CIF 102 FIF 102 Session 3 Modelling the system

1
CIF 102 / FIF 102 Session 3Modelling the system
2
Outcomes

• To understand how to create a conceptual model of
  the system
• To understand how to capture the data required
• To have a basic understanding of and be able to
  apply some of the techniques used in Soft Systems
  Analysis and design methodology (SSADM)

3
Introduction

• SSADM
• What is a Data Flow Diagram?
• Why do we use DFDs?
• The Elements
• Conventions
• Decomposition and Abstraction
• Process and Data Stores
• External Entity
• Data Flow
• The Levels
• Rules

4
S.S.A.D.M.

• S.S.A.D.M. - Structured Systems Analysis and
  Design Method
• Uses different techniques to model a system
• Data Flow Diagrams
• Entity Relational Model (Logical Data Stores)
• Normalisation

5
What is a Data Flow Diagram?

• Known as DFDs
• A way to model a real world situation
• They model the real world situation which aids
  discussion and clarity amongst users, analysts
  and designers.

6
Why do we use DFDs?

• It is a method used to transform the physical
  view and convert it into a logical view.
• The physical view - all documents involved
• The logical view - the data they contain
• All models such as SSADM aim to communicate with
  the user the analysts understanding of the scope
  of the required system

7
Constructing DFDs

• The Document Flow Diagram
• The Context Diagram
• Draw the external entities and data stores
• Level 1 Physical DFD
• Example Flatmates media system Physical DFD

8
Levelling

• Levels determine the amount of information shown
• Context diagrams show environment
• Each level shows more info. than the last
• DFDs are expanded or decomposed into levels.
• Separating each process into sub processes
• Uncovers more and more detail - we will be
  dealing with level 1 only

9
Conventions

• Balancing
• Process at lower level should have identical data
  flows if they flow out of a process
• Modelling Data Stores
• Only use DATA STORES used within this process on
  the diagram
• Numbering
• 1 - 1.1 - 1.1.1
• 1.2 - 1.2.1
• Labels
• Should carry as much meaning as possible

10
The Elements

• The four main elements of DFDs notation
• Data Flows, with a label to indicate what data is
  flowing
• Processes, that handle the data
• Data stores, within the system (diary, filing
  cabinet or computer file)
• External entities, outside sources of data

11
Process and Data Stores

• A process is made up of Data Stores

Destination (Place or Name)
Process Number
Process description Should be descriptive,
starting with a verb.
Can be M for manual or D for computer base data
stores.
Name of Store
M1
12
External Entity

• Is anything outside the system that is of
  interest to the system. Can be a person, a
  company or another system.

Customer a
Outside entity shows the Name and a lowercase
alpha character is used to uniquely identify it.
Customer a
If an outside entity is repeated for the purpose
of neat layout a line is added across the top.
13
Data Flow

• Is shown by a line with an arrowhead, indicating
  the direction of the flow of data.
• Each data flow should be named to indicate what
  data is being passed.
• Nouns or adjectives only no verbs are permitted.

14
The Levels

• Context - Overview - contains only one process
• Level 1 - Utilises all four elements
• Level 2 - A breakdown of a level 1 process
• Level 3 - A breakdown of a level 2 process
• There is no rule as to how many levels of DFD
  that can be used.

15
Rules

• It is a method for Identifying major processes
• Sequence not important - getting the Process
  correct is
• Context or Level 0 - Identifies the system/
  boundary/External Links
• Level 1 - Overview of function
• Level 2 - Breakdown to Understand

16
Constructing DFDs

• Draw a document flow diagram of the current
  system
• Draw a system boundary around the parts of
  interest for your database
• Draw a context diagram
• Identify processes (transactions) in the system
• Complete the Level 1 current physical DFD

17
The context diagram

• Examine the diagram to see which agencies are
  going to contain data stored in your system
• Put a box around this part- this is the context
  diagram
• There are other entities-we need to separate
  those external to the system
• The entities in the bubbles are external to the
  system we are going to design and data about them
  will not be stored by the system. The flatmates
  are all owners-what are the entities in the
  flatmates (owner) system?

Store
Friend
Owner
Maintains book and DVD data
18
Activity

• From this description of part of the current
  system, draw the external entities, process
  diagrams and datastores
• --------------------------------------------------
  ---------------------------------
• At present when each flatmate buys a book or DVD,
  they store the receipt in a filing cabinet. When
  a flatmate borrows a book or DVD she/he notes the
  name on a post-it. When someone borrows a book or
  DVD she/he notes the name and item on a post-it.
• The flatmates would like a system for a computer
  that will allow them to keep track of items- who
  a particular book or DVD belongs to and who has
  borrowed which item.

19
Draw the external entities and the datastores
Owner
1
Friend
Sales receipts
M1
Maintains records of bought items
Owner
2
Store
Keeps track of items lent to and borrowed from
friends.
Collection of post-it notes
M2
20
Connect the external entities and the data stores
Sales record data
Owner
receipt
1
Store a
M1
Sales receipts
Maintains records of bought items
Who owns what data
Owner
2
Verbal request
Loan data
Keeps track of items lent to and borrowed from
other flatmates
Friend b
Collection of post-it notes
M2
Returned item data
M1 and M2 are manual storage systems you can
also have digital storage denoted by D1 etc.
These data stores will eventually be the tables
in your database. The replacement logical,
electronic database will have entities that
relate to these tables. The external entities are
not represented in the system friend could be
someone not a flatmate.
21
What are the links between the DFDs and the
Entity Relation Diagram?

• Look at the ERD from last session

Owner
owns
owns
Is owned by
is owned by
DVD
DVD
Book
is lent to
is lent to
Borrowed by
Borrowed by
DVDs lent out
Books lent out
22
Linking the DFDs to the ERD

• The post-it notes and sales receipts become data
  entered into the DVD and Book tables.
• The process diagrams become processes within the
  system
• so for example Process 2 maintain records will be
  the process for entering data into the books or
  DVDs lent out tables.

23
Summary of logical database design
methodology(adapted from the Chapter 17,the
PerfectPets worked example in Database Solutions,
Connolly and Begg, 2005)

• Identify entities
• Identify relationships
• Identify attributes for each entity
• Identify data types for each attribute
• Determine the primary keys for each entity
• Check the model supports the transaction
  requirements (processes in the system).
• Create tables
• Check tables support transactions

24
Design first!

• The last slide showed guidelines for working on a
  new database like your group assignment. You can
  hack together a set of linked tables without
  going through this process but then find that
  after a lot of effort something major has to be
  changed and you may have lost a lot of time.
• The moral is design it properly from the start.
• We did not go through the process for the
  flatmates system because you needed to quickly
  learn the basics of using the Microsoft Access
  interface.
• You may be able to think of better ways to
  construct the logical data model for the
  flatmates system.

25
Further reading and practical work

• Tutorial session 5 from the booklet.
• You can read the PerfectPets case study in
  Connolly and Begg for a detailed step-by-step
  example of how to design a database
• Note that the diagrams use the 1 notation for
  one to many relationships and similar notation
  for other types of relationships. This is because
  it has been constructed using a proprietary
  design package.
• You can use either notation for your assignment.