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Sequence Diagrams
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UML sequence diagrams sequence diagram: an "interaction
diagram" that models a single scenario executing in the system perhaps 2nd most used UML diagram (behind
class diagram)
relation of UML diagrams to other exercises: use cases -> sequence diagrams
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Key parts of a sequence diag.
participant: an object or entity that acts in the sequence diagram sequence diagram starts with an unattached
"found message" arrow
message: communication between participant objects
the axes in a sequence diagram: horizontal: which object/participant is acting vertical: time (down -> forward in time)
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Sequence diag. from use case
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Representing objects squares with object type, optionally
preceded by object name and colon write object's name if it clarifies the diagram object's "life line" represented by dashed
vert. line
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message (method call) indicated by horizontal arrow to other object write message name and arguments above arrow
dashed arrow back indicates return different arrowheads for normal / concurrent
(asynchronous) methods
Messages between objects
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Lifetime of objects creation: arrow with
'new' written above it notice that an object
created after the start of the scenario appears lower than the others
deletion: an X at bottom of object's lifeline Java doesn't explicitly
delete objects; they fall out of scope and are garbage-collected
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Indicating method calls activation: thick box over object's life
line; drawn when object's method is on the stack either that object is running its code, or it is
on the stack waiting for another object's method to finish
nest to indicate recursionActivation
Nesting
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Indicating selection and loops
frame: box around part of a sequence diagram to indicate selection or loop if -> (opt) [condition] if/else -> (alt) [condition], separated by horiz. dashed
line loop -> (loop) [condition or items to loop over]
[balance <> 0]opt [balance < 100.00]
[balance >= 100.00]
alt
[balance < 0]loop
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linking sequence diagrams if one sequence diagram is too large or refers to
another diagram, indicate it with either: an unfinished arrow and comment a "ref" frame that names the other diagram when would this occur in our system?
Verify customer credit
refCustomer Info
Approved?
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Example sequence diagram
sd Example
loop
StoreFront Cart Inventory
AddItemReserveItem
PlaceItemInOrder
Checkout
ProcessOrderConfirmOrder
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Flawed sequence diagram 1
What's wrong with this sequence diagram? (Look at the UML syntax and the viability of the scenario.)
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Flawed sequence diagram 2
What's wrong with this sequence diagram?
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Flawed sequence diagram 3
What's wrong with this sequence diagram?
:Computer :PrintServer :Printer :Queue
print(file)[if printer free] print(file)
[else] enqueue(file)
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Why not just code it? Sequence diagrams can be somewhat
close to the code level. So why not just code up that algorithm rather than drawing it as a sequence diagram? a good sequence diagram is still a bit above the level
of the real code (not EVERY line of code is drawn on diagram)
sequence diagrams are language-agnostic (can be implemented in many different languages
non-coders can do sequence diagrams easier to do sequence diagrams as a team can see many objects/classes at a time on same
page (visual bandwidth)
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Sequence diagram exercise 1
Let's do a sequence diagram for the following casual use case, Start New Poker Round :
The scenario begins when the player chooses to start a new round in the UI. The UI asks whether any new players want to join the round; if so, the new players are added using the UI.
All players' hands are emptied into the deck, which is then shuffled. The player left of the dealer supplies an ante bet of the proper amount. Next each player is dealt a hand of two cards from the deck in a round-robin fashion; one card to each player, then the second card.
If the player left of the dealer doesn't have enough money to ante, he/she is removed from the game, and the next player supplies the ante. If that player also cannot afford the ante, this cycle continues until such a player is found or all players are removed.
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Sequence diagram exercise 2
Let's do a sequence diagram for the following casual use case, Add Calendar Appointment :
The scenario begins when the user chooses to add a new appointment in the UI. The UI notices which part of the calendar is active and pops up an Add Appointment window for that date and time.
The user enters the necessary information about the appointment's name, location, start and end times. The UI will prevent the user from entering an appointment that has invalid information, such as an empty name or negative duration. The calendar records the new appointment in the user's list of appointments. Any reminder selected by the user is added to the list of reminders.
If the user already has an appointment at that time, the user is shown a warning message and asked to choose an available time or replace the previous appointment. If the user enters an appointment with the same name and duration as an existing group meeting, the calendar asks the user whether he/she intended to join that group meeting instead. If so, the user is added to that group meeting's list of participants.
Data Flow Diagrams
Where do they fit in?
Analysis (What do we do?) Fact finding
investigate business process and the current system
modelling the current and required systems
deliverables - requirements specification logical models of the
required system
Life Cycle Phases Planning Feasibility Study Analysis Design Code and Unit
test
3
Data Flow Diagrams (DFD)
DFDs describe the flow of data or information into and out of a system what does the system do to the data?
A DFD is a graphic representation of the flow of data or information through a system
4 Main Elements
external entity - people or organisations that send data into the system or receive data from the system
process - models what happens to the data i.e. transforms incoming data into outgoing data
data store - represents permanent data that is used by the system
data flow - models the actual flow of the data between the other elements
Notation
Process box
D Data Store
ExternalEntity
Data Flow
• Data Flow
• Process
• External Entity
• Data Store
4
Levelled DFDs
Even a small system could have many processes and data flows and DFD could be large and messy use levelled DFDs - view system at different
levels of detail one overview and many progressively greater
detailed views
Level 0 - Context Diagram
models system as one process box which represents scope of the system
identifies external entities and related inputs and outputs
Additional notation - system box
System boxExternalentity
Data flow out
Data flow in
Level 1 - overview diagram
gives overview of full system identifies major processes and data flows
between them identifies data stores that are used by the
major processes boundary of level 1 is the context
diagram
Level 2 - detailed diagram
level 1 process is expanded into more detail
each process in level 1 is decomposed to show its constituent processes
boundary of level 2 is the level 1 process
Other Notation
Duplicates marked by diagonal line in corner
System Boundary Elementary Processes - star in
corner Process that is levelled - dots on top
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Rules for DFDs
Numbering
Labelling
Balancing
Numbering
On level 1 processes are numbered 1,2,3…
On level 2 processes are numbered x.1, x.2, x.3… where x is the number of the parent level 1 process
Number is used to uniquely identify process not to represent any order of processing
Data store numbers usually D1, D2, D3...
Labelling Process label - short description of
what the process does, e.G. Price order Data flow label - noun representing the
data flowing through it e.G. Customer payment
Data store label - describes the type of data stored
Make labels as meaningful as possible
Balancing and data stores
Balancing any data flows entering or leaving a
parent level must by equivalent to those on the child level
Data stores data stores that are local to a process
need not be included until the process is expanded
Data Flows
Allowed to combine several data flows from lower level diagrams at a higher level under one data flow to reduce clutter
Flows should be labelled except when data to or from a data store consists of all items in the data store
Joe’s YardJoe’s builders’ suppliers has a shop and a yard. His system is
entirely manual. He has a stock list on the wall of his shop, complete with prices. When a builder wants to buy supplies, he goes into the shop and picks the stock items from the list. He writes his order on a duplicate docket and pays Joe, who stamps the docket as paid. The builder takes the duplicate docket and he goes to the yard and hands it to the yard foreman. The yard foreman gets the ordered items from the yard and gives them to the builder. The builder signs the duplicate docket and leaves one copy with the foreman and takes one copy as a receipt. Every week, Joe looks around the yard to see if any of his stock is running low. He rings up the relevant suppliers and reorders stock. He records the order in his order book, which is kept in the yard. The yard foreman takes delivery of the new stock and checks it against what has been ordered. He pays for it on delivery and staples the receipt into the order book. At the end of every month, Joe goes through all the dockets and the order book and produces a financial report for the shareholders.
Draw a context level DFD and a level-1 DFD for this system.
Context Diagram
Find the people who send data into the system Often data is part of a PHYSICAL transaction When handing a bar of chocolate to a
shopkeeper, you are handing him/her a barcode. Find the people who get data out of the
system. The only data you need is data that is
transformed or sent completely out of the system – not data that is handled by an operator within the system.
Joe’s YardJoe’s builders’ suppliers has a shop and a yard. His system is
entirely manual. He has a stock list on the wall of his shop, complete with prices. When a builder wants to buy supplies, he goes into the shop and picks the stock items from the list. He writes his order on a duplicate docket and pays Joe, who stamps the docket as paid. The builder takes the duplicate docket and he goes to the yard and hands it to the yard foreman. The yard foreman gets the ordered items from the yard and gives them to the builder. The builder signs the duplicate docket and leaves one copy with the foreman and takes one copy as a receipt. Every week, Joe looks around the yard to see if any of his stock is running low. He rings up the relevant suppliers and reorders stock. He records the order in his order book, which is kept in the yard. The yard foreman takes delivery of the new stock and checks it against what has been ordered. He pays for it on delivery and staples the receipt into the order book. At the end of every month, Joe goes through all the dockets and the order book and produces a financial report for the shareholders.
Draw a context level DFD and a level-1 DFD for this system.
Context diagram
Joe'sYard
Joe Customer
Supplier
Shareholders
Docket &Payment
Signed docket
Supply needs
Supply invoiceSupply order& payment
financialreport
Level-1 DFD processesJoe’s builders’ suppliers has a shop and a yard. His system is
entirely manual. He has a stock list on the wall of his shop, complete with prices. When a builder wants to buy supplies, he goes into the shop and picks the stock items from the list. He writes his order on a duplicate docket and pays Joe, who stamps the docket as paid. The builder takes the duplicate docket and he goes to the yard and hands it to the yard foreman. The yard foreman gets the ordered items from the yard and gives them to the builder. The builder signs the duplicate docket and leaves one copy with the foreman and takes one copy as a receipt. Every week, Joe looks around the yard to see if any of his stock is running low. He rings up the relevant suppliers and reorders stock. He records the order in his order book, which is kept in the yard. The yard foreman takes delivery of the new stock and checks it against what has been ordered. He pays for it on delivery and staples the receipt into the order book. At the end of every month, Joe goes through all the dockets and the order book and produces a financial report for the shareholders.
Verbs from script Has (passive) Buy supplies Picks stock items Writes order Pays joe Stamps docket Takes docket to yard Hands it to foreman Gets items Gives them to builder
Builder signs docket Takes copy as receipt Looks around yard and
reorders Records order in order
book Foreman takes delivery –
checks Foreman pays supplier Staples receipt to order
book Produces financial report
Remove passive verbs and queries
Passive: has stock list Buy supplies
Picks stock items (views list)
Writes orders Pays joe Stamps docket
Customer then Takes docket to yard Hands it to foreman
Gets items Gives them to builder Builder signs docket Takes copy as receipt
Joe then Looks around yard and
reorders Records order in order book
Foreman takes delivery – checks Foreman pays supplier Staples receipt to order book
Joe Produces financial report
Level 1 current physical
Customer
Buysupplies
20
getitems
12
Reordersupplies
21
Restock
22
*
Producefinancialreport
Joe's Office5
Joe
Orderbook
M3
DocketM1
Money
stock
Shareholders
Supplier
Foreman
*
Taketo
yard
11Supply needs
financialreport
Docket &Payment
Supply order
Docket
Signed docket
Signed docket
Signed docket
Payment
required stock
required stock
completeddocket copy
completeddocket copy
Supply order
supplies
supplies
Payment Payment
Supplierreceipt
Supplierreceipt
completeddocket copy
buildersignature
Buy Supplies
Buy supplies20 DocketM1
Money
Customer
*
WritesOrder
20.1
*
PaysJoe
20.2*
StampDocket(signatu-
re)
20.3
Docket
Payment
Docket
Payment Docket
Get Items
get items12
Foreman
stock
*
Give itemsto customer
12.1*
Get buildersignature
12.2
*
Givecopy
as receipt
12.3
Customer
Signed docket
required stock
buildersignature
Signed docket
completeddocket copy completed
docket copy
Reorder supplies
Reorder supplies21
OrderbookM3
Joe
*
Reorderfromyard
21.1
*
Recordorder
21.2
Supply order
Supply needs
Supply needs
Restock
Restock22
stock
Orderbook
M3
Supplier
Money
*
Takedelivery
22.1
*
Paysupplier
22.2
supplies
Payment
Supplierreceipt
Supply order
supplies
Payment
Supply order