ACSC 155 System Analysis and Design 3. Process Modelling

Dr. Stephania Loizidou HimonaDr. Stephania Loizidou Himona 11

ACSC 155 ACSC 155 System Analysis and DesignSystem Analysis and Design

3. Process Modelling3. Process Modelling


Process ModellingProcess Modelling

A technique used to organize and document A technique used to organize and document the system’s processesthe system’s processes

Decomposition Diagram Decomposition Diagram Data Flow Diagrams (DFDs)Data Flow Diagrams (DFDs)


Decomposition DiagramDecomposition Diagram

A tool used to depict the breaking of a A tool used to depict the breaking of a system into subcomponents.system into subcomponents.

A technique that helps the analyst visualize A technique that helps the analyst visualize and define hierarchies of objects (a and define hierarchies of objects (a hierarchical tree structure). The "thing" hierarchical tree structure). The "thing" represented by the parent object in the represented by the parent object in the diagram is composed of the "things" diagram is composed of the "things" represented by the child objects.represented by the child objects.


Decomposition diagrams can be used to Decomposition diagrams can be used to show hierarchies of processes, functions, show hierarchies of processes, functions, requirements, goals, entities, organizations, requirements, goals, entities, organizations, problems, families or virtually anything else. problems, families or virtually anything else.

They are a very powerful and general tool They are a very powerful and general tool that allows us to partition large, complex that allows us to partition large, complex groups of things into smaller, more groups of things into smaller, more manageable groups of things. manageable groups of things.


The first thing you need to create a The first thing you need to create a decomposition diagram is a decomposition diagram is a rootroot. . The root generally represents a complex The root generally represents a complex abstraction of some kind. abstraction of some kind. Then, that root is decomposed into one or Then, that root is decomposed into one or more more childrenchildren. The children, when taken as . The children, when taken as a whole, make up the entire root. a whole, make up the entire root. Each child is then decomposed into its Each child is then decomposed into its children. children. The process continues until the diagram The process continues until the diagram shows the desired level of detail.shows the desired level of detail.



Another exampleAnother example


Data Flow Diagrams (DFDs)Data Flow Diagrams (DFDs)

DFDs show how data flows around an DFDs show how data flows around an information system.information system.

They are a simple and powerful graphic They are a simple and powerful graphic technique which is both easily updated and technique which is both easily updated and easily understood by users.easily understood by users.

One of the main diagrammatic techniques of One of the main diagrammatic techniques of SSADM (Structured System Analysis and SSADM (Structured System Analysis and Design Methodology).Design Methodology).


DFDsDFDs

A hierarchical set of diagrams which is used A hierarchical set of diagrams which is used to define:to define:

- - the boundary of the system to be developedthe boundary of the system to be developed

- the information flow to and from the system- the information flow to and from the system

- data flows within the system- data flows within the system

- the functions used by the system- the functions used by the system


DFDs consists of the following DFDs consists of the following symbolssymbols

ProcessProcessShows a transformation of data, also referred Shows a transformation of data, also referred

to as a functionto as a function

nn is the number of the process, indicates the is the number of the process, indicates the level of the processlevel of the process

PNPN is the process name is the process name

n

PN


Data FlowData Flow

Data flow/Physical flow of dataData flow/Physical flow of data

External EntityExternal Entity

Source and/or destination of data (person, Source and/or destination of data (person, organisation unit, system or another organisation unit, system or another organisation interacting with the system). organisation interacting with the system).

DFName

EN


Data StoreData Store

Storage of dataStorage of data

DSn – Data Store numberDSn – Data Store number

DSName – Data Store nameDSName – Data Store name

Plus, some additional symbols that show Plus, some additional symbols that show optionality and repetitions of various optionality and repetitions of various symbolssymbols

DSn DSName


RulesRulesP. – processP. – process

D.S. – Data StoreD.S. – Data Store

E.E. – External EntityE.E. – External Entity

Plus, additionally, each data store must have at least Plus, additionally, each data store must have at least one input flow and one output flow (read and one input flow and one output flow (read and write)write)


How is DFD developed?How is DFD developed?

Identify inputs & outputsIdentify inputs & outputs Label all data flows Label all data flows Label all processes Label all processes Identify data storesIdentify data stores Label all External EntitiesLabel all External Entities Start again (be prepared)Start again (be prepared)


Data Flows betweenData Flows between

External entity and Process External entity and Process Data Store and Process Data Store and Process Process and ProcessProcess and Process

Note: Information (Data) held for any amount Note: Information (Data) held for any amount of time between processes is called a of time between processes is called a DataStore. DataStore.


ExampleExample DFD Level 0 (Context Diagram)DFD Level 0 (Context Diagram)

0

FIT DVD CLUB

SYSTEM

MEMBER

SUPPLIER

ACCOUNTING DEPARTMENT

Incomes Report

Application Form

New movies Info

Request for new movies

Request for movie availability and info

Results from customer inquiries

Member card

Account Statement

Payment

Rental info & receipt

Request for income reportLetter for overdue movies

and late return fees


Level 1 DFD Level 1 DFD (General Case)(General Case)


Data Flow Diagrams – Numbering RulesData Flow Diagrams – Numbering Rules


Level 1 DFD Level 1 DFD (example part)(example part)


Level 2 DFDLevel 2 DFD

Each Process box in the Top Level Each Process box in the Top Level diagram diagram will itself be made up of a number will itself be made up of a number of processes, and will need to be of processes, and will need to be decomposed as a second level diagram. decomposed as a second level diagram.

Basically, one for each process.Basically, one for each process.


Level 2 DFD Level 2 DFD (Example – (Example –

Decomposition of Process 1)Decomposition of Process 1)


Each box in a diagram has an identification Each box in a diagram has an identification numbernumber derived from the parent - in the top left derived from the parent - in the top left corner. corner. (The Context level is seen as box 0).(The Context level is seen as box 0).

Any box in the second level decompositionAny box in the second level decomposition may be decomposed to a third and then a fourth may be decomposed to a third and then a fourth level. level. (Very complex systems may possibly require decomposition of (Very complex systems may possibly require decomposition of

some boxes to further levels).some boxes to further levels). Decomposition stopsDecomposition stops whenwhen a process box can a process box can

be described with an be described with an Elementary Process Elementary Process DescriptionDescription using ordinary English. using ordinary English. (Later on the process (Later on the process will be described more formally as a Function Description using, for will be described more formally as a Function Description using, for example, pseudo code).example, pseudo code).


SummarySummary

A DFD is also known as a Process Model.A DFD is also known as a Process Model. Process Modelling is an analysis technique Process Modelling is an analysis technique

used to capture the flow of inputs through a used to capture the flow of inputs through a system (or group of processes) to their system (or group of processes) to their resulting output).resulting output).

The model is fairly simple in that there are The model is fairly simple in that there are only four types of symbols – process, only four types of symbols – process, dataflow, external entity and datastore.dataflow, external entity and datastore.


Process Modelling is used to visually Process Modelling is used to visually represent what a system is doing. It is much represent what a system is doing. It is much easier to look at a picture and understand easier to look at a picture and understand the essence than to read through verbiage the essence than to read through verbiage describing the activities. describing the activities.

System Analysts after talking with various System Analysts after talking with various users will create DFD diagrams and then users will create DFD diagrams and then show them to users to verify that their show them to users to verify that their understanding is correct.understanding is correct.

The process models can be created to The process models can be created to represent an existing system as well as a represent an existing system as well as a proposed system.proposed system.


Process – an activity or a function that is Process – an activity or a function that is performed for some specific reason; can be performed for some specific reason; can be manual or computerised; ultimately each manual or computerised; ultimately each process should perform only one activityprocess should perform only one activity

Data Flow – single piece of data or logical Data Flow – single piece of data or logical collection of information like a billcollection of information like a bill

Data Store – collection of data that is Data Store – collection of data that is permanently storedpermanently stored

External Entity – a person, organisation, or External Entity – a person, organisation, or system that is external to the system but system that is external to the system but interacts with itinteracts with it


Be aware of the basic rules for Process Be aware of the basic rules for Process Modelling:Modelling:- A series of data flows always starts or ends at an - A series of data flows always starts or ends at an external agent and starts or ends at a data store. external agent and starts or ends at a data store. Conversely, this means that a series of data flows Conversely, this means that a series of data flows cannot start or end at a processcannot start or end at a process- A process must have both data inflows and - A process must have both data inflows and outflowsoutflows- All data flow must be labelled with the precise - All data flow must be labelled with the precise data that is being exchangeddata that is being exchanged- Process names should start with a verb and end - Process names should start with a verb and end with a nounwith a noun- Data flows are named with descriptive nouns- Data flows are named with descriptive nouns


Rules - continuedRules - continued

- A data store must have at least one data inflow- A data store must have at least one data inflow- A data store cannot go between an external - A data store cannot go between an external agent and a data store, but a process must be in agent and a data store, but a process must be in betweenbetween- A data flow cannot go between two external - A data flow cannot go between two external entities, but a process must be in betweenentities, but a process must be in between- A data flow cannot go between two data stores, - A data flow cannot go between two data stores, but a process must be in betweenbut a process must be in between- External agents and data flows can be repeated - External agents and data flows can be repeated on a process model in order to avoid lines crossing on a process model in order to avoid lines crossing but do not repeat processes but do not repeat processes

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ACSC 155 System Analysis and Design 3. Process Modelling