Data Flow Modelling Concepts  Data Flow Diagrams  External Entities, Data Stores, Processes and Data Flows  Elementary Process Descriptions  Levelling  Document Flow Diagrams

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 Sample 1Sample 2

Key Definitions  Logical process models describe processes without suggesting how they are conducted  Physical models include information about how the processes are implemented

Data Flow Diagrams aid communication Manager e Supplier d Purchase Order Cabinet M1 Stock File M2 * Receive Stock 2 Stock Clerk Manager e * Sell Stock 3 Cashier Customer a Purchase Order Stock List P.O. Matched Orders Orders Purchase Order Delivery Bought Goods Stock List Sold Goods Delivered Goods Order Stock 1 Stock Clerk M2 Stock File ProcessesData Stores External Entities

Data Flow Diagrams DFD Notation external entitydata flow processdata store  The DFD is a diagram that consists principally of four symbols, namely the external entity, the data flow, the process and the data store  Additionally, a physical flow can be shown on the DFD of the current system

Data Flow Diagrams External Entities d Supplier

Data Flow Diagrams Data Flows Cosmetics Goods Customer Details Data Flow (usual) Bi-directional Flow (rare) Flow Between External Entities (for convenience) Resource Flow (for convenience)

Data Flow Diagrams Process Sell Stock Cashier3

Data Flow Diagrams Data Stores D3 Suppliers Stock File M1T1 Unpaid InvoicesD1 Orders Digitised Manual Transient Duplicate

Data Flow Diagrams Decomposition

 Any process on a DFD may be broken up into several sub-processes which, when viewed collectively, make up that process  Thus for example we may break-up process 1 of the Small Stock System into that shown on the next slide: Data Flow Diagrams Decomposing Data Flow Diagrams

1 Order Stock Purchase Order Cabinet M1 Manager e Stock File M2 * Produce Stock List 1.1 * Record Purchase Order 1.2 Stock List Purchase Order Stock List Data Flow Diagrams Decomposing Data Flow Diagrams

levelling  The decomposition of a DFD into lower level DFDs is known as levelling  The DFD that shows the entire system is known as the ‘top level’ or ‘level 1’ DFD  The DFDs that contain more detailed views of the level 1 processes make up ‘level 2’ DFDs  Any level 2 process that is further decomposed gives rise to a level 3 DFD and so on Data Flow Diagrams Decomposing Data Flow Diagrams

Elementary Process Description System: Small Stock DFD Type: Current Process Name: Record Purchase Order Process Id: 1.2 Managers give the stock clerk a ready-made purchase order. The stock clerk places this order in the Purchase Order Cabinet. It is the managers’ responsibility to send the order directly to the supplier they have chosen. Each purchase order contains product information taken from the supplier’s price list. The date after which a delivery of goods will be unacceptable is also included. Data Flow Diagrams Decomposing Data Flow Diagrams

* Order Record Purchase 1.2 Data Flow Diagrams Decomposing Data Flow Diagrams

Data FlowData ItemRemarks Stock Listproduct name quantity in stock Purchase Ordersupplier name supplier address supplier’s product code product name quantity ordered purchase order date latest acceptable delivery date Purchase order contains one ‘supplier name’ but many ‘product name’ Data Flow Diagrams Context Diagrams

Data Flow Diagrams Starting from the Context Diagram  To develop a Context Diagram we carry out the following tasks: (i)Identify all sources and recipients of data from the system, i.e. external entities (ii)Identify the major data flows to and from the external entities (iii)Convert each source or recipient into an external entity symbol (iv)Add the data flows between each external entity and a single box representing the entire system

Supplier s Delivery Note r Purchase Order s Delivery Details s Invoice Purchaser s P.O. Quantities r Stock Report Customer r Dispatch Note Sales & Marketing s Customer Order rMatched C.O. #1 AccountsrMatched Invoices External Entity S or R Data Flow Data Flow Diagrams Starting from the Context Diagram

ZigZag Warehouse System Accounts e Customer d Purchaser b Sales and Marketing c Delivery Details Delivery Note Supplier a Purchase Order P.O.Quantities Despatch NoteMatched Invoice Customer Order Matched C.O. Copy #1 Invoice Payment Stock Report Data Flow Diagrams Starting from the Context Diagram

Document Flow Diagrams i. Identify all recipients and sources of documents, whether inside or outside the system boundary ii. Identify the documents that connect them iii. Convert each source and recipient into an external entity symbol iv. Add data flow arrows to represent each connecting document v. Add the system boundary to exclude the external entities identified in the context diagram

Document Flow Diagrams Supplier Invoice P.O.Clerk Supplier Delivery Times Stock Clerk Stock Clerk Stock Report Purchaser Stock Clerk Stock Report Despatch Supervisor Despatch Clerk Despatch Note Customer Customer Customer Order Sales & Marketing Sales & MarketingCustomer OrderDespatch Clerk Despatch ClerkDespatch ReportDespatch Supervisor Despatch Super.Matched Dsp RepDespatch Clerk Despatch ClerkMatched CO #1Sales & Marketing …. Source Document Recipient

Document Flow Diagrams Despatch Clerk Despatch Supervisor Customer Order Matched Despatch Rpt Despatch Report Matched C.O. Copy #1 Sales and Marketing

Data Flow Diagrams Converting Document Flow Diagrams What process generates this document flow? What process receives this document flow? Is the document stored by a process? Where is the document stored? Is the document created from stored data? What business activity triggers the process? Is the document a source of new data? To transform the Document Flow Diagram into a DFD we follow each document flow in turn, asking the following questions:

Manager e Supplier d Customer a Bought Goods Purchase Order P.O. Stock List Small Stock System Delivery Matched Orders Data Flow Diagrams Context Diagrams  A level higher than level 1, showing the whole system as a single process with external entities around it, is also possible:

Data Flow Diagrams Decomposing Data Flow Diagrams

Decomposition Diagram

 We can now follow each flow into and identify the elementary process responsible for it  A grouping of these elementary processes can then give us a first glimpse of the system’s Data Flow Model Data Flow Diagrams Starting from the Context Diagram

Check Delivery Place Goods in Delivery Dock Allocate Stock Location Remove Goods from Delivery Dock Store Goods in Depot b Supplier 2 Goods Receiving Check Delivery 3 Stock Keeping Store Stock M2 StockM1 Purchase Orders Matched P.O. New Stock P.O. Copy Delivery Note Data Flow Diagrams Converting Business Activity Models T2 Matched P.O.’s

 For a process to be complete, it needs to have both an input and an output (shown by data flows going into and coming out of it) Data Flow Diagrams Tips 2 Goods Receiving Check Delivery Matched P.O. T2 Matched P.O.’s b Supplier 2 Goods Receiving Check Delivery Delivery Note b Supplier 2 Goods Receiving Check Delivery Delivery Note T2 Matched P.O.’s Matched P.O.

Data Flow Diagrams Tips 2 Do something with it M2 A data store f Someone Something f Someone Something M2 A data store Same something WHY?

Creating Use Cases

Elements of a Use Case  Trigger -- event that causes the scenario to begin  External trigger  Temporal trigger  All possible inputs and outputs  Individual steps  Show sequential order  Show conditional steps

Scenario Template (Use Case)

Creating Data Flow Diagrams

Steps in Building DFDs  Build the context diagram  Create DFD fragments for each scenario  Organize DFD fragments into level 0  Decompose level 0 DFDs as needed  Validate DFDs with user

A DFD Fragment Example

DFD – Common Errors Error

DFD – Packet Concept

A Second DFD Fragment Example

Level 0 Tips  Generally move from top to bottom, left to right  Minimize crossed lines  Iterate as needed  The DFD is often drawn many times before it is finished, even with very experienced systems analysts

Composite & Elementary Flows

Tips for Level 1 and Below  Sources for inputs and outputs listed at higher level  List source and destination of data flows to processes and stores within each DFD  Depth of DFD depends on overall system complexity  Two processes generally don’t need lower level  More than seven processes become overly complex and difficult to read

Flows to & from Data Stores

Illegal Data Flows

Validating the DFD  Syntax errors  Assure correct DFD structure  Semantics errors  Assure accuracy of DFD relative to actual/desired business processes  User walkthroughs  Role-play processes  Examine lowest level DFDs  Examine names carefully

1. Use cases hold functional requirements in an easy-to-read text format 2. They make a good framework for non- functional requirements & project details. 3. Use cases show only the Functional req’ts. 4. Design is not done only in use case units. How should we gather requirements? Use cases?

Use cases do not collect formulae, state, cardinality, performance, uptime,...  Examples: 1. Order cost = order item costs * 1.06 tax 2. Promotions may not run longer than 6 months. 3. Customers only become Preferred after A customer has one and only one sales contact. 5. Response time is Uptime requirement is Number of simultaneous users will be...

Scenarios What if...? “Knows how to...” Use the scenarios directly to Design and Test the system.

Summary  The Data Flow Diagram (DFD) is an essential tool for creating formal descriptions of business processes and data flows.  Use cases record the input, transformation, and output of business processes.  Eliciting scenario descriptions and modeling business processes are critically important skills for the systems analyst to master.