Systems Analysis & Design Data Flow Diagrams Continued

Data Flow Diagrams – Levels Many different DFD’s can be produced We can have… High (general view) of the system Lower level more detailed view of the system Every higher level diagram can be decomposed (exploded) into separate lower level diagrams Processes on the lower level diagram may be further exploded into additional diagrams

Data Flow Diagrams – Context Diagram Context Diagram includes only one process, ‘the system’ all external entities data flow to and from external entities Context diagram explodes to produce diagram 0

Data Flow Diagrams – Context Diagram Some external entity Library System Explode into Diagram 0

Data Flow Diagrams – Level 0 shows major processes & subsystems within the context diagrams single process data flow to and from external entities data stores Processes can be exploded into child DFD’s

Data Flow Diagrams – Level 0 Diagram 2 Customer Account 1 Record new borrower 3 Item Returns 4 New Item Buying 5 Item Lending Explode into Child Processes

Data Flow Diagrams – Child DFD’s Each higher level process can be decomposed/exploded into a child DFD Explode to the level where a detailed specification can be written for each process Processes can be exploded into child DFD’s

Data Flow Diagrams – Level 1 Diagram Customer 5.3 Checking out Check Out Loan request Update database 5.1 Customer account details Customer database Update Catalogue Catalogue database Customer details 5.2 Check for overdue items & fines Explode into Child Processes

Data Flow Diagrams – Reiteration Context Diagram: This is the highest level and shows the overall system and its interaction with the external environment Level 0 Diagram: This shows the major subsystems and their interactions Level 1 Diagram: Shows the processes that make up each of the major subsystems Level 2 Diagrams: Shows the details of each process within a level 1 diagram

Data Flow Diagrams – Reiteration Processes: Each process on one level gets "exploded" to show the detail at another level. These pairs are referred to as parent-child pairs. Numbering: Each process in Level 0 gets a number. Each child inherits the number. Each processes on the child DFD maintain the parent number, and begin their numbering as a number after the decimal point. E.g. 5.1, 5.2 are child processes of a parent process numbered 5 Each process must have a unique numbers.

Data Flow Diagrams – Extra Info Exploding: For each process on one level that gets "exploded" , no new data can be added within the child diagram. Inputs/Outputs: The inputs to a process must be sufficient to produce the all the outputs. Additional Symbols: To avoid having data flow lines crossing, you can repeat data stores and external entities on any diagram. You can use an additional symbol to show this repeated item, such as a repeated line for a store or a diagonal line for an external entity

Kell College Library Services The following activities are currently supported by manual brown-card procedures; Taking a book out on loan involves taking the book and your brown library tickets to the issue/return desk where the information is recorded. Returning items involves handing the books back to the desk,paying any fines due and receiving you library tickets back. Reserving a book involves filling in a reservation form and passing to the staff. The library member will then be notified by post when a copy of the reserved item has come in.

Kell College Library Services Activities cont’d; If books are overdue the library sends a reminder to the member concerned and the member will be warned that they cannot take any more books out of the library until they return the overdue book and pay any fines owed. The library stocks many book titles. For each book title there may be many copies, of which some may be either on loan or on the shelves or being repaired. The brown card system maintains details of all book titles and the number of copies . This information is stored using paper records The brown card system maintains details of all library members. This information is stored using paper records

Kell College Library Services Task 1 Create an Event table listing the activities that occur in the library. Here is a fragment of an event table.. Establish the major inputs and outputs of the system, the source and destination and represent the inputs/outputs using a context diagram. Event Description Event Trigger Trigger Source Actions Output Response Output Destination Customer wishes borrow an item Lending request Customer View customer details and check out items Update borrowers database and item database Borrowers database and Item database

Kell College Library Services Task 2: Using your event table…. Create a list of all external entities you can think of that may interact with the system Create a list of all the independent processes you think the system will have to carry out. Create a list of all independent data stores you think the system may have to support Once you are satisfied with your lists, start numbering your processes. I.e. Process Number Process Name 1. Joining 2. Validate 3. Lending

Kell College Library Services Task 3 Using the processes, external entities and data stores you listed in task 2… Choose an event from the event table and draw a data flow diagram for that event only. Select one of the processes from your DFD and draw exploded child DFD for it

Data Flow Diagrams – CASE Tools Computer aided software engineering CASE tools allow us to Draw and edit attractive diagrams easily Automatically maintain parent/child numbering and relationships Document processes, data stores, data flows and external entities Automatically generate text documents Examples of CASE tools are Select and SSADM

Data Flow Diagrams – CASE Tools Once you have sketched out your context and level 1 data flow diagrams, you will practice using a CASE tool. ` Here is a screen shot