Learning Dfd

CSE1204 - Information Systems 1

Data Flow Diagrams

Levelling (last lecture recap) Process Modelling Using Function Decomposition

Levelling DFDs

1 2 Context diagram 3 Level zero diagram 3.1 3.2 Diagram 3 (level 1) Further diagrams or process descriptions 2.1 2.2 Diagram 3 (level 1)

Levelling of DFDs

Context

Level 0

1

2

3

4

Level 1 Level 2

1.1

1.2

1.3

2.1

2.2

3.1

3.2

4.1

4.2

4.3

1.1.1 1.1.2

3.2.1

3.2.2

1

Guidelines for Levelling DFDs

External communication: external agents represent entities in the environment of our information system external agents are outside the scope of our information system we do NOT model interactions between external agents we do NOT allow external agents to interact directly with data stores

Guidelines for Levelling DFDs

numbering: when a process is decomposed, its diagram is given the same number as that process balancing of levelled DFDs: all data flows entering and leaving a process must appear on the corresponding diagram which decomposes that process external agents: are only included on the two diagrams which represent the entire system, i.e. the context and level zero diagrams

Guidelines for Levelling DFDs

the access to data stores across levels of diagrams must be consistent: • the direction of accesses must match and all accesses on higher level diagrams must appear on corresponding lower level diagrams • a data store is first shown on the highest level diagram where it is accessed by more than one process • it can then appear on all lower level diagrams where it is accessed

2

Guidelines for Levelling DFDs

the access to data stores across levels of diagrams must be consistent:

1 1.1 1.2 1.3 2

Guidelines for Levelling DFDs

How many levels should be in a set of DFDs?

each diagram usually has between 3 and 7 processes level the diagrams until bottom level or primitive processes are reached: primitive processes have only 1 or 2 inputs...