Systems AnalysisRequirements structuring Process Modeling

Representing the system with data flow diagrams

Logic Modeling Represents the logic of the DFD’s processes

Data Modeling Represents the contents and structure of the

DFD’s data flows and stores

Process Modeling

Graphically represent the processes that capture, manipulate, store and distribute data between a system and its environment and among system componentsData flow diagrams (DFD) Graphically illustrate movement of data

between external entities and the processes and data stores within a system

Process Modeling

Modeling a system’s process Utilize information gathered during

requirements determination Structure of the data is also modeled in

addition to the processes

Deliverables and Outcomes Set of coherent, interrelated data flow

diagrams

Deliverables and outcomes

Context data flow diagram (DFD) Scope of system

DFDs of current system Enables analysts to understand current system

DFDs of new logical system Technology independent Show data flows, structure and functional

requirements of new system

Project dictionary and CASE repository

Data Flow Diagram Symbols

Data Flow

Data Store

Process

Source/Sink

DFD Mechanics

Data Flow Depicts data that are in motion and moving

as a unit from one place to another in the system.

Drawn as an arrow Select a meaningful name to represent the

data

DFD Mechanics

Data Store Depicts data at rest May represent data in

File folder Computer-based file Notebook

Drawn as two horizontal parallel lines The name of the store as well as the

number are recorded in between lines

DFD Mechanics

Process Depicts work or action performed on data

so that they are transformed, stored or distributed

Drawn as a circle Number of process as well as name are

recorded

DFD Mechanics

Source/Sink Depicts the origin and/or destination of the

data Sometimes referred to as an external entity Drawn as a square symbol Name states what the external agent is Because they are external, many

characteristics are not of interest to us

DFD Syntax

Context Diagram A data flow diagram (DFD) of the scope of an

organizational system that shows the system boundaries, external entities that interact with the system and the major information flows between the entities and the system

Level-O Diagram A data flow diagram (DFD) that represents a

system’s major processes, data flows and data stores at a higher level

Context Diagram:Automated food ordering system

Level-0 diagram:Automated food ordering system

DFD Guidelines

Inputs to a process are always different than outputs The same input may go in and out of a process

but the process also produces other new data flows as a result of manipulating the inputs

Objects always have a unique name In order to keep the diagram uncluttered, you

can repeat data stores and data flows on a diagram

Incorrect DFD (process is shown as a sink)

Decomposition of DFDs

Functional decomposition Act of going from one single system to many

component processes Repetitive procedure Lowest level is called a primitive DFD

Level-N Diagrams A DFD that is the result of n nested

decompositions of a series of sub processes from a process on a level-0 diagram

Level 1

Level 2

Balancing DFDs

When decomposing a DFD, you must conserve inputs to and outputs from a process at the next level of decomposition

Unbalanced DFDs

Decomposing a Process

Use Case

A set of activities the system performs to produce some output result.

Describe the tasks in the process

Identify the triggers for each task

List all inputs and outputs

Building use cases

1. Identify the major use casesAsk “Who?”, “What?”, “When?” and “Where?” about

the tasks and their inputs and outputs

2. Identify the major steps within each use case

Ask “How?” about each use case

3. Identify the major inputs and outputs for each step

Ask “How?” about each step

Data Flow Diagramming Rules

ProcessA. No process can

have only outputs (a miracle)

B. No process can have only inputs (black hole)

C. A process has a verb phrase label

Data StoreD. Data cannot be moved

from one store to another.

E. Data cannot move from an outside source to a data store

F. Data cannot move directly from a data store to a data sink

G. Data store has a noun phrase label

Data Flow Diagramming Rules

Source/SinkH. Data cannot move

directly from a source to a sink

I. A source/sink has a noun phrase label

Data FlowJ. A data flow has only one

direction of flow between symbols.

K. A fork means that exactly the same data go from a common location to two or more processes, data stores or sources/sinks

Data Flow Diagramming Rules

Data Flow (Continued)L. A join means that exactly the same data come

from any two or more different processes, data stores or sources/sinks to a common location

M. A data flow cannot go directly back to the same process it leaves

N. A data flow to a data store means updateO. A data flow from a data store means retrieve or

useP. A data flow has a noun phrase label

Advanced DFD Rules: Balancing

Composite data flow can be spilt into components data flows at the next levelInputs to a process must be sufficient to produce the outputAt the lowest level, new data flows may be addedTo avoid having data flow lines cross each other, can repeat data stores or sources/sinks on a DFD

Types of DFDsCurrent physical Process labels include the users and technology involved Data flows and stores are labeled with the name of the actual

physical media

Current logical Physical aspects of the system are removed

New logical Additional functions Obsolete functions removed Inefficient flows re-organized

New physical Defer until several alternative solutions have been considered.

Conservation of Data

Data stays at rest until moved by a process

Processes cannot consume or create data Black hole error (no output) Miracle error (no input)

Guidelines for Drawing DFDs

1. Completeness DFD must include all components

necessary for system Each component must be fully described

in the project dictionary or CASE repository

Guidelines for Drawing DFDs

2. Consistency The extent to which information contained

on one level of a set of nested DFDs is also included on other levels

3. Timing Time is not represented well on DFDs Best to draw DFDs as if the system has

never started and will never stop.

Guidelines for Drawing DFDs

4. Iterative Development Analyst should expect to redraw diagram

several times before reaching the closest approximation to the system being modeled

5. Primitive DFDs Lowest logical level of decomposition Decision has to be made when to stop

decomposition

When to Stop

Rules for stopping decomposition When each process has been reduced to a

single decision, calculation or database operation

When each data store represents data about a single entity

When the system user does not care to see any more detail

When to Stop (continued)

When every data flow does not need to be split further to show that data are handled in various ways

When you believe that you have shown each business form or transaction, on-line display and report as a single data flow

When you believe that there is a separate process for each choice on all lowest-level menu options

Final DFD Notes

  when to show a direct data flow between processes and when to decouple these with a data store

  what activities to encompass with each process

  how to distinguish processes from sinks and sources

Using DFDs as Analysis Tools

Gap Analysis The process of discovering discrepancies

between two or more sets of data flow diagrams or discrepancies within a single DFD

Inefficiencies in a system can often be identified through DFDs

Using DFDs in Business Process Reengineering

Characteristics of a DFD that imply areas for improvement:

Processes that simply collect and pass on information (rather than transforming data)

Collecting the same information into several processes

Placing untransformed data into data stores (causing unknown delays in processing this data)

Cycles or loops that have no apparent termination

Logic Modeling

Data flow diagrams do not show the logic inside the processesLogic modeling involves representing internal structure and functionality of processes depicted on a DFDMethods Structured English Decision Tables & Trees

Modeling Logic with Structured English

Modified form of English used to specify the logic of information processes

Uses a subset of English Action verbs Noun phrases No adjectives or adverbs

No specific standards

Modeling Logic with Structured English

Similar to programming language Sequence statements Conditional statements (choice) Iterative statements (repetition)

Modeling Logic with Decision Tables

A matrix representation of the logic of a decision

Specifies the possible conditions and the resulting actions

Best used for complicated decision logic

Modeling Logic withDecision Tables

Consists of three parts Condition stubs

Lists condition relevant to decision Action stubs

Actions that result for a given set of conditions Rules

Specify which actions are to be followed for a given set of conditions

Modeling Logic with Decision Tables

Indifferent Condition Condition whose value does not affect which action

is taken for two or more rules

Standard procedure for creating decision tables Name the condition and values each condition can

assume Name all possible actions that can occur List all rules Define the actions for each rule Simplify the table

Decision trees

Depicts a decision as a connected series of nodes and actions

Each node is a decision point with at least two branches

Branches are the available options for each node

Endpoints are the actions

Which logical model?Criteria SE D Table D Tree

Portraying simple logic

Better Worst Best

Portraying complex logic

Better Best Worst

Transforming conditions & actions into sequence

Best Worst Best

Checking consistency & completeness

Worse Best Best