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• CSC 450

Slides adapted from slides created by Robert B. France

State Models

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State Models A state model specifies the life histories of

objects in terms of the sequences of operations that can occur in response to external stimuli. For example, a state model can describe how an object

responds to a request to invoke one of its methods. A state model consists of state diagrams that

each describes how an object responds to external stimuli.

A state diagram describes behavior in terms of sequences of states that an object can go through in response to events.

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Simple Example: Telephone Object

Idle

Active

lift receiver/ get dial tone

caller hangs up / disconnect

state

event

Activity that is performed during transition: it executes instantaneously

initial state

• When the object is created, it moves into the Idle state.

• If a “lift receiver” event is received the object moves from the Idle to the Active state. During the transition the “get dial tone” activity is executed.

• If a “caller hangs up” event occurs when in Active state, the object moves to the Idle state and the “disconnect” activity is performed during the transition.

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Key Concepts An event is a significant or noteworthy

occurrence at a point in time. Examples of events: sending a request to invoke a

method, termination of an activity. An event occurs instantaneously in the time scale of

an application. A state is a condition of an object during its

lifetime. For example, a student is in the registered state after

completing course registration. A state is an abstraction of an object’s attribute values

and links For example, a bank account is in the Overdraft state

when the value of its balance attribute is less than 0.

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Key Concepts - 2 A transition occurs when an event causes an

object to change from its current (source) state to a target state. For example, if a student is in the registered state and

then drops out of the program then the student is in the “not registered” state.

The source and target states can be the same. A transition is said to fire when the change from source

to target state occurs. A guard condition on a transition is a boolean

expression that must be true for a transition to fire

An activity is a behavior that is executed in response to an event.

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top

Basic UML State Diagram

ReadyReady

stop

/ctr := 0stop

StateState

EventEvent

ActivityActivity

Initial state

Initial state

TransitionTransition

Final stateFinal state

DoneDone

“top” state“top” state

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Guards Conditional execution of transitions

guards (Boolean predicates) must be side-effect free

SellingSelling

UnhappyUnhappy

HappyHappy

bid [(value >= 100) & (value < 200)] /sell

bid [value >= 200] /sell

bid [value < 100] /reject

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Event Types ChangeEvent: Occurs when a condition expressed as a boolean

expression becomes true. when condition

Examples: when(temperature > 80); when(balance < 0)

SignalEvent: Occurs when an signal is sent or received, where a signal is a one-way transmission of information from one object to a target object. Event(param1,param2,…)

Examples: lift receiver; hang-up telephone

TimeEvent: Passage of a specific period of time after a designated event, or an occurrence of a specified time instance. Passage of time since entry to current state - after (time period) ,

e.g., after (10 secs), after (5 secs since exit of state A) Occurrence of a time instance – when(due-date=Feb 28, 2005,

11pm)

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Object Behavior - General Model Typical object lifecycle

HandleRequest

HandleRequest

initializeObject

initializeObject

TerminateObject

TerminateObject

Wait forRequest

Wait forRequest

void:offHook (); {busy = true; obj.reqDialtone(); … };

Handling depends on specific request type

Handling depends on specific request type

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Object Behavior and State Machines Reflecting object lifecycle in a state diagram

HandleEvent

InitializeObject

TerminateObject

Wait forEvent

on

off

Lamp On

Lamp On

Lamp Off

Lamp Off

off

on/print(”on”)

stop

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anActiveObject

#currentEvent : Event

+ start ( )+ poll ( )+ stop ( )

Active Objects and State Diagrams

created

ready

start/^master.ready()

poll/^master.ack()

stop/

poll/defer

ready

created

start start/^master.ready() ready

If object is in the created state and a start event occurs, it sends a method invocation event (request to invoke ready()) to the object named master and moves to state ready.

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Active Objects: Dynamic Semantics

Run-to-completion model: • Events are queued• An object processes events in a queue one at a

time.• An object cannot suspend handling of an event to

handle another event

ActiveObject:

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Active1Active1 Active2Active2

The Run-to-Completion Model A high priority event for (another) active object

will preempt an active object that is handling a low-priority event on a uniprocessor

hi

hi

lo

Event is queued because object is handling the lo event

Object, Active1, has its activity suspended so that the Active2 object can respond to hi

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State Representations

StateName

StateNameAtt1 : Attrib1

entry/action

exit/action

event/action

do/action

activity executed when state is entered

activity executed when state is exited

activity executed when event occurs while in state

activity executed while in state

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State Syntax

A state symbol can have one or more compartments (all optional): Name compartment Internal transition compartment

contains internal actions or activities Format: event(args) [condition] / action entry / action (invoked implicitly) exit / action (invoked implicitly) do / machine name (invokes a nested state

machine)

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State Syntax

Entry and exit activities cannot have arguments and cannot have guard conditions.

Activity expressions can use object attributes and links, and parameters of events on incoming transitions.

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State Entry and Exit Actions

LampOnLampOn

entry/lamp.on();

exit/lamp.off();

e1

e2

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Resulting activity sequence:printf(“exiting”);printf(“to off”);lamp.off();

Resulting activity sequence:printf(“exiting”);printf(“to off”);lamp.off();

Order of Actions: Simple Case

printf(“exiting”);printf(“needless”);lamp.off();

printf(“exiting”);printf(“needless”);lamp.off();

off/printf(“needless”);

off/printf(“to off”);LampOffLampOff

entry/lamp.off();

exit/printf(“exiting”);

LampOnLampOn

entry/lamp.on();

exit/printf(“exiting”);

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Internal Transitions

Self-transitions that bypass entry and exit actions

LampOffLampOff

entry/lamp.off();

exit/printf(“exiting”);

off/null;

Internal transitiontriggered by an “off” event

Internal transitiontriggered by an “off” event

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ErrorError

entry/printf(“error!”)

State (“Do”) Activities

Forks a concurrent thread that executes until: the action completes or the state is exited through an outgoing transition

do/while (true) alarm.ring();

“do” activity“do” activity

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Static Conditional Branching Merely a graphical shortcut for convenient

rendering of decision trees

[(value >= 100) & (value < 200)] /sell

[value >= 200] /sell

[value < 100] /reject

SellingSelling

UnhappyUnhappy

HappyHappy

bid

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bid /gain := calculatePotentialGain(value)

SellingSelling

UnhappyUnhappy

HappyHappy

Dynamic Conditional Branching Choice pseudostate: guards are evaluated at the

instant when the decision point is reached

[(gain >= 100) & (gain < 200)] /sell

[gain >= 200] /sell

[gain < 100] /reject

DynamicchoicepointDynamic

choicepoint

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Hierarchical State Machines Graduated attack on complexity

states decomposed into state machines

LampFlashingLampFlashingflash/

1sec/1sec/

FlashOffFlashOff

entry/lamp.off()

FlashOnFlashOn

entry/lamp.on()off/

LampOffLampOff

entry/lamp.off()

LampOnLampOn

entry/lamp.on()

on/

on/on/

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“Stub” Notation Notational shortcut: no semantic

significance

LampFlashingLampFlashingflash/

on/

FlashOn

FlashOff

off/

LampOffLampOff

entry/lamp.off()

LampOnLampOn

entry/lamp.on()

on/ on/

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LampFlashingLampFlashing

1sec/1sec/

FlashOffFlashOff

entry/lamp.off()

FlashOnFlashOn

entry/lamp.on()off/

LampOffLampOff

entry/lamp.off()

LampOnLampOn

entry/lamp.on()

on/

Group Transitions Higher-level transitions

flash/

on/

Default transition tothe initial state in LampFlashing

Default transition tothe initial state in LampFlashing

Group transition: when on occurs in any LampFlashing state

a transition is made to LampOn

Group transition: when on occurs in any LampFlashing state

a transition is made to LampOn

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Completion Transitions Triggered by a completion event

generated automatically when an immediately nested state machine terminates

CommittingCommitting

Phase1Phase1

Phase2Phase2CommitDoneCommitDone

completion transition (no trigger)

completion transition (no trigger)

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LampFlashingLampFlashing

off/

FlashOffFlashOff

FlashOnFlashOn

Triggering Rules Two or more transitions may have the same

event trigger innermost transition takes precedence event is discarded whether or not it triggers a transition

on/

on/

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State Syntax - Submachines

State name:submachine submachine is the name of a (nested) state machine

that has an initial and final state. Execution of the nested machine begins in the initial

state. When final state of the nested machine is reached then

the exit action of the parent state is executed (if it exists).

Dispense cash: Cash Dispenser

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UML 2.0: Entry/Exit Points Encapsulation of submachines

definition of exit point

ReadAmountSM

selectAmount

EnterAmount

ok

abort

aborted

amount

otherAmount

abort

againdefinition of

entry point

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Entry/Exit Points: Usage

VerifyCard

OutOfService

acceptCard

ReleaseCardVerifyTransaction

outOfService

releaseCard

ReadAmount :ReadAmountSM

aborted

use of exit point

use of entry point

rejectTransaction

again

ATM

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Deferred Events Events can be retained if they do not trigger a

transition The event is put to the top of the queue; it must be

handled in the next event handling cycle

off/

LampOnLampOn

entry/lamp.on()

on/

LampOffLampOff

entry/lamp.off()

off/defer

Deferred eventDeferred event

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Order of Actions: Complex Case Same approach as for the simple case

S1exit/exS1

S1exit/exS1

S11exit/exS11

S11exit/exS11

S2entry/enS2

S2entry/enS2

S21entry/enS21

S21entry/enS21

initS2E/actE

Actions execution sequence:

exS11 exS1 actE enS2 initS2 enS21

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Composite States

A state can be decomposed in two ways: AND decomposition: object state decomposed

into two parallel components. OR decomposition: object state decomposed into

a network of states.

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Orthogonality Multiple simultaneous perspectives on the same entity

Child

Adult

Retiree

age

Poor

Rich

financialStatus

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Orthogonal Regions Combine multiple simultaneous descriptions

Child

Adult

Retiree

age

Poor

Rich

financialStatus

Poor

Rich

financialStatus

Child

Adult

Retiree

age

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OutlawOutlaw

LawAbidingLawAbiding PoorPoor

RichRich

financialStatuslegalStatus

Orthogonal Regions - Semantics All mutually orthogonal regions detect the same events and

respond to them “simultaneously” usually reduces to interleaving of some kind

robBank/ robBank/

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Catch-22

“There was only one catch and that was Catch-22, which specified that a concern for one's safety in the face of dangers that were real and immediate was the process of a rational mind. Orr was crazy and could be grounded. All he had to do was ask; and as soon as he did, he would no longer be crazy and would have to fly more missions. Orr would be crazy to fly more missions and sane if he didn't, but if he was sane he had to fly them. If he flew them he was crazy and didn't have to; but if he didn't want to he was sane and had to.” – Joseph Heller

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Catch22Catch22sanityStatus flightStatus

Interactions Between Regions Typically through shared variables or awareness

of other regions’ state changes

(flying)/

Crazyentry/sane := false;

Crazyentry/sane := false;

Saneentry/sane := true;

Saneentry/sane := true;

requestGrounding/

Flyingentry/flying := true;

Flyingentry/flying := true;

Groundedentry/flying := false;

Groundedentry/flying := false;

(sane)/

(~sane)/

sane : Booleansane : Boolean

flying : Booleanflying : Boolean

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Transition Forks and Joins For transitions into/out of orthogonal

regions:

StaffMember

StaffMember

employee

ChildChild AdultAdult RetireeRetiree

age

ManagerManager

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Common Misuse of Orthogonality Using regions to model independent objects

Child

Adult

Retiree

Child

Adult

Retiree

Person1 Person2

Person1 Person2

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An Example

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Subsystem design modeling wrap-up What model should we start developing first?

Interaction model then class model Interaction models helps determine class responsibilities Focus first on interaction models that realize use cases

Class model and interaction model in parallel When should we use state and activity models

Use state models if system is reactive A reactive system maintains an ongoing interaction with its

environment. Air traffic control system Programs controlling mechanical devices such as a train, a plane. Ongoing processes such as a nuclear reactor.

Use activity models to define class methods

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State Model Exercises

Develop a state machine for a Hotel Reservation system.

Develop a state machine for a student record system.

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______________________Devon M. Simmonds

Computer Science Department

University of North Carolina Wilmington

_____________________________________________________________

Qu es ti ons?

The End

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State Model exercise Complete state machine for a student record system.

Applicant Acceptedaward [accepted] /

Rejected

award [rejected] reapply

Registered

register

addCourse

addCourse

Dismissed

program-terminated

reapply

Graduateprogram-completed

Inactiveunregistered

register

after: one semester

no-reapplication

no-reapplication

award-presented

Active

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State Model exercise Develop a state machine for a Hotel

Reservation system.

Input Bookingbooking request

Reservationreservation request

Cancellation

cancellation request

success

cancellation request query

transaction complete query

query query