IS3320 Developing and Using Management Information Systems Lecture 16: Data-Flow Diagrams 1 (Intro to Context-Level diagrams) Rob Gleasure

IS3320 Today’s lecture  What are DFDs?  Exercise

What are DFDs? DFD stands for Data Flow Diagram A DFD is a representation of a system that captures all of its important component pieces, as well as the interfaces between these components DFDs focus on the movement of data between components

So… how is that different from flowcharts? FlowchartsDFDs Flow of control/activityFlow of data Flows are sequentialFlows are parallel Flows are bound within a single time unit, e.g. over a minute, week, month, etc. Flows can happen at any time, including on demand

Why use DFDs? Use cases  Allows us to identify the key actors and uses of a system Flowcharts  Allows us to break down uses into sequential algorithmic processes, including how these processes are effected by decisions and conditions DFDs  Allows us to model the data and system resources available, as well as how these are required by different actors during different processes Used during analysis and design!

How do we create a DFD? A DFD is made up of four components ComponentSymbol (a la gane and Sarson) Processes External entities Data flows Data stores

Components: Processes Work or actions performed on data (inside the system) Receives input data and produces output  Labels should be verb phrases

Components: Processes Rules  Can have more than one outgoing data flow or more than one incoming data flow  Can connect to any other symbol (including another process symbol)

Rules  Needs to have input and output Components: Processes Incorrect!

Components: Data Store Is used in a DFD to represent data that the system stores  Labels should be noun phrases

Components: Data Store Incorrect! Rule Must have at least one incoming and one outgoing data flow

Components: External Entity External entity that is origin or destination of data (outside the system) Is the singular form of a department, outside organisation, other IS, or person  Labels should be noun phrases Note, sometimes the following terms are used:  Source – Entity that supplies data to the system  Sink – Entity that receives data from the system

Rule Must be connected to a process by a data flow Components: External Entity

Incorrect!

Components: Data Flow A path for data to move from one part of the IS to another  Arrows depicting movement of data Can represent flow between process and data store by two separate arrows

Components: Data Flows Summary of rules for data flows MAY connect a process to another process MAY connect a process to an external entity MAY connect a process to a data store May NOT connect an external entity to another external entity May NOT connect an external entity to a data store May NOT connect a data store to another data store

Spot the errors

Levels of DFDs DFDs are constructed separately across several layers, each of which serves a slightly different function  Context-level Shows system boundaries, the data flows between external entities and the system  Level-1 Shows system’s major processes, data flows, and data stores at a high level of abstraction (not detailed)  Level-2, Level-3, …, Level n Iteratively break down a system description down into finer and finer detail New DFDs must be balanced at each stage to ensure they are complete and correct

Steps in Constructing Context Level DFD 1. Read the narrative/collection of narratives describing the system 2. Create your main system process 3. Identify the external entities from the text (usually these will be among the nouns in the narrative) 4. Identify and name the data flows to and from these external entities to your main system process

Example Context Level DFD

Exercise Consider an ATM interface system  Users enter their account details and password  Users view balance  Users retrieve cash  ATM checks account details and balance against bank system  ATM updates the bank database when cash is distributed  ATM checks quantity of each bank note available from cashbox  ATM retrieves bank notes from cashbox  ATM updates quantity of each bank note available from cashbox

Want to read more? Sullivan, L.H. (2006). +Just Enough Structured Analysis, Chapter 9: Dataflow Diagrams  /512878e6e4b02e5615b4c5ed/ /Yourdon%20DF D.pdf Data Flow Diagrams: An Agile introduction 