WEEK 8: PROCESS MODELING 4 (HIERARCHIES OF DFDs) (not in the textbook)
NOTE: Developing hierarchies of DFDs is a requirement for both the assignment and the project. It is highly recommended that you come to this class prepared to work on the assigned cases. You also need to bring to class Mini Marche system level DFD distributed in the previous week (DFD-3)
• LEVELS OF DFD’S: DESCRIPTIONS, NUMBERING OF THE PROCESSES AND CONTENTS:
Level
Description
Process numbering
Content
Requirement for the project
Context level
Models the system’s interactions with its environment (all external entities that interact with the system & external data stores, if any)
0.0
1 process,
all external entities, all I/O, external data stores (if any)
Only 1
Level-0
Models the interaction all the sub-systems of the entire system have with each other and the external entities
1.0, 2.0, 3.0, …
1 process representing each subsystem,
all external entities,
all I/O,
all shared data stores,
all interfaces (data flows between subsystems)
Only 1
Level-1
(example subsystem 2)
Models the interaction the processes within a single sub-system have with each other and the data stores and external entities that interact with this specific sub-system
2.1, 2.2, 2.3, …
All processes within P2.0,
all data flows within P2.0 and going into or out of P2.0,
all data stores within P2.0 or shared with P2.0,
all external entities interacting with P2.0
1 for each subsystem
Level-2
(example for P2.3)
2.3.1, 2.3.2, 2.3.3, …
All processes within P2.3,
all data flows within P2.3 and going into or out of P2.3,
all data stores within P2.3 or shared with P2.3,
all external entities interacting with P2.3
As needed
• STEPS IN DRAWING HIERARCHIES OF DFDs FOR YOUR PROJECT
• Identify shared DSs (they will appear in the level-0 diagram)
• Identify internal DSs (they will appear only in the DFD level they appear)
• Identify the processes within each subsystem level-1 DFD
• Now that you have identified the boundaries of each level-1 DFD in the above steps, you can draw each level-1 DFD separately
• Draw level-0 DFD
• Draw context level DFD
• GUIDELINES FOR DRAWING HIERARCHY OF DFD’S
• Note that Visible Analyst software automatically numbers processes as they are entered from top to bottom and left to right.
• You may postpone representing error and exception handling to the level-1 DFDs if it is necessary to reduce the clutter in context and level-0 DFDs.
• System level DFD is NOT included in the hierarchy of DFDs because it is only a working document. It is discarded once you understand the current system sufficiently to move on to modeling it with a hierarchy of DFDs
• EXAMPLE OF HIERARCHIES OF DFDS: MINI-MARCHE CASE CONTEXT AND LEVEL-0 DFDs
Context Level DFD
Level-0 DFD
• BALANCING
• The number of inputs to the system should be the same at each level of the DFD. The same is true for outputs from the system. For example, if there are 10 Inputs to the system at the context level DFD, there should be 10 inputs to all the processes at the level-0, etc.
• The only exception to this rule is the inputs and outputs introduced in a higher level of the DFD to represent the errors and exceptions.