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IS1101 Sample DFD

 

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A data flow diagram illustrates the flow of data through an information system. This means the data has to be inputted by a person(s) and received by others. For example, when you (EXTERNAL AGENT) use an ATM (PROCESS/SYSTEM) for a bank to withdraw money, request balance, pay bills etc. you do not communicate directly with the bank staff (EXTERNAL AGENT) although you know they will eventually be able to see that you withdrew money, viewed your balance or paid a bill. You will insert your bank card, enter a pin number and choose an option (DATA FLOWS INTO THE SYSTEM). Once you have withdrawn money for example (DATA FLOWS OUT OF THE SYSTEM), the details of the transaction are recorded - your account number, the date, the amount and the ATM machine (DATA FLOWS TO A DATA STORE). These may not be viewed immediately by the bank staff, they may be saved for future use. Instead they may be saved to a file or a table in a database (DATA STORES). Bank staff may also provide inputs into the system (applying bank charges to your account etc.) and receive outputs (customer activity reports/ATM cash shortage notices etc.).

 

Lets look at an example:

Argos has a store in Patrick Street, Cork. Customers come into the store and browse the catalog of products. Once they have located the product they would like to purchase, they can take the unique identification number each product has and enter it into the inventory devices on the shop floor. By entering the number, the system will be able to show the customer the details of the product, whether it required home delivery and how many are left in stock. If the customer wants to purchase the product, they fill in a slip of paper with the identification code and the desired quantity and queue for the check-out and give it to the sales assistant. At the check-out, the sales assistant will take the slip and input the identification number into the system via their keyboard. If it shows that there are still some of the product in stock, the customer will be asked for payment (cash, laser or credit card). Once the transaction is complete it is saved to the transaction file and the customer will be given a receipt which also shows what counter they should collect the product at and an approximation of how long they will be waiting. In the stock room as transactions occur, staff are notified of the product and quantity and will search the stock room and bring the product to the counter for collection. They will input into the system that the order has been filled and that the customer can collect it (the product catalog will be amended for stock level); the overhead screen will indicate to the customer that they may collect their order and at which counter. The customer may then pick up the product and leave the store. Periodically, as products are purchased the stock will run out (stock levels will be checked) and will have to be reordered from the supplier. Argos uses a wide number of suppliers so they keep a list of their contact details and the products they supply. Argos records all details relating to orders in the product-order file, this can be used to verify incoming orders later. The supplier receives a batch order for products periodically which they will fill and notify Argos of the delivery time and cost.

Ok. have you read it thoroughly?

Lets look at it again but we will underline any process, external agent, data flows and data stores identified in the narrative. External agents will be red; Processes will be blue; data flows will be underlined and data stores will be green.

Argos has a store in Patrick Street, Cork. Customers come into the store and browse the catalog of products. Once they have located the product they would like to purchase, they can take the unique identification number each product has and enter it into the inventory devices on the shop floor. By entering the number, the system will be able to show the customer the details of the product, whether it required home delivery and how many are left in stock. If the customer wants to purchase the product, they fill in a slip of paper with the identification code and the desired quantity and queue for the check-out and give it to the sales assistant. At the check-out, the sales assistant will take the slip and input the identification number into the system via their keyboard. If it shows that there are still some of the product in stock, the customer will be asked for payment (cash, laser or credit card). Once the transaction is complete it is saved to the transaction file and the customer will be given a receipt which also shows what counter they should collect the product at and an approximation of how long they will be waiting. In the stock room as transactions occur, staff are notified of the product and quantity and will search the stock room and bring the product to the counter for collection. They will input into the system that the order has been filled and that the customer can collect it (the product catalog will be amended for stock level); the overhead screen will indicate to the customer that they may collect their order and at which counter. The customer may then pick up the product and leave the store. Periodically, as products are purchased the stock will run out (stock levels will be checked) and will have to be reordered from the supplier. Argos uses a wide number of suppliers so they keep a list of their contact details and the products they supply. Argos records all details relating to orders in the product-order file, this can be used to verify incoming orders later. The supplier receives a batch order for products periodically which they will fill and notify Argos of the delivery time and cost.

Note: we only identified one process at this early stage. We will come back and look at this narrative again when we go to build a System Level DFD.

CONTEXT DATA FLOW DIAGRAM:

Assumptions:

SYSTEM LEVEL DATA FLOW DIAGRAM

Okay - lets read the narrative again to identify sub-processes (we do this so we can explode/decompose our context dfd).

Ask the question: What are the different, distinct actions occurring within this narrative?

We will underline possible sub-processes below (using a different colour each time):

Argos has a store in Patrick Street, Cork. Customers come into the store and browse the catalog of products. Once they have located the product they would like to purchase, they can take the unique identification number each product has and enter it into the inventory devices on the shop floor. By entering the number, the system will be able to show the customer the details of the product, whether it required home delivery and how many are left in stock. If the customer wants to purchase the product, they fill in a slip of paper with the identification code and the desired quantity and queue for the check-out and give it to the sales assistant. At the check-out, the sales assistant will take the slip and input the identification number into the system via their keyboard. If it shows that there are still some of the product in stock, the customer will be asked for payment (cash, laser or credit card). Once the transaction is complete it is saved to the transaction file and the customer will be given a receipt which also shows what counter they should collect the product at and an approximation of how long they will be waiting. In the stock room as transactions occur, staff are notified of the product and quantity and will search the stock room and bring the product to the counter for collection. They will input into the system that the order has been filled and that the customer can collect it (the product catalog will be amended for stock level); the overhead screen will indicate to the customer that they may collect their order and at which counter. The customer may then pick up the product and leave the store. Periodically, as products are purchased the stock will run out (stock levels will be checked) and will have to be reordered from the supplier. Argos uses a wide number of suppliers so they keep a list of their contact details and the products they supply. Argos records all details relating to orders in the product-order file, this can be used to verify incoming orders later. The supplier receives a batch order for products periodically which they will fill and notify Argos of the delivery time and cost.

Now, it is possible to go into more detail, but this is only a System Level diagram and if more detail (decomposition) is required it can be done in subsequent diagrams (Level 2 - one for each subprocess identified in Level 1 DFD).

Lets give each of these sub-processes are proper name:

We will also include the data stores which we identified earlier. The external agents and inflows/outflows remain the same. A sample System Level DFD is displayed below:

- notice the 4 subprocesses identified

- notice the use of the data stores (they have data flows associated which were not displayed in the Context DFD because they were internal to the system).

- There is nothing else new in this diagram. 

The difficult part of a System Level DFD is identifying the sub-processes. 

If you have questions about the text above or questions about DFDs or ERDs please e-mail the lecturer.

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