quartz/content/notes/sequence-diagrams.md

title	aliases	tags
sequence-diagrams		info201

equence diagrams document a sequence of particpant interactions required to carry out a use case

• actor <-> object
  • actors are outside the system
  • objects are otside the system
  • via a method call
    • might get a result
• object <-> object
• lifetime of interactions and objects
  • when they are created updated destroyed
• time is a key aspect
  • use-case-diagrams dont have order

These diagrams are:

• detailed, low level, bottom up
• behavioural diagram
  • not structural
• common in industry
  • along with class diagrams
• need to be designed and read alongside corresponding class diagrams
  • e.g., class diagrams with inform sequences diagrams and vice versa
  • back and forth process

General overview example: annotated example

• time goes from top to bottom
  • however no specific time units
• can have actors as participants
  • but not usually
  • existence of actor usualy indicates a sequence is owned by a use case
• interactions are indicated by messages (solid arrows)
  • e.g. actor to main menu
    • actor clicks a button
    • menu reacts
    • etc
  • messges are synchronous
    • i.e., thing sending message must wait for result
  • always method calls (or something that equated to a method call)
• participants are supposed to be instances of classes
  • however we are usually more interested in the class name
• the dashed lines are lifelines
  • can also be solid
  • basically indicate the existenc of something
  • e.g., Thingform gets destroyed, thingfinder and thing remain throughout
• the rectangles (activation bars) indicate when an a thing is doing somethin
  • caused by incoming message
  • ended by a return
  • these can have sub activations
    • i.e., nested
  • these can be self-activations
  • implcit: not all methods return something

relevant slide:

Messages

example

• direction
  • <- or ->
  • easier to under stand if most messages are ->
  • however this is not always possible
    • same object used by multiple other objects
    • an object calls back to the object that called it
• can be conditions (guards) example
  • only sent if condition is true
  • able to approximate if-then-else using multiple branches with exclusive conditions
  • this is better done in activity diagram
• looping messages example
  • an asterisk idicated looping
    • repeat message until condition id false
    • send messge to each object in a collection
  • may also be better in activity diagram

Interaction frames (UML 2.x)

example

• loop frame
  • any kind of loop
  • replaces * notation
• opt frame
  • optional or conditional processing
  • can replace [] notation
• alt frame
  • if-then-else
  • can replace [] notation

one thing that can cause complications is

• when something can a top level loop which is waiting for input.
• a cancel anytime option

Basic process of creation

• identity participants of a use case (dont always need to use a use-case diagram)
  • use use case to create first version of the activity diagram. as you implement the code update the class and activity diagrams
• identify messges required to carry out use case
• for each message
  • it is always sent
  • is it sent conditionally
  • is it sent multiple times
• assemble messages in correct sequence and attach to relevant lifelines/activations
• add returns where necessary

Case study ATM

bank is developing a new ATM system for their customers

scope and requirements

• each customer has one or mor accounts
• transaction types are
  • view balance
  • withdraw cash
  • deposit funds
• the customer can cancel at any point before final confirmation
• customer authenticates by inserting bank card and entering four digit pin

process

• choose account
• choose amount
• check customer funds
• check amount in cash dipenser
• results
  • withdraw amount
  • dispense amount
  • remind user

this diagam is probably too general for this case

note navigability of domain

sequence diagram

• part 1
• part 2
• part 3
• part 4

full diagram