Upload
bennett-jones
View
217
Download
1
Tags:
Embed Size (px)
Citation preview
Slide 1 of 24
System Development Tools, Techniques and Methods
Structured Process Modelling
Slide 2 of 24
What we will cover
Process Modelling in principle Data Flow Diagrams in principle Symbols and Semantics Process Modelling in action Data Flow diagrams in practice Other process modelling techniques
Slide 3 of 24
Context Diagrams and DFDs
Symbols System (for context only) External entity Data Flow Process Data Store System boundary (Material Flow)
Definitions in a moment
Slide 4 of 24
When process models are used Current physical
the how of now Current logical
the what of now Proposed logical
the what of the new Proposed physical
the how of the new
Slide 5 of 24
Current physical process model
two volunteers please one student one bar steward
Slide 6 of 24
External Entity
Person, organisation or system outside the system being considered
Communicates only with system process via data flows
Student
Serve bottle of Bud
1 Bar stewardRequest for bottle of Bud
Student
Slide 7 of 24
Data Flow
The direction of flow of data from source to recipient Always has a description
Serve bottle of Bud
1 Bar stewardRequest for bottle of Bud
Student
Request for bottle of Bud
Key element in determining data needed by and in system
Slide 8 of 24
Process
where something happens to data (transformed, validated, stored …)
always has a number always has a description (strong verb, strong
noun, maybe qualifier) always has a location/executor
1 Bar steward
Serve bottle of Bud
Slide 9 of 24
Data Store
a location where data is stored may be manual or computer (M or D) always has a number and descriptor
Till M1
Serve bottle of Bud
1 Bar Steward
Till M1
Money offered
Change notification
Slide 10 of 24
Material Flow
A newish, non-purist representation of the movement of material
Student Serve bottle of Bud
1 Bar stewardBottle of Bud
Slide 11 of 24
System (context only)
Represents the system as a whole may include non-computer functions shows no detail of internal processes or
data stores used only for context diagrams
UCAS Admissions System
Slide 12 of 24
Oversimple context diagram
BACS
Money and account details
Employee
Timesheet
Personal and account details
Money
Tax rules
NI rules
Other legal stuff
Statutory bodies
Payroll System
Payslip
Statutory payments and account details
Slide 13 of 24
Another Context Diagram
Customer
Video HireSystem
Membership Application Request
Membership Details
Returned Video
Fee
Hire Video
Membership Application Response
Membership card
Slide 14 of 24
Levelling
Top Level - Context diagram (also known as Level 0 DFD)
Next level - Level 1 DFD Next level - Level 2 DFD and so on you are unlikely to need more than 3! Numbering of lower level processes links
processes between levels
Slide 15 of 24
Levelling - Graphically
Context
Level 1
Level 2
L
e
v
e
l
3
Slide 16 of 24
Textual descriptions
Every symbol represented on a process model should have a more detailed textual description linked to it
Can be achieved in CASE tool Can be achieved with other, linked text (which
is what you will do for your assignment) Provides explanatory detail
Slide 17 of 24
Example of levelling
Level 0 Video Hire System
Level 1 Process 1 Administer Membership
Level 2 Process 1.1 Receive membership
application
Slide 18 of 24
Data dictionary
A precise and unambiguous way of storing data about the data identified in data flows and data stores on DFDs
Links DFDs and ERMs (you will understand more about this before
Christmas) Provides the basis for ER Modelling Needed for SAD assignment 2 – see Britton and
Doake’s Structure Analysis book, Chapter 4
Slide 19 of 24
Problem/Requirements list
A numbered list of all the problems with the current system and (usually hence) requirements of a new system
Does not differentiate between whether issue is a problem or a requirement
Serves as a check-list when conceiving and designing new system
Helps to prevent problems and requirements becoming lost in the depths of analysis and design methods
Slide 20 of 24
Things we haven’t covered
rules for building models “sensibleness” checks for DFDs
Britton and Doake Chapter 4 detail of textual descriptions
Tutorials, self guided study, Valachi, George and Hoffer and assignment
Slide 21 of 24
Health warning
Process modelling only makes sense when you do it (it seems trivial otherwise)