ASWEC 2006 Proposed Notation for Exception Handling in UML 2 Sequence Diagrams Oddleif Halvorsen Øystein Haugen

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Proposed Notation for Exception Handling in UML 2 Sequence Diagrams

Oddleif Halvorsen

Øystein Haugen

Apr 10, 2023 Halvorsen / Haugen

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What are UML 2 Sequence Diagrams?

Describe interaction sequences Emphasize the interaction

between objects Partial descriptions May be used to verify

interaction properties with respect to a specification

Send event Receive event

Asynchronous message


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Why the Need for Exception Handling?

”This basic description of an object cannot be cluttered up with all of the details needed for handling the contingencies.”

- Terry Winograd (1979)


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Challenges wrt. Seq. Diagrams and Exceptions

How to visually isolate exception handling from a sequence diagram without disturbing the current semantics?

Sequence diagrams are often used to describe distributed and multithreaded environments– The scope of exception handling in modern programming

languages is normally limited to one call stack Sequence diagrams describes sets of traces

– There is no call stack


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Example Scene – Withdrawal of money from an ATM

Happy Day Scenario– Enter a card– Give the correct pin – Select a legal amount to withdraw– Receive the money and the card

Exceptions– Unrecognized card– Wrong pin– Selected too high amount to

withdraw– ATM is out of money– Lost connection with the bank– Unable to deliver the money– Unable to give back card– ...

sd Withdrawal

:Bank:User :ATM

Code(cid, pin)

OK

refEnterPin

Cardid(cid)

msg(”Select amount”)

Amount(selectedAmount)

Withdraw(selectedAmount)

Money(selectedAmount)

card

sd EnterPin

:User :ATM

msg(”Enter PIN”)

Digitloop(4)


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Handling exceptions with current standard

sd Withdrawal

:Bank:User :ATM

Code(cid, pin)

OK

refEnterPin

Cardid(cid)





card

sd Withdrawal

:Bank:User :ATM





card

alt [ correctPin == true ]

msg(”Card taken.”)

loop

Code(cid, pin)

refEnterPin

Cardid(cid)

[ correctPin != true && attempts < 3 ]

correctPin = false; attempts = 0;

alt OK(MaxAmount)

correctPin = true

msg(”Wrong PIN”)attempts++

loop [ selectedAmount <= MaxAmount ]



msg(”To high amount”)

NOK

Ensuring correct pin.

Preventing the user from withdrawing more than allowed.

No valid pin given.


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Our Waysd Withdrawal

:Bank:User :ATM

Code(cid, pin)

OK

refEnterPin

Cardid(cid)





card

The normal flowsd Withdrawal

:Bank:User :ATM

Code(cid, pin)

OK(maxAmount)

refEnterPin

Cardid(cid)





card

Adding exception flow without obstructing the

normal flow

exceptionInvalidPin() [pin != cid.pin]

exceptionTooHighAmount(selectedAmount, maxAmount)[ selectedAmount > maxAmount]

Reference to a diagram that handles

the exception.

Guard that triggers the exception.


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Dynamic Gate Matching

sd InvalidPin

:Bank

NOK

sd PinNotOK

:ATM:User

refEnterPin

Code(cid, pin)

sd NewCode

:Bank

Code(cid, pin)

exceptionInvalidPin() [ pin != cid.pin ]

return

exceptionNoMoreAttempts() [ OutOfAttempts ]

NOKmsg(”Wrong PIN”)

critical

Recovered from the exception.

If a gate can be matched it is combined to complete the trace


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Dynamic Gate Matching (continued)

ATM

:CardReader

:CashDispenser:Keyboard

:Controller

User-Reader

User-Keyboard

ATM-bank

User-Cash

:Screen

User-Screen

Withdrawalsd EnterPinsd

PinNotOKsd

NewCodesd

InvalidPINsd

NoMoreAttemptssd

The normal flow diagrams

Exception handlingwith dynamic gate

matching


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The Diagram Stack

Example stack– The user enters wrong pin

twice

– Correct pin the third time

Each frame represent a new separate layer of diagrams.

Move between frames through the use of exception and return.

sdWithdrawal

sdInvalidPin

sdPinNotOk

sdNewCode

exception

0

1

sdInvalidPin

sdPinNotOk

sdNewCode

2

exception return

return

First attempt, wrong pin.

Second attempt, wrong pin.

Third attempt, correct pin.

The trace can now continue with the

normal flow.


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Trace Semantics

Without exceptions: U = <!m, ?m> seq <!n, ?n> Disregarding the exception: U = a seq <q> seq b seq d Recoverable exception:

– b is a subtrace such that b contains no events on the same lifeline as that of q– c [[e]] i.e. c is a trace in the exception– Provided d is not empty, it starts with an event on the same lifeline as q– U = a seq <q> seq (b par c) seq d where a,b,c,d are (sub)traces and seq is trace

concatenation and par represents all possible merge combinations

a: events executed before

the exception

b: events enabled before the exception

<q>: trigger

d: events after the exception

Exceptione() [guard]

c: exception


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Conclusion

Properties of our concepts of exception in UML Seq. Diag– Compact notation– Clear visual separation between the normal flow and the

exception handling– Underpinned by precise semantics– May describe exception handling in synchronous as well as

multithreaded and distributed scenarios

Documents

ASWEC 2006 Proposed Notation for Exception Handling in UML 2 Sequence Diagrams Oddleif Halvorsen Øystein Haugen