software engineering-ii sir zubair sajid. 3 data flow diagrams (dfd) dfds describe the flow of data...
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Software Engineering-II
Sir Zubair Sajid
3
Data Flow Diagrams (DFD) DFDs describe the flow of data or
information into and out of a systemwhat 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 messyuse levelled DFDs - view system at different
levels of detailone overview and many progressively
greater detailed views
Level 0 or context diagram shows a system as a single process with inputs and outputs flowing to or from external entities.
Level 1 DFD will split up that single process into subsystems and show more detail about the data flows and data stores.
Level 2 DFD may decompose a single subsystem even further.
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
5
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 storesdata 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
Context Diagram Find the people who send data into the
systemOften data is part of a PHYSICAL transactionWhen 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.
Context diagramsa top-level DFD shows the least amount of detail and is known as a Level 0 DFD or context diagram.
Customer
Order
Invoice
Process orderOut-of-stock notice
Picking ListWarehouse
Order rejection notice
Context diagram
Joe'sYard
Joe Customer
Supplier
Shareholders
Docket &Payment
Signed docket
Supply needs
Supply invoiceSupply order& payment
financialreport
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 SuppliesBuy 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 suppliesReorder supplies21
OrderbookM3
Joe
*
Reorderfromyard
21.1
*
Recordorder
21.2
Supply order
Supply needs
Supply needs
Restock
Restock22
stock
OrderbookM3
Supplier
Money
*
Takedelivery
22.1
*
Paysupplier
22.2
supplies
Payment
Supplierreceipt
Supply order
supplies
Payment
Supply order