Structuring system requirements: process modeling Chapter 8

Three views for structuring data requirements Process view: –the sequence of data movement and handling operations within the system (chapter 8)  process modelling Logic and timing view: –the rules by which data are transformed and manipulated and an indication of what triggers data transformation (chapter 9 & 12)  Logic modelling Data view: –the inherent structure of data independent of how or where it is processed (chapter 10)  Data modelling

Key Definitions A process model is a formal way of representing how a business operates Data flow diagramming shows business processes and the data that flows between them Logical process models describe processes without suggesting how they are conducted Physical models include information about how the processes are implemented

Objectives Understand the logical of processes modeling through studying examples such as data flow diagrams (DFD) Learn basic mechanisms of drawing and revising data flow diagram (DFD) Use flow diagrams as tools to support the analysis of information

Modelling for three main reasons 1.Facilitate understanding a system 2.Process modelling involves graphical representation of function and processes which capture, manipulate, store and distribute data between a system and its environment, and between component within a system 3.It supports communication with a systems analysts, end-users, and other roles

Reading a DFD

DFD Elements

DFD Shapes from Visio Visio 5.x Visio 2000

DFD – Practical Example Launched Dec. 11, 1998, the Climate Orbiter plunged too steeply into the Martian atmosphere Sept. 23, 1999, and either burned up or crashed. In an initial failure report released Oct. 15, 2000 the review board blamed the navigation error on a communications foul-up between NASA's Jet Propulsion Laboratory and prime contractor Lockheed Martin.

Data Flow Diagram (DFD) DFD is one of several notations that are called structured analysis technique It is a graphical tool that allows analyst-users to depict the flow of data in an IS It concentrates on the movement of data between processes DFD evolves four symbols for process modelling –Data flows –Data stores –Process –Source / sink

Data flows –They are groups of data that move and flow through a system from one place to another. –Include names of all individual elements of data moving –Data flow is represented by “arrow” E.g. results of grading mark Current score Final score Final grades

Data stores –They refer to data at rest that may represent one or many physical locations. –Each data store has a label and number (number of files location) –Data store is represented by a rectangle E.g. data about students (paper-based or floppy disk) Class/ grad file D1

Processes –They are the action performed on data so that they are transformed (manual or automatic), stored or distributed E.g. calculate class mark –Each process includes a name & process number –It is represented by rectangle with rounded corners Calculate Class Grad 1.0

Sources / sinks –They are the origin (sources) and/or destinations (sink) of the data, i.e. that supplies or receive data –They refer to external entities (or outside) to a system; boundaries of the system; boundaries –Each source/ sink has a name that states what the external agent is, e.g. students –A source (sink) is represented by square Teacher

Example of Data Flow Diagram Process Source / sink Data stores (files) Data flow Current grade Final grades Test scores Current score Final scoreFinal grades Student Calculate Class Grad Calculate University Grad Class/ grad file Student files D1 D2 Register’s office Teacher Save Open

Items not considered in DFD –Interaction between source (sink) –Actions of source (sink) are black box –How to redesign or control the source (sink) –How to provide source (sink) direct access to data,

Clarification of the concept source (sink) –Any organisation unit or other organisation which send or receive data (information) from the system you are analysing (e.g. supplier) –A person inside or outside the business supported by the system (e.g. customer) –Another system with which the system, you are analysing, exchanges data (information)

Context diagram of DFD Final grades Test scores Current score Final score Final grades Student Grading system Register’s office Teacher Context diagram is an overview of an organisational system that show the system boundaries, external entities that interact with the system, and the major information flows between the entities and the system

Level-0 diagram DFD could have many levels diagrams: level-0, level-1, etc. Level-0 diagram is a data flow diagram that represents a system’s major processes, data flows, and data stores at a high level of detail. Level-n A DFD that is the result of n nested decomposition of a series of sub processes from a process on a level 0 diagram –Example of level-0 diagram Current grade Final grades Test scores Current score Final scoreFinal grades Student Calculate Class Grad Calculate University Grad Class/ grad file Student files D1 D2 Register’s office Teacher Save Open

Context diagram of MacDonald food ordering system CustomerKitchen Restaurant manager Customer order ReceiptFood order Management report Food ordering system 0

Level-0 diagram Exercise in the classroom –Generate the level-0 diagram by detailing the main process –Food ordering system (process 0) could be split into three sub processes Receive and transform customer food order Update good sold file Update inventory file Produce management report

Rules of DFD / decomposition of process 4.0 Functional decomposition is an iterative processes of breaking the description of a system down into finer and finer details which creates a set of charts in which one process in a given chart is explained in greater details on another chart Produce management report 4.0 Daily inventory amount Daily Good sold amounts Management report Access good sold & inventory data 4.1 Inventory data Daily Goods Sold amounts Prepare management report 4.3 Aggregate good sold & inventory data 4.2 Good sold data Aggregate data Management reportDaily inventory amount

Rules of DFD / balancing Balancing is the conservation of inputs and outputs to a DFD process when that process is decomposed to lower levels

Which of the two diagrams is balanced or unbalanced? Produce report 4.0 Daily inventory amount Daily Good sold amounts Management report Access data 4.1 Inventory data Daily Goods Sold amounts Prepare report 4.3 Aggregate data 4.2 Good sold data Aggregate data Management report Produce report 4.0 Daily inventory amount Daily Good sold amounts Management report Access data 4.1 Inventory data Daily Goods Sold amounts Prepare report 4.3 Aggregate data 4.2 Good sold data Aggregate data Management report Daily inventory amount A B

A DFD Fragment Example

A Second DFD Fragment Example

Four types of DFD used in system development Current physical –Processes label include the name of people and technology used to process data –Data flow and data store include the name of actual physical media in which data flow or in which data are stored (file location, PCs, etc.) Current logical –Physical model is reduced to only essential part, i.e. reduced to data & processes that transform them (what and how) New logical –Models is the same as current logical model if users were happy about current functionality of the system (there were errors during implementation) –Others, new logical models include new functions according to users’ wishes New physical –Reflect decisions about which functions to automate or to leave manually

Output/deliverable of process modeling Process Modeling Input Information gathered from: interview, questionnaire, JAD sessions, etc. Output ( DFDs) Context Data Flow Diagram DFD of current physical system DFD of current Logical system (technology impendent) DFD of new logical system Description of each DFD components

Using DFD in the analysis process & guideline Correctness Completeness Inconsistency Timing Primitive DFD

Correctness and Completeness Correctness: it refers to drawing a correct DFD (see rules e.g. balancing, functional decomposition) Completeness: it refers to whether you have included in your DFDs all the components necessary for the system you are modeling. –Examples of completeness There are entities that are not connected to any thing else There is a process with only input and without output

Example of completeness: where is the error? Update inventory file Update good sold file Produce management report CustomerKitchen Good sold file Inventory file Receive & transform customer food order Good sold Inventory sold Format data Daily good sold amount Daily inventory amount

Consistency Consistency: it refers to whether or not the depiction of the system shown in at one level of a nested set of DFD is compatible with the depiction of the system shown in at other level –Example of inconsistencies Existing of a level-1 diagram with no high level -0 diagram Existing of a data flow at level-0 diagram but not at level-1 diagram

Example of inconsistencies: where is the error? (already seen) Produce report 4.0 Daily inventory amount Daily Good sold amounts Management report Level-0 diagram Aggregate data Access data 4.1 Inventory data Daily Goods Sold amounts Prepare report 4.3 Aggregate data 4.2 Good sold data Management report Level-1 diagram Daily inventory amount

Other guidelines DFD does not show timing, draw diagrams as if the system has never started and will never stop Iterative development: DFD recognize that system requirement determination & structuring are interacting Primitive DFD: it refers to the lowest level of DFD decomposition. When does a DFD decomposing stop? Very important idea: there is no good or bad modelling!!! Drawing DFD is facilitated by CASE upper tools (generating processes and diagrams) DFD can be used as an analysing tool to check correctness, inconsistencies and completeness DFD can be used as a tool to support Business Process Re-engineering (BPR) –Example of CASE tool, developed in SIMNET, used to generate processes (different from DFD) –It consists to generate processes manually and the system converts the process into a coding system

Context Diagram Shows the context into which the business process fits Shows the overall business process as just one process Shows all the outside entities that receive information from or contribute information to the system

Relationship Among DFD levels

Decomposition Diagram

Level 0 Diagram Shows all the processes that comprise the overall system Shows how information moves from and to each process Adds data stores

Level 1 Diagrams Shows all the processes that comprise a single process on the level 0 diagram Shows how information moves from and to each of these processes Shows in more detail the content of higher level process Level 1 diagrams may not be needed for all level 0 processes

Level 2 Diagrams Shows all processes that comprise a single process on the level 1 diagram Shows how information moves from and to each of these processes Level 2 diagrams may not be needed for all level 1 processes Correctly numbering each process helps the user understand where the process fits into the overall system

Data Flow Splits and Joins A data flow split shows where a flow is broken into its component parts for use in separate processes Data flow splits need not be mutually exclusive nor use all the data from the parent flow As we move to lower levels we become more precise about the data flows A data flow join shows where components are merged to describe a more comprehensive flow

Alternative Data Flows Where a process can produce different data given different conditions We show both data flows and use the process description to explain why they are alternatives Tip -- alternative data flows often accompany processes with IF statements

Your Turn At this point in the process it is easy to lose track of the “big picture”. Describe the difference between data flows, data stores, and processes. Describe in your own words the relationship between the DFD and the ultimate new application being developed.