Sequence Diagrams

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Sequence Diagrams
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Sequence Diagrams
Related Terms Interaction Diagrams Event Trace
Diagram Scenarios Message Diagram
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Scenarios

• A scenario is a sequence of events that occurs
  during one particular execution of a system.
• A scenario can include all events in the system
  or can only include these events that are
  generated by a certain object in that system.
• A scenario can be a historical record of
  executing or simulating the execution of a system
  or an object.

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Example Scenario for a Phone Line

• Caller lifts receiver
• dial tone begins
• caller dials digit (5)
• dial tone ends
• caller dials digit (5)
• caller dials digit (5)
• caller dials digit (1)
• caller dials digit (2)
• caller dials digit (3)
• caller dials digit (4)

• called phone begins ringing
• ringing tone appears in calling phone
• called party answers
• called phone stop ringing
• ringing tone disappears in calling phone
• phones are connected
• called party hangs up
• phones are disconnected
• caller hangs up

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Event Trace Diagram

• Each event transmits information from one object
  to another.
• Example
• dial tone begins event transmits a signal from
  the phone line to the caller.
• In Event Trace
• Identify sender and receiver objects of each
  event.
• Event Trace Diagram (ETD) shows the sequence of
  events and the object exchanging these events.
• ETD shows object as a vertical line and each
  event as a horizontal line from the sender to the
  receiver.

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Event Trace Diagram for a Phone Call
Caller
Callee
Phone line
caller lifts receiver
dial tone begins
dials (5)
dial tone ends
dials (5)
dials (5)
dials (1)
dials (2)
dials (3)
dials (4)
phone rings
ringing tone
answers phone
tone stop
ringing stops
phone connected
phone connected
callee hangs up
connection broken
connection broken
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Sequence Diagrams
Definition A sequence shows a series of
messages exchanged by A selected set of objects
in temporally limited situation, With emphasis on
the chronological course of events. Objects are
shown by vertical lifelines. This highlights
The chronological sequence of the message. Time
runs from top to bottom.
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Scenarios

• Stories about how the business works
• Typical sequences that happen, and variations on
  the basics
• Usually structured with numbered steps and calls
  to sub-sequences
• Can be illustrated with sequence charts
• Scales --- level of detail of the steps
• Daily scale --- "my pay was credited to my
  account yesterday, so I went to get some cash
  today"
• Detail scale --- "to get money from your account,
  you insert your card into a cash machine, select
  'cash', "
• Grand scale --- "A good way of making money is to
  run a bank. Provide accounts with higher rates of
  interest in exchange for less accessibility. They
  should be able to get cash from the most
  accessible accounts at any time of day or night."
• Choose the level you're working at

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Sequence charts for scenarios

• Helps
• identify the different participants and their
  interactions
• show the sequence of events
• Each event can be expanded to more detailed
  sequences
• and each object to more detailed objects

get cash
Payday
joEmployee
joCustomer
CashMachine
lourdesBank
lourdesBank
ibmEmployer
insert card
pay
req cash
payslip
req details
get cash
cash
get cash
joCustomer
Card Reader
Dispenser
Keyboard
insert card
req cash
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Using Scenarios

• Stories illustrated by sequence diagrams

Payday
joEmployee
lourdesBank
ibmEmployer
IBM transfers money to Jo's account at her Bank
pay
IBM notifies Jo that she has been paid
payslip
get cash
Jo can now collect cashfrom her Bank.
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Benefits of scenarios

• Enable dialogue with non-technical users
• Initial exploration of the domain

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Try it

• Write some scenarios for a Library
• No software, just the people and things
• Write a scenario (or two) to show what happens
  when Jo wants to borrow Trainspotting, but Pat
  and Chris already have the only copies. How does
  Jo eventually get to read it?
• Notice that a sequence diagrams vertical bars
  are objects, not types you can have more than
  one object of the same type

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Technique Spot nouns

• Nouns gt candidate types
• Elephant, Airplane, Meeting, Flight
• Questions you could ask at any one moment gt
  associations and attributes
• fred is an Elephant
• What colour is fred? What Airplane does fred own?
  What Meetings is fred chairing this month? What
  meetings is fred attending this month?
• m42 is a Meeting
• Who is chairing m42 ? Where will m42 be held?
  When will m42 take place?
• Verbs gt use-cases
• But usually prefer to do them on a separate
  diagram
• Fred flies to Athens m42 is held by fred the
  board
• Some of them turn into objects fred meets jo gt
  meeting

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Technique Scenarios -gt snapshots

• Snapshots can be used to exercise scenarios
• Scenario a story about a series of occurrences
• For each step in the scenario, modify the
  snapshot
• ensure there is a way you can record all the
  relevant information in the associations and
  attributes
• ensure that all information relevant to enquiries
  is represented in the associations and attributes
• Update the type model to reflect improvements you
  made while going through the scenario

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Process writing scenarios

• Ask users for typical stories about tasks in the
  domain
• Envision with snapshots, cartoons, screen shots,
  play-acting
• "rich pictures"

MegaBank
Each of Jo'scards identifiesan Account.
123DAccount
joCustomer
345CAccount
Jo wants moneyfrom her currentaccount.
FastBuck
welcome
She inserts thecard in an ATM.This associates
the ATM to her current account. Any further
operations at themachine are recorded against
her current account.
20 Debit date17/5/99
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How UML is used software specification

• Showing which activities the software supports
• Sometimes also useful to note those it doesnt
  support

use-case diagram with system boundary
Room Management System
check in
check out
Receptionist
Person
pay bill
? uml-c-p77
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Scenario with sequence diagram

• Interactions between actorsThis is a form of
  instance diagram it just shows one example of
  what could occur
• Used to work out use-case implementation at high
  level
• No internal details of system

Sequence diagram
jo Person
chris Receptionist
rms Room Manager System
use-case check in
Person arrives at desk,gives name. R gets map of
available Rooms, chooses one. R enters Persons
details on Room Management system against chosen
room. R gives key to Person.
arrive
get room map
choose room
get details
person details
time
confirm room assignment
key
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Try it ...

• Make a system boundary diagram for a system to
  support a library
• Create two scenarios for people checking a book
  out of a library, and another for returning a
  book
• List the operations the system is required to do
• Write the dictionary descriptions for the
  operations

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Object oriented or component based design

• Designs consist of interacting parts
• each containing interacting parts ...
• Design is about dividing up the task between the
  parts

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Collaborating objects

• Functions of software system are divided between
  objects
• Each object performs those sub-tasks most
  relevant to it
• Objects interact to achieve overall system goals
• Sending messages invoking functions
• Objects have to send each other messages to ask
  for other parts of the job to be done
• A good OO design partitions the tasks cleanly, so
  that no object does work that should properly be
  done by another
• decoupling
• flexible, easily changeable software
• separable, generic components that can be
  assembled in many ways to make many end products
• Bad design results in poor decoupling no OO
  benefits

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Distribution of tasks issues

• Applying for a job

0 thinks I need a job
10 (later) thinks I need a widgetgrinder
1 chris.acceptApplication (myApplication)
jo Person
pat Manager
chris Secretary
2
Application
intray WorkTray
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Distribution of tasks issues

• Appropriate distribution of responsibilities
• changes have more restricted impact
• objects can be reused elsewhere

0 thinks I need a job
10 (later) thinks I need a widgetgrinder
11 chris.nextApplicn()
1 chris.acceptApplication (joApplic)
jo Person
pat Manager
chris Secretary
12 intray.get()
2 intray.put(joApplic)
11 intray.get()
joApplic Application
intray WorkTray
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UML collaboration diagrams

• Messages shown atop instance diagrams
• associations references stored within one object
  linking to another
• could be pointers in memory, database keys,
  URLs, ...
• messages invocations of a function from one
  object to another
• could be function calls, database queries, http
  (web page) requests,
• These diagrams don't show the precise nature of
  the implementation

2 needWidgetgrinder( )
repeated with other receiver instances
1 needJob( )
1.2 acceptApplication (joApplic)
2.1 nextApplicn( )
jo Person
chris Secretary
pat Manager
boss
1.2.1 put(joApplic)
association
2.1.1 get()
message
1.1 new
1 intray
2.1 intray.get()
instance creation
intray WorkTray
joApplic Application
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UML Sequence diagrams

• Different view of collaborations
• Tools translate directly between the two syntaxes
• Good for showing assignment of responsibility
• not so good at associations
• also weak at control flow variations

jo Person
chris Secretary
intray WorkTray
pat Manager
1 needJob( )
instance creation
1.1 new
joApplic Application
repeated
1.2 acceptApplication (joApplic)
1.2.1 put(joApplic)
message
stores joApplic
2 needWidgetgrinder( )
2.1 nextApplicn( )
2.1.1 get()
optional
return joApplic