Interaction FragmentOperandsThe Alternative Fragment has multiple mutually exclusive guards in brackets. Theoperand associated with the true guard is executed. This structure is like the “CASE”or “if-then-else-if” constructs of the programming language
11 Depicting an Break Fragment sd sampleclientorderinventoryInteractionFragmentOperationName:(Break)break[ ! good_status]error_msg( )Alt[new_cust=yes]cr_custinfo()The guardexpression ofNOT good_status[new_cust= no]get_custinfo()Cr_order( )The Break Fragment has a single operand which is processed if the guard is “true,”and the rest of the processing in the diagram is not performed. It is like the “break”construct in programming language.
12 Depicting an Loop Fragment sd sampleorderinventoryn_it = check_items()checking formore itemsInteractionFragmentOperationName:Loop (min,max)createiteratormore = has_item (n_it)Loop[more]Process_item()The guardexpression of[ more]more = has_item(n_it)The Loop Fragment is expressed as Loop(min,max). The loop is performed atleast min times and at most max times. If neither min or max is specified, then min=0and max is unlimited. If the loop is performed min times but less than max, then it isperformed again as long as the guard is true. The default value of guard is true.
13 Some Sequence Diagram Guidelines Pick a design level (based on the classes in the static model) and “be consistent” at that level through out the interaction diagram.Put the sender of the first message leftmostPut pairs of entities that interact heavily next to each otherPosition the entities to shorten the message arrowsPosition the entities to make the message arrows go from let to rightSuppress return arrows as much as possible when using execution occurrences
14 Some Thoughts on Designing Design is not a sequential process but much more iterative: (“Component/Interaction” Co-Design)Design (generate) the components in terms of entities (with class model and express in class diagram)Design the interactions among the classes (express in sequence diagram)Design is not a single level process, but more top-down: (Outside-In Design)Top may be viewed as external (requirement level)Down may be viewed class model and interactions representing deeper levels of solutionsIterate the above as we evaluate, alter, and improve the model(See pages example in your book)And we progressively move into more details (inwards)
15 Some details on evaluating interaction alternatives (Example) waterHeatercntrl is constantlypolling the clock with a fixedrate efficient for waterHeatercntrl?clock is constantly checking time and notifieswaterHeatercntrl when the time arrives, then waterHeatercntrl takes action.SD pollingSD notificationclockwaterHeatercntrlwaterHeatercntrlclockloopnotifylooptime=getTimetime=getTimeoptopt[time = right][time = right]takeActiontakeActionWhich one would you pick and why ?Also, note the synchronous message creates an execution occurrence
16 On Control MechanismIn designing, one of the issue is on “point of control,” or the controller, which makes decisions and directs other components.There are three major ways to establish control:Centralized control where all decisions are made by one or two entities and the rest of the entities receives directions from themDelegated control where only the main decisions are made by one or two “main” entities, other decisions are delegated to lower level entities and coordinated among the entities.Dispersed control where decision making is spread out widely, with no easily identifiable coordinating entity or entities.
17 Centralized ControlShould be used only when the solution is small and only a few decisions are involved. (easy to find control point)Lots of drawbacks:Centralized control can be “bloated” and too big to manageMay be less cohesion when too many varieties of decisions are being madeMay increase coupling between the controller and other entities which merely act as data store or simple functionsInformation hiding can NOT be easily achievedCentral contrl
18 Heuristics to Avoid Centralized Control Avoid interaction design where most messages originate from single componentKeep components small so that there can not be a “bloated” controller. (This is not very different from the traditional advise on keeping modules small how small is small? --- cohesion?)Make sure that operational responsibilities are not assigned to just a few components.Make sure operational responsibilities are consistent with data responsibilities. (what happens if they are not? you may have less cohesion among methods in a class )Look at diagram on page 386: it is an over-centralized control design:- AutoCycle delegates nothing and is coupled with all other objects- it lacks cohesion in that it is doing all types of details- it is too big in size because it contains all the low level activities
19 Delegated Control Control is in more entities – smaller in size Information hiding is easier with different control pointsIncreased cohesion with delegated points of controlEach controller is coupled to less entities. (but over-all # of couplings may not decrease)(Note: the 1st to 4th object interaction is asynchronous)Delegated contrl
20 Heuristics for Delegated Control Delegated control is the ideal case we are after.Ensure that each component is responsible for “high level” tasks and as much of the lower ( more detailed, less functional, just different functional areas, etc.) level tasks are delegated as possible.The lower level tasks may be performed in a more collaborative manner among several other components.Look at diagram on page 387, where AutoCycle delegates someresponsibilities to Zone. This is a much less coupled and a more cohesivedesign along with a certain amount of encapsulation of information.
21 Dispersed ControlToo many controls and hard to figure out the interactions:Too much interactions among the entities – high coupling among the parts and possibly very low cohesion within each entity.
22 Heuristics for avoiding Dispersed control Basically, avoid situations where every component is sending a lot of messages to other components.Ensure that there is not an over-delegation, where each component is responsible for too a small portion of the whole and there are a lot of components involved in accomplishing anything.
23 Control and Communications centralized &wheeldispersed &all-memberdelegated &hierarchical[(n x (n-1))/ 2]potential coupling(n-1) potential couplingbut deceiving because - -?(n-1) potential couplingbut deceiving because - -?
24 Law of Demeter for OO Interaction Design An operation (method M) of an object, Obj, should send messages only to the following:Within the object, Obj, itselfMethods within ObjAttributes of Obj (its instance variables)Argument of the operation (parameters of method M, which may be some object)Elements of a collection that is an argument of the operation or an attribute of the object, Obj.Objects created within the operation (objects instantiated within the method M)Global Classes or objectsNote that objects that are returned by messages sent to other object is not included.“ Talk only to your immediate neighbors”The Law of Demeter is meant to help in : (1) information hiding;(2) lessening centralized control
25 Example from page 375 of text Design a water heater controller based on:Caldera is a smart water heater controller that attaches to the thermostat of a water heater and provides more efficient control of water temperature to save money and protects the environment.Caldera sets the water heater thermostat high when hot water is much in demand and sets it low when there is no much demand. For example:Caldera can be told to set the thermostat high on weekday mornings and evenings and all day on weekends.And low during the middle of the week days and nights.Caldera can be told to set the thermostat high all the time in case of illness or other needs.Caldera can be told to set the thermostat low all the time in case of vacation or some other prolonged absence from house.
26 Your Caldera Design may progress as follows: heaterControllerthermostatset_temp1. Class Modelsd CalderaheaterControllerthermostat2. ClassInteractionsset_temp( )
27 Your Caldera Class Design (further Refinement) ? heaterControllerthermostatset_tempnotify_timespecial_setnotify_dateclockmanualcalendar3. Further “Refined” Class Model
28 Your Caldera Interaction Design (further Refinement) ? sd CalderaheaterControllerthermostatCalendarClockmanualNotify_date( )Set_temp( )Notify_time( )Special_set( )Set_temp( )4. Refined Class Interactions in Sequence Diagram
29 Further Evaluate and Improve the Caldera Design Consider the notion of adding another entity to represent the notion of “load scaling” or “temp scaling” which traps the inputs from clock and calendar and sends the controller a binary high or low signal.Consider the manual override to go directly to thermostat and be equal to the controller.Draw the Class diagram and the Sequence diagram for these concepts,evaluate and see if they are indeed improvements:- cohesion- coupling- size