Sequence Diagrams

Introduction

• A Sequence diagram depicts the sequence of actions that occur in a system.

• The invocation of methods in each object, and the order in which the invocation occurs is captured in a Sequence diagram.

• This makes the Sequence diagram a very useful tool to easily represent the dynamic behavior of a system.

• A Sequence diagram is two-dimensional in nature. On the horizontal axis, it shows the life of the object that it represents, while on the vertical axis, it shows the sequence of the creation or invocation of these objects.

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 object

– Synchronous message where the sending object suspends action and waits for the response to the message

– Asynchronous message where the sending object continues with its operations without waiting for the response

– A return of control from the synchronous message

– A creation of a new entity

(filled head)

(open head)

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Example 1: Withdraw savingScenario:1. He enters his card into an ATM (automated teller machine). 2. The ATM machine prompts “ Enter PIN”. 3. The customer enters his PIN. 4. The ATM (internally) retrieves the bank account number from the

card. 5. The ATM encrypts the PIN and the account number and sends it

over to the bank. 6. The bank verifies the encrypted account and PIN number. 7. If the PIN number is correct, the ATM displays, “Enter Amount”. 8. Draws money from the bank account and pays out the amount.

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Withdraw savingUse Case: Withdraw SavingActors: Bank, Client

Flow of Events:1. Bank client insert ATM card into ATM machine2. ATM machine request PIN code3. Bank client enter PIN code4. Verify PIN code entered with saving account5. ATM machine request amount if the PIN is valid6. Bank client enter the required amount7. Process the transaction in the client account8. ATM machine dispense cash9. ATM machine print receipt when the transaction completed

Alternative Flow of Events• Invalid PIN code entered. Indicate error message. Return step 3.

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Identifying Classes– Bank Client – ATM Card– ATM Machine– Savings Account– Cash– Message

Client ATM-Machine SavingAccount

Insert ATM card

Request PIN

Enter PIN code

Verify PIN code

PIN valid

Request amount

Enter amount

Process transaction

Transaction successful

Dispense cash

Print receipt

Example 2: Buying TicketsAn interaction between a customer, ticket seller, database and printer.

Example 3: Logging inLet's start with the simple example above: a user logging onto the system.

The Logon use case can be specified by the following step:1. Logon dialog is shown2. User enters user name and password3. User clicks on OK or presses the enter key4. The user name and password are checked and approved5. The user is allowed into the system Alternative: Logon Failed - if at step 4 the user name and password are not

approved, allow the user to try again

Example 4: Course Enrolment System

Example 5: Product Order

Example 6: Search Engine

QUESTIONS