University of Sunderland ISIC 1 Data Flow Diagrams - Part 2 Hierarchical DFDs

University of Sunderland ISIC 1 Temporary Data store T1 Can use T to indicate a temporary data store usually manual eg in tray, where orders are placed before filing

University of Sunderland ISIC 1 Hierarchical DFDs DFDs are hierarchical Top-down stepwise refinement is employed Processes are expanded into sub-processes to show more and more detail Each expansion is a new level Highest level is level 0 aka a CONTEXT DIAGRAM Lower levels are numbered 1,2,3 etc, No limit to the number of levels allowed, but level 3 is usually enough

University of Sunderland ISIC 1 Components of a Data Flow Model Overall process Entity Level 0 Level 1 Process 1 2 3

University of Sunderland ISIC 1 Components of a Data Flow Model Overall process Entity Level 0 Level 1 Level 2 Process sub Process sub Process sub Process

University of Sunderland ISIC 1 Components of a Data Flow Model Overall process Entity Level 0 Level 1 Level 2 Level 3 Process sub Process sub Process sub Process sub-sub Process sub-sub Process sub-sub Process

University of Sunderland ISIC 1 Overall process Entity Level 0 Level 1 Level 2 Level 3 Level 4 Process sub Process sub Process sub Process sub-sub Process sub-sub Process sub-sub Process Components of a Data Flow Model

University of Sunderland ISIC 1 Level 0 DFDs - Context Diagrams Level 0 DFDs, aka Context diagrams are the highest level DFD, but not always required or drawn Level 0 DFDs depict the system as a single process Since there is only one process, there is no need to number the process

University of Sunderland ISIC 1 Level 0 DFDs – Context Diagrams The process description at Level 0 gives an overview of the system i.e. the activities on the Level 1 DFD Level 0 diagrams do not show Data stores External entities should be shown and referenced

University of Sunderland ISIC 1 Example Context Diagram Sales, Despatch & Warehouse Accounts Customer Supplier Order Delivery note Order acknowledgement Weekly credit limit list Copy delivery note Re-order card Goods received note Goods received note a b c

University of Sunderland ISIC 1 GOODS RECEIVED NOTE (GRN) –A document produced when goods are received into the factory. It will usually accompany goods to any inspection and is used to check against invoices before payment..

University of Sunderland ISIC 1 delivery note Definition Document accompanying a shipment of goods that lists the description, grade, and quantity of the goods delivered. A copy of the delivery note, signed by the buyer or consignee, is returned to the seller or consignor as a proof of delivery.

University of Sunderland ISIC 1 Level 1 DFDs First real DFD, i.e. always included Show processes, external entities, data stores physical resources if required Number all processes 1,2,3 etc External entities must have the same references as Level 0 (if it was drawn)

University of Sunderland ISIC 1 Example - Level 1 DFD a b c b

University of Sunderland ISIC 1 Level 2 DFDs If a process cannot be expanded any further, put an asterisk in the lower right hand corner *

University of Sunderland ISIC 1 Example - Level 2 DFD In our example, just process 3 Stock control can be expanded to a Level 2 diagram In the Level 2 diagram, the Stock control process is broken down into 4 sub-processes Processes are numbered 3.1, 3.2, 3.3 etc Process 3 has a Data store, called Stock, which was not present at Level 1, so its ID is M3/1, the first data store within process 3

University of Sunderland ISIC 1 Level 2 DFD - Stock Control b bc

University of Sunderland ISIC 1 Level 3 DFDs Notice, from our Level 2 DFD, sub process 3.3 does not have an asterisk, so it can be expanded further...

University of Sunderland ISIC 1 Level 3 DFD - Monitor Stock Levels

University of Sunderland ISIC 1 Level 3 DFDs Note processes are now numbered 3.3.1, etc New data store has the ID M3.3/1 All these Level 3 sub processes cannot be expanded further You will not be asked to go beyond Level 2 in this Module

University of Sunderland ISIC 1 Good Practise Level 0 diagram shows the whole system as just one process Level 1 diagram has one process for each department Level 2 shows what happens within each department Level 3 could show processes performed by a single named job role – eg warehouse supervisor This is not always possible, but is very effective when it can be done

University of Sunderland ISIC 1 Drawing DFD Tips Identify data flows, data stores and processes, as described in the last lecture Draw processes first, down the centre of the page Add external entities down the left hand side Draw data stores down the right hand side Finally add the data flows Flow lines should not cross, use duplicate symbols to avoid crossing lines and keep DFD neat and tidy...

University of Sunderland ISIC 1 Multiple Instances on a Single DFD For each instance of the duplicate element, add a line to the symbol, like so … Use duplicates to keep your DFD clear and easy to follow.

University of Sunderland ISIC 1 Duplicate Processes Is possible, but should be avoided ie duplicate external entities and data stores Don’t duplicate Processes, unless you really have to Duplicated processes complicate DFDs Which defeats the objective

University of Sunderland ISIC 1 Drawing DFDs in this way encourages a step-by-step approach, that should result in diagrams which are better laid out and are easier to read. You need to know the terminology and conventions for exams. Practice interpreting case studies and drawing the DFDs. Read through the examples in Lejk and Deeks.

University of Sunderland ISIC 1 Drawing DFDs in this way encourages a step-by-step approach, that should result in diagrams which are better laid out and are easier to read. You need to know the terminology and conventions for exams. Practice interpreting case studies and drawing the DFDs. Read through the examples in Lejk and Deeks.

University of Sunderland ISIC 1 Rules for drawing DFDs Processes: No process can have only output No process can have only input If you think a process has only input or output, then it must be an external entity (source/sink) A process name must contain a verb

University of Sunderland ISIC 1 Rules for drawing DFDs Data Stores: Data cannot move directly from one data store to another data store - data must be moved by a Process Data cannot move directly from an external entity to a data store or vice versa - data must be moved by a process The name of a data store must contain a noun

University of Sunderland ISIC 1 Rules for drawing DFDs External Entities: Data cannot move directly from one external entity to another. It must be moved by a process, otherwise don’t show the data flow on the diagram The external entity name must contain a noun

University of Sunderland ISIC 1 Rules for drawing DFDs Data flow: A data flow has only one direction of flow between symbols. It may flow in both directions, but these are usually shown as 2 separate arrows since they happen at different times A data flow cannot go directly back to the same process it leaves A data flow to a data store means update (append, delete or change) A data flow from a data store means retrieve or use A data flow label will contain a noun