Large Scale Systems Design
G52LSS
Lecture 15 – Constructing DFDs
•Steps
to Construct DFDs
•Examples of Creating DFDs
Learning outcomes: describe the steps to approach the construction
of multi-level DFDs; understand the relationship between DFDs and
use case analysis; develop context diagrams; develop Level 0 DFDs;
decompose DFDs; engage in discussions when creating DFDs.
University of Nottingham
School of Computer Science
Large Scale Systems Design
Dr Dario Landa-Silva
1
Steps to Construct DFDs
1. Build the context diagram
2. Create the level 0 diagram using (if necessary) the
requirements definition, use case diagrams, use cases,
user stories, summary of business activities, etc.
3. Decompose level 0 processes into level 1 DFDs
4. Decompose level 1 processes into level 2 DFDs and
decompose further if needed
5. Balance and validate DFDs to ensure completeness
and correctness
University of Nottingham
School of Computer Science
Large Scale Systems Design
Dr Dario Landa-Silva
2
Important Considerations When Constructing DFDs
• Keep
focus on modelling flow and storage of data
• Identify
processes that transform data
• Maintain
the balance between diagrams
• Assess
the need for decomposition
• Follow
conventions to name DFD elements
• Think
of data flows as ‘data in motion’
• Think
of data stores are as ‘data at rest’
• Think
of processes as actions performed on data
• Do
not worry about how data is produced or used by
the external entities
University of Nottingham
School of Computer Science
Large Scale Systems Design
Dr Dario Landa-Silva
3
Creating the context diagram
• Identify
‘actors’ or ‘user roles’ (external entities) from any
documentation available
• Identify the information that each external entity sends to
the system and receives from the system
Creating the level 0 DFD
• Identify
the major functions of the system (processes) from
any documentation available
• Identify
which processes produce the data sent to the
external entities and which processes require the data
produced by the external entities
• Identify
data flows and data stores and their interaction
• Data flow to data store means ‘update’, data flow from data
store means ‘retrieve’
University of Nottingham
School of Computer Science
Large Scale Systems Design
Dr Dario Landa-Silva
4
Creating level 1, level 2, etc. DFDs
• Functional
decomposition in an iterative process which
results in DFDs that give more details about a process
• The
lowest level DFDs are called primitive DFDs
• Describing
a process can help to decide whether it is
necessary to decompose the corresponding DFD
• If
a process requires to carry out several tasks (logical
functions), it is a good candidate for decomposition
• Ensure the conservation of inputs and outputs (data flows)
when decomposing processes (balancing)
• Data
splits and data joins can be used in the decomposition
process to provide more detailed view of how data is
transmitted
• Ensure
completeness, consistency, no timing dependence,
iterative development, creation of primitive DFDs
University of Nottingham
School of Computer Science
Large Scale Systems Design
Dr Dario Landa-Silva
5
Example 15.1 Creating a multi-level DFD for this scenario.
University Registration System. The system should enable
staff of each academic department to examine the modules
offered by their department, add and remove modules, and
change the information about them (e.g. the maximum
number of students permitted). It should permit students to
examine currently available modules, add and drop modules
to and from their schedules, and examine the modules for
which they are enrolled. Department staff should be able to
print a variety of reports about the modules and the students
enrolled in them. The system should ensure that no student
takes too many modules and that students who have any
unpaid fees are not permitted to register (students can verify
their fee paying status). Note: assume that a fees data store is
maintained by the university’s financial office and this data
store is accessed by the registration system but the fees data
store is not modified by the registration system.
University of Nottingham
School of Computer Science
Large Scale Systems Design
Dr Dario Landa-Silva
6
Example 15.1 (cont.) The Context Diagram
module enrolment
module offering list
0
module drop
enrolment report
Department
Staff
enrolment
report request
available
module request
University
Registration
System
financial status request Student
available modules
module
offering change
student schedule
financial status
financial
status request
financial
status
Finance Staff
University of Nottingham
School of Computer Science
Large Scale Systems Design
Dr Dario Landa-Silva
7
module drop
module offering change
1
module offering list
Dept
Staff
2
module
enrolment
request
Module Offering
3
Produce
Module
Enrolment
Reports
D3
module enrolment
student schedule
student
schedule
Enrolment
financial
status request
financial
status request
Verify
Financial
Status
financial status
payments update
Example 15.1 (cont.)
The Level 0 DFD
financial
status
4
enrolment
information
enrolment report
University of Nottingham
School of Computer Science
Student
available modules
module information
enrolment report request
available
modules request
Maintain
student
enrolments
module
offering
update
module
offerings
D2
available
modules
Maintain
department
module
offerings
Finance
Staff
D1
Large Scale Systems Design
Dr Dario Landa-Silva
payments history
request
Fees
8
Example 15.1 (cont.) Level 1 DFD
Process 2 (maintain student enrolments) of the Level 0
DFD can be decomposed to a number of child processes.
The corresponding use case and user story can be used to
guide the decomposition.
Examples of possible child processes are:
2.1 Validate student identity
2.2 Find fee paying status
2.3 Find available modules
2.4 Enrol student in module
2.5 Drop student from module
2.6. Find student schedule
University of Nottingham
School of Computer Science
Large Scale Systems Design
Dr Dario Landa-Silva
9
available modules
username
D4 Student Users
2.1
Validate
student
identity
new user details
user record
list of available modules
login message
available modules
2.3
Find
user
available registration
modules
fee paying history
available
module request
password
user ID
2.2
Find fee
paying status
user
registration
2.4
Enrol student
in module
module
enrolment
user registration
enrolment
add module request
student schedule
2.6
Find student
schedule
user registration
drop module
request
student schedule
University of Nottingham
School of Computer Science
2.5
Drop student
from module
student
schedule
drop
module
drop
module enrolment request
Large Scale Systems Design
Dr Dario Landa-Silva
10
Example 15.2 Creating a multi-level DFD for this scenario.
Health Club Membership System. When members join the
health club, they pay a fee for a certain length of time. Most
memberships are for one year, but memberships as short as 2
months are available. Throughout the year, the health club
offers a variety of discounts on their regular membership
prices (e.g. two memberships for the price of one for
Valentine’s Day). It is common for members to pay different
amounts for the same length of membership. The club wants
to mail out reminder letters to members asking them to
renew their memberships 1 month before their memberships
expire. Some members have become angry when asked to
renew at a much higher rate than their original membership
contract, so the club wants to track the price paid so that the
manager can override the regular prices with special prices
when members are asked to renew.
University of Nottingham
School of Computer Science
Large Scale Systems Design
Dr Dario Landa-Silva
11
The system must track these new prices so that renewals can
be processed accurately. One of the problems in the health
club industry is the high turnover rate of members. While
some members remain active for many years, about half of
the members do not renew their memberships. This is a major
problem, because the health club spends a lot in advertising
to attract each new member.
The manager wants the system to track each time a member
comes into the club. The system will then identify the heavy
users and generate a report so that the manager can ask
them to renew their memberships early, perhaps offering
them a reduce rate for early renewal. Likewise, the system
should identify members who have not visited the club in
more than a month, so the manager can call them and
attempt to re-interest them in the club.
University of Nottingham
School of Computer Science
Large Scale Systems Design
Dr Dario Landa-Silva
12
Example 15.2 (cont.) The Context Diagram
member details
update request
Member
renew details request
renewal info
club visit info
University of Nottingham
School of Computer Science
0
Health Club
Membership
System
delete member request
new member request
renew details request
current membership
Club
Manager
club usage report
Large Scale Systems Design
Dr Dario Landa-Silva
13
Example 15.2 (cont.) The Level 0 DFD
delete member request
update request
member details
1
Maintain
Members
Details
member record
member updates
D1 Members
member to delete
member record
Member
renewal offer
renew details
request
club visit info
3
Maintain
Club Usage
Records
2
Maintain
Memberships
Details
renew details request
Club
Manager
new membership details
current membership
details
renewal details
D2 Memberships
club usage report
usage details
club visit
University of Nottingham
School of Computer Science
current membership
new member request
D3 Club usage
Large Scale Systems Design
Dr Dario Landa-Silva
14
Example 15.2 (cont.) Level 1DFD
From the above Level 0 DFD, a good candidate for further
decomposition is process 2: Maintain memberships details.
This is because of the various types of memberships,
discounts and offers mentioned in the scenario narrative.
Examples of possible child processes are:
2.1 Validate renewal request
2.2 Find membership status
2.3 Find available membership types
2.4 Validate renewal payment
University of Nottingham
School of Computer Science
Large Scale Systems Design
Dr Dario Landa-Silva
15
Additional Reading
Chapter 6 of (Dennis et al., 2006)
Chapter 7 of (Kendall and Kendall, 2005)
University of Nottingham
School of Computer Science
Large Scale Systems Design
Dr Dario Landa-Silva
16