IS1101 - Process Modelling Lecture

Process Modelling

Model – representation of reality
Logical and Physical
Process Modelling – technique for organising & documenting the structure and flow of data through a system.
Data Flow Diagram (DFD) – tool that depicts the flow of data through a system and the work or processing performed by that system.

A model is a representation of reality. Just as a picture is worth a thousand words, most models are pictorial representations of reality


Types of Models

Logical models show what a system is or does. They are implementation independent; that is, they depict the system independent of any technical implementation.

Physical models show not only what a system is or does, but also how the system is (to be) physically and technically implemented. They are implementation dependent because they reflect technology choices.

Why use Logical Models?

Logical models remove biases that are the result of the way the system is currently implemented, or the way that any one person thinks the system might be implemented.
Logical models reduce the risk of missing business requirements because we are too preoccupied with technical results.
Logical models allow us to communicate with end-users in nontechnical or less technical languages.

When to draw Process Models:

Strategic systems planning

Enterprise process models illustrate important business functions.

Business process redesign

“As is” process models facilitate critical analysis.
“To be” process models facilitate improvement.

Systems analysis (primary focus of this course)

Model the existing system including its limitations
Model the target system’s logical requirements (meaning processes and data flows needed regardless of how the system will be implemented)
Model candidate technical solutions (physical DFDs only)
Model the target technical solution (physical DFDs only)

Differences between System Flowcharts and Data Flow Diagrams

Processes on DFDs can operate in parallel (at-the-same-time)
    Processes on flowcharts execute one at a time
DFDs show the flow of data through a system
    Flowcharts show the flow of control (sequence and transfer of control)
Processes on one DFD can have dramatically different timing
    Processes on flowcharts are part of a single program with consistent timing

Process Modelling and DFDs

Process modelling is a technique for organizing and documenting the structure and flow of data through a system’s processes, and/or the logic, policies, and procedures to be implemented by a system’s processes.

A data flow diagram (DFD) is a tool (and type of process model) that depicts the flow of data through a system and the work or processing performed by that system.
DFDs have become a popular tool for business process redesign.


Standard symbols in a Data Flow Diagram


Sample System Level DFD:


Simple System Level DFD (from Whitten & Bentley, 2001)

System Thinking

Systems thinking is the application of formal systems theory and concepts to systems problem solving.
DFDs are a tool that supports systems thinking.

Problems typically solved in university are much smaller and simpler than those encountered in the real world.

Systems thinking is a technique that will pay off as problem size and complexity grow.


Process Concepts

A process is work performed on, or in response to, incoming data flows or conditions.

A System is a Process!



Decomposition is the act of breaking a system into its component subsystems, processes, and sub-processes. Each level of abstraction reveals more or less detail.


A decomposition diagram or hierarchy chart shows the top-down, functional decomposition of a system.

Types of Logical Processes:

A function is set of related and ongoing activities of a business.
An event (or transaction) is a logical unit of work that must be completed as a whole (as part of a function).

An elementary process (or primitive process) is a discrete, detailed activity or task required to respond to an event. Usually, several such tasks must be completed to respond to an event.



Whitten & Bentley (2006)


Data Flows & Control Flows

A data flow represents an input of data to a process, or the output of data from a process.
A data flow may also be used to represent the creation, reading, deletion, or updating of data in a file or database (called a data store).

A composite data flow is a data flow that consists of other data flows.
A control flow represents a condition or non-data event that triggers a process.
Used sparingly on DFDs.


Data Flow Packet Concept

Basically, we try to summarise the data flows.

Data Flows to and from Data Stores:


Examples of Illegal Data Flows:

Whitten & Bentley (2006)

What is an External Agent?

An external agent defines a person, organization unit, or other organization that lies outside of the scope of the project but that interacts with the system being studied.
External agents define the “boundary” or scope of a system being modelled.
As scope changes, external agents can become processes, and vice versa.
Almost always one of the following:
Office, department, division inside the business but outside the system scope.
An external organization or agency.
Another business or another information system.
One of your system’s end-users or managers

What is a Data Store?

A data store is an inventory of data.
Frequently implemented as a file or database.
A data store is “data at rest” compared to a data flow that is “data in motion.”
Almost always one of the following:
Persons (or groups of persons)
Events (about which data is captured)
Concepts (about which data is important)
Data stores depicted on a DFD store all instances of data entities (depicted on an ERD)

Modern Structured Analysis

Draw a context DFD to establish initial project scope.
Draw a functional decomposition diagram to partition the system into subsystems.
Create an event-response or use-case list for the system to define events for which the system must have a response.
Draw an event DFD (or event handler) for each event.
Merge event DFDs into a system diagram (or, for larger systems, subsystem diagrams).
Draw detailed, primitive DFDs for the more complex event handlers.
Document data flows and processes in the data dictionary.


Sample Context Data Flow Diagram (source: Whitten & Bentley, 2006)

Sample System Level Data Flow Diagram


Other Process Techniques

Problems with Natural English [Source: Adapted from Matthies, Leslie, The New Playscript Procedure, (Stamford, CT: Office Publications, Inc. 1977)]

Structured English

Structured English is a language and syntax, based on the relative strengths of structured programming and natural English, for specifying the underlying logic of elementary processes on DFDs.

Sample of Structured English

Policies and Decision Tables

A policy is a set of rules that governs some process of the business.
A decision table is a tabular form of presentation that specifies a set of conditions and their corresponding actions (as required to implement a policy).



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