IS3320 Developing and Using Management Information SystemsLecture 18: Data-Flow Diagrams 3 – Level 1 ModellingRob Gleasure

R.Gleasure@ucc.iewww.robgleasure.com

IS3320

Today’s lecture level 1 modelling Exercise

Recap on DFDs Processes on DFDs can operate in parallel (unlike flowcharts which execute one at a

time)

DFDs show the flow of data through a system (unlike flowcharts which show the flow of control)

Processes on a DFD can have different timing, e.g. daily, weekly, event-related (unlike flowcharts where they are part of a single program with shared timing)

Iterative DFD creationOnce we have activities identified…

1.Construct Context Level DFD(identifies external entities and processes)

2.Construct level 1 DFD (identifies manageable sub-processes and data stores)

3.Construct Level 1- n DFD (more detailed breakdown of sub-processes…)

Moving Beyond the Context-Level While the context-level provides an overview, we really need more detail for effective system analysis and

design Each new level breaks apart each process and “decomposes” it into sub-processes to create a more

detailed DFD.

As we descend to more and more detailed levels of a DFD, there is a danger of forgetting what level you are on. Each level has different numbering for processes

Context Diagram Process labelled “0” level 1 Processes labelled 1.0, 2.0, 3.0, . Level 1 Processes labeled 1.1, 1.2, 1.3, . Level 2 Processes labeled 1.11, 1.12,...

Moving Beyond the Context-Level The steps can be thought of as follows

1. Consider the main functional processes involved in the systema) For a large organisation, this may be a breakdown by departmental

function, e.g. manufacturing, sales, marketing, etc.b) For a smaller system, this may be more activity-oriented, e.g.

purchasing, producing, recording Replace the main system process from the context-level DFD with these sub-processes

Moving Beyond the Context-Level

2.Link these sub-processes to the individual data-flows identified at the context-level

3.Add any additional data-flows that are required to represent data flow between sub-processes and other sub-processes*

Note: the added level of detail may also bring to light new data flows between sub-processes and external entities (in which case they should also be added to the context-level DFD)

Moving Beyond the Context-Level

5. Consider the types of data that must be stored by the system to support these data flows and add data stores for each of these types of data

6. Add data-flows to and from these data stores

An Example: Floyd’s Book Shop Floyd’s Bookshop system handles interactions between customers, staff members, and suppliers.

Customers order books and, once the books have been provided, pay for them. They may also register for discounts, in which case their details and discounts are stored in a customer records database. Staff members enter time-off requirements as they occur to management, as well as their hours worked at the end of every month to payroll. Management informs them of the work schedule at the beginning of the month and updates staff records as necessary. These staff records, as well as the sales records are also used by management to create reports). Management will also collect signed hours sheets and pass them to payroll for processing. Payroll pays staff on the last work day of each month and also updates staff records. Staff members inform Supply Dept. about book shortages and receive books from them once they have arrived. Supply Dept. pass orders to suppliers, as well as receiving books from them, checking and updating inventory and sales records as necessary. Supply then pass a request for procurement to management, who will send this payment directly to suppliers and process receipts.

Context-Level DFD for Floyd’s

Step 1: Decompose Floyd’s into Sub-Processes

Step 2: Add Data Flows between Floyd’s Sub-Processes

Step 3: Add Data-Flows to External Entities to Floyd’s Sub-Processes

Step 4: Add Data Stores to Floyd’s

Step 4: Add Data Stores to Floyd’s

Some Rules of Thumb Any level 1 DFD should probably have a max of 6 - 7 processes (if

you need more, you should probably leave them for the next level)

Processes should be named after verbs (with exceptions, e.g. the context level may be the system name, org. functions, etc.)

Data stores and external entities should be named after nouns

More levels aren’t necessarily better, stop once you feel you are communicating enough detail

Either model the movement of physical goods or don’t, just be consistent (‘logical DFD’ or ‘physical DFD’)

Exercise In groups of 2-3, draw Context-Level and level 1 DFDs for the following narrative

Imagine an online auctioneering system. Sellers contact the auctioneers in person, and an object is listed in an upcoming auction. Information about the listing is stored in a listings file. Information about the seller is stored in a sellers file. Every 2 weeks, the auctioneering system automatically launches a new online auction on the system, based on the newly listed objects. When a registered buyer sees an item they would like and wishes to make a bid, they enter the details into the auction system. They may also enter queries, which sellers typically answer through the system. Items receiving most bids are tweeted automatically, linking back to the listed items on the system. Once an auction ends, the seller is notified automatically of the winning bid, as is the responsible bidder, and the seller ships the item to the bidder directly. This sale is recorded in a list of completed sales, which also links to the listings and sellers files. Reports are automatically generated for management based on these files.