Sequence Diagrams in Software Development
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)
OOAD
11
Example 1: Withdraw saving
Scenario:
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.
12
Withdraw saving
Use Case:
Withdraw Saving
Actors:
Bank, Client
Flow of Events:
1.
Bank client insert ATM card into ATM machine
2.
ATM machine request PIN code
3.
Bank client enter PIN code
4.
Verify PIN code entered with saving account
5.
ATM machine request amount if the PIN is valid
6.
Bank client enter the required amount
7.
Process the transaction in the client account
8.
ATM machine dispense cash
9.
ATM machine print receipt when the transaction completed
Alternative Flow of Events
•
Invalid PIN code entered. Indicate error message. Return step 3.
OOAD
13
Identifying Classes
–
Bank Client
–
ATM Card
–
ATM Machine
–
Savings Account
–
Cash
–
Message
Example 2: Buying Tickets
An interaction between a customer, ticket seller, database and printer.
Example 3: Logging in
Let'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 shown
2. User enters user name and password
3. User clicks on OK or presses the enter key
4. The user name and password are checked and approved
5. 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
Sequence diagrams depict the sequence of actions in a system, capturing the invocation of methods in objects. They are a valuable tool for representing dynamic system behavior. Message arrows in sequence diagrams indicate communications between objects, illustrating synchronous and asynchronous message interactions. An example scenario of a withdrawal process at an ATM demonstrates the use of sequence diagrams in real-life applications.
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Presentation Transcript
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 (filled head) Asynchronous message where the sending object continues with its operations without waiting for the response (open head) A return of control from the synchronous message A creation of a new entity
Example 1: Withdraw saving Scenario: 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. OOAD 11
Withdraw saving Use Case: Actors: Withdraw Saving Bank, Client Flow of Events: 1. Bank client insert ATM card into ATM machine 2. ATM machine request PIN code 3. Bank client enter PIN code 4. Verify PIN code entered with saving account 5. ATM machine request amount if the PIN is valid 6. Bank client enter the required amount 7. Process the transaction in the client account 8. ATM machine dispense cash 9. ATM machine print receipt when the transaction completed Alternative Flow of Events Invalid PIN code entered. Indicate error message. Return step 3. 12
Identifying Classes Bank Client ATM Card ATM Machine Savings Account Cash Message OOAD 13
ATM-Machine SavingAccount Client 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 Tickets An interaction between a customer, ticket seller, database and printer.
Example 3: Logging in Let'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 shown 2. User enters user name and password 3. User clicks on OK or presses the enter key 4. The user name and password are checked and approved 5. 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
