1 Structured Analysis Techniques

2 Data Flow Diagrams

3 Process models  Process models show the overall process and the processes that are supported by the system  A systems analysis process model consists of data flow diagrams (DFDs).  A data flow diagram (DFD) depicts the flow of data through a system and the work or processing performed by that system.

4 Process models  Data flow diagrams (DFDs)  The DFD allows the system to be partitioned into independent units so that they, and thereby the system, can be more easily understood.  The graphical aspect of DFDs means that they can be used both as static pieces of documentation and as a communication tool.  DFD's contain four components:  processes  data flows  data stores  sources and sinks

Goods Purchase - Level 1 Order Goods Prepare check Receive delivery note Verify invoice order delivery note invoice Supplier Orders File Delivery Notes Invoice Filecheque Shop goods_request Supplier (Source) (Dataflow) (Process) (Datastore) (Sink)

6 Process models  DFD components  Processes  The rectangles represent Processes or work to be done. A process is work performed on, or in response to, incoming data flows.  A process must have at least one data flow into it and one data flow out of it.  Processes cannot exist independently.  External agents  The circles represent - the boundary of the system. An external agent defines a person, organization unit, other system or other organization that lies outside the scope of the project but that interacts with the system being studied  External agents provide the net inputs into a system and receive net outputs from a system.

7 Process models  DFD Components (cont.)  data flows  The arrows represent, inputs and outputs, to and from the processes.  A data flow is also used to represent the creation, deletion or updating of data in a file or database.  A good analogy is that of a conveyor belt in a factory which takes data from one 'worker' to another. Each 'worker' then performs some process which may then result in another data flow on the conveyor belt.

8 Process models  DFD Components (cont.)  data stores  The open-ended boxes represents files or databases etc.. Data stores correspond to all instances of a single entity in a data model.  A data flow from a data store to a process indicates that data are to be “read” for some specific purpose.  A data flow from a process to a data store indicates that data are to be created, deleted, or updated in/from that data store.

9 Illegal data flows Agent 1 Agent 2 file Agent 3 file Agent 3 file

10 Process models  DFD Leveling  DFDs allow the analyst or user to look at the system at different levels of detail. Even in quite small systems a level 1 DFD may contain many processes and look cluttered.  To overcome this it is usual to 'break down' the level one DFD into a set of DFDs, a practice known as levelling.  Context diagrams  A Context DFD represents a system at a high level of detail in terms of its inputs from external entities and its outputs to external entities.  A context diagram comprises one process box for the entire system, together with the external entities and the data flows that pass between them and the system.

11 Process models  Level 1 diagrams.  A level 1 DFD shows the system (as initially represented in a context diagram) in more detail.  A level 1 diagram must have exactly the same inputs and outputs as the context diagram since it represents the same system but in more detail.  The processes modelled in a level 1 diagram (and also in any further sub-levels), represent a breakdown of the activities which make up the system.  Level 2 diagrams  A level 2 diagram may exist for every process on a level 1 diagram. That is, each level 2 diagram relates to a single process on the corresponding level 1 diagram.

1 2 3 Sub- Process Sub-sub- Process Elementary Process Description -Decision tables -Decision Trees -Structured English Level 0 Level 1 Level 2 Level 3 Level 4 Overall Process

13 Process models  Advantages of DFDs  Since they are easily understood by users, they are easier to validate for correctness. It is therefore easy to determine whether requirements are correct.  DFDs allow the analyst to abstract to whatever level of detail is required so that it is possible to examine a system in overview and at a more detailed level.  The DFD specifies the system at a logical level rather than a physical level. The benefit of this separation is that the users can specify their requirements without any restriction being imposed of a design nature, for example, the exact hardware requirements.

14 Example 1  Description of a Purchasing System  The purchasing system receives requests for goods from the shop manager.  The purchasing system orders goods from a Supplier.  When goods are ordered, the order is sent to the Supplier, and details are stored in the orders file.  When the Supplier sends a delivery note this is recorded in the delivery-note file.  When the Supplier sends an invoice this is verified by reading the delivery-note file, and then recording the details in the invoice file.  Cheques are then prepared by reading the invoice file, checking this against the orders file, and then sending out a check to the Supplier.

15 Example 1 ProcessInputOutputExternal Agents Data Store Order Goods goods request order; order dets shop; supplier order-file Receive Delivery Note delivery note note details supplier delivery notes verify invoice invoice invoice details supplier invoice-file prepare cheque invoice details; cheque supplier invoice-file; order details order-file

16 Example 1 Goods Payment System delivery-note invoice order Supplier Shop goods_request cheque Goods Payment System - Context Level

Goods Purchase - Level 1 Order Goods Prepare check Receive delivery note Verify invoice order delivery note invoice Supplier Orders File Delivery Notes Invoice Filecheque Shop goods_request Supplier

18 Example 2  Another example - Order verification system  A customer sends in an order form containing details of their order, and their membership number. A check is made to ensure that they are a member.  After the order is verified for membership credentials, a check is made to verify that the items ordered are produced by the company.  If the order is valid it is used to create an invoice, which is sent on to the Order Fulfilment System.

19 Customer Order verification system Order Fulfilment order_form Invoice Example 2  Order Verification System - Context Level

20 Customer Verify club membership Verify item ordered Create invoice Order Fulfilment Member file Items file order_form member_dets verified order valid order items_dets invoice Example 2  Order Verification - Level 1