Introduction to Information Systems: In this section of the course we start to think of the computer as just being a component in a system which may contain one or many computers linked together. An Information System is a system that takes data and adds value to it to make it into (more useful) information. Data might be a list of football scores or the daily temperatures for Perth over the year. This data becomes information when say it is sorted; ranked or having something done to it so that it has more meaning. Telephone books and catalogues are examples of paper based information systems (why?). (Data) (Information) We move from a general information system to a Computer Based Information System Examples of modern Information Systems would include an on line footy tipping competition; the School s Administration System; a banks EFTPOS system. (Can you explain why each of these are a computer based information system?) We now start the journey on designing an Information System. The person who starts the process is a Systems Analyst. They study the existing / proposed system to document what data is used and what happens to it as it moves through the system. Step 1 what happens to the data associated with the system. Modern systems design is characterised by considering the function of the system first, then deciding what physical implementation of this should be used. Data Flow Diagrams help by concentrating on the data, and do not have any relationship to a program's flow of control, or to the physical characteristics of the system. The data flow diagram (DFD) is the fundamental modelling tool for the systems analyst. It represents the essential functions of an information processing system in a highly abstract way using a minimum set of symbols. The emphasis is upon how to read, understand and hence draw data flow diagrams. Originally sourced from Newman College 6/4/2001 - Data Flow Diagrams- edited FDS for Computer Science 2009 1
Components of the DFD A system is defined as an interrelated set of components and methods, all working together to achieve some common purpose. A system also has a boundary distinguishing the system from its environment. An information processing system has three principal purposes: transmission, storage, and transformation of information. Thus an appropriate abstraction for an information processing system would depict it in terms of just three types of components: one which shows the movement of information, one which shows the storage of information, and one which shows the transformation of information. These components are the data flow, data store, and process (also called transform), respectively. A Data Flow Diagram models a specific information processing system by showing the relationships among components of these three types. During the analysis phase of development process, Data Flow Diagrams are useful for communicating with the user. Basic Definitions A data flow diagram is a network model of an information processing system. The arcs of the network represent data flows, and the nodes represent data stores, transforms, or selected elements of the environment. A data flow is a movement of information within the system or across the system boundary. A data flow which crosses the system boundary to enter the system is called a net system input or simply system input (data source). A data flow which crosses the boundary to leave the system is called a net system output or system output (data sink). A data store is a time-delayed repository of information, where data is kept temporarily or permanently. Avoid thinking of them in computer "file" terms as there are many quite different forms. EG the Canteen (system) uses a box to store the lunch orders till processed this is a data store. A process, or transform, is a transformation that inputs one type of data and outputs a different type. It is a conversion of data from one form to another. A process cannot output data without getting some in; data cannot be created in a process. EG in the Canteen the orders are taken from the Orders datastore and processed by sorting them into say Staff and Student lunch orders. To show the connection between a system and its environment, a fourth type of element is used. A source (origin) is a person or organisation or system outside the system under consideration that provides information to the system under study in the form of an incoming data flow. A sink (destination) is a person or organisation or system outside the system under study that receives a system output. Sources and sinks are collectively called external entities. These are often the users, suppliers of commodities, or customers of the system. Symbols There are two major schools of Data Flow Diagramming. One is that of Yourdon and De Marco the other is that of Gane and Sarson. Students are required to be able to draw diagrams in the standard set of conventions used for this course. Originally sourced from Newman College 6/4/2001 - Data Flow Diagrams- edited FDS for Computer Science 2009 2
The data flow lines are arched and free flowing, with arrowheads for direction and MUST be labelled eg new member data. Used in TEE course (based on DeMarco & Yourdon) External Entity Process Data Flow Data Store A data flow is a movement of information from one point within the system to another and has a direction. If we view the data flow as connecting the points between which it moves, we see it as an interface between a process and a file, or a process and another process, or between the system and an external entity. From the standpoint of a process, a data flow is an input or output. A data flow entering or leaving a data store is information stored or retrieved by an access to that data store. These aspects of information flow are shown explicitly on a data flow diagram. Data is what is important not the way it is implemented, e.g. a message delivered to you could be via phone lines, computer network, or the mail. The message would be the data, not how it got there. Examples of data stores in a non-automated system include: a tax table used in calculating federal income tax deductions from employee pay packets, a collection of invoices for a customer determining credit card entries on the monthly statement, and customer orders in a restaurant that are waiting to be filled as soon as a cook is available. The ability to change the form or content of information permits an information processing system to support a business or other organisation. Using the term transform for the system components which derive outputs from inputs emphasises the act of changing one set of data flows into another. However, transforms are sometimes called by a variety of other names such as process or function. In this context process is synonymous with transformation, as shown in the familiar phrase inputprocess-output. Transformations are the parts of the system that do the work. Associated with each transformation is a procedure or set of instructions for making its inputs into outputs. A data flow diagram merely Originally sourced from Newman College 6/4/2001 - Data Flow Diagrams- edited FDS for Computer Science 2009 3
shows which transforms are required in the system; the procedures themselves are not shown in a DFD. Logical (what happens to the data) vs. Physical (how do we implement it) You can keep to logical work in two main ways: 1. Avoid showing material (objects) flow; and instead show the data / information related to it. The materials have data like numbers, prices, and other information. This is what matters, not the actual substance. 2. Get rid of processes which are only physical alterations. These are often just change of information carrying medium, such as punching a source document onto cards. The data is the same, just the medium carrying it has changed. Drawing the DFD You will often be working from a description of the system, whereas in real systems work the analyst would be working from his own notes from the data collected from the business. Read this description carefully. Look for mentions of documents and displays. These show data flows. Documents which accumulate or have to wait for processing may show a data store. Note documents which cross the system boundary are net inputs or net outputs. Data stores will be indicated by mentions of files, manual or automated, but also by documents waiting to be processed as a group, and collections of data need by the system to do processing, such as tax tables, stock reference lists, price lists. Draw up a table as you do this detailed reading of the description. One important decision you have to make while doing this is to locate the system boundary. Where is the system boundary? How do we select and isolate the system we wish to represent from the complexity of the environment in which it is embedded and with which it interacts? Systems themselves come in hierarchies. Every system can be viewed as part of a more inclusive system. There is always a temptation to expand our area of study to obtain a more comprehensive perspective. You have to draw the line somewhere. You can't be concerned with the entire world and to try to solve all its problems in a DFD. The scope of our system should be sufficiently inclusive and our system description should be sufficiently detailed to serve our purpose, but no more. Avoid the temptation to put in too much. In practice, where to locate the system boundary is often one of the most difficult decisions an analyst has to make. The boundary defines the limits of the system. Inside the boundary is the system, the part of the world to be studied, described, structured, and usually, changed. Outside the boundary is the environment or context, the part of the world to be ignored except for a few important interactions at the boundary between the system and the environment. The top level (level 0) diagrams that we draw should show the system, not its environment, but selected interactions between them. Naming System Components Names or labels on a data flow diagram serve two purposes. Formally, they identify the individual components; from a practical point of view, they also communicate as much specific information about the system as possible. Every constituent of a data flow diagram must have a unique name to distinguish it from the other components of the same type. Originally sourced from Newman College 6/4/2001 - Data Flow Diagrams- edited FDS for Computer Science 2009 4
Appropriate names for the components of a data flow diagram are assigned as follows: Since data flows are moving packets of data, they take on the name of these packets. Since data stores are resting places for packets of data, they take on the name of the data that reside there. Avoid names like "sequential file", or "sorted magnetic tape file" which often come from reading written descriptions of systems. Give it a name showing what data it contains and some way of locating it eg Secretaries In Tray. Since processes change incoming data flows into outgoing data flows, their names state the activity involved in the transformation and perhaps incorporate the names of the inputs and outputs. A verb must appear as a principal part of the name. Since source and sinks are producers and consumers of data flows respectively, they take on the name of the outside entity they represent a person, an organisation, or another system. It is customary to hyphenate the names of data flows and data stores. Remember that processes, in addition to being named, are often identified uniquely by numbers. Allowed connections among components The movement of information interconnects processes and data stores. It also connects the system to the sources and sinks of information in the system's environment. Data flows are thus the required interfaces between other types of system components. The conventions for permissible connections are shown below. Data flows may connect a process to any other type of system component. Data stores, sources, and sinks may not be connected to each other directly, but require an intervening process. Direct flow between external entities, is by definition, outside the system and is not included in a DFD. Allowed connections among components See Curriculum Council notes for example of Context Diagrams and Level O diagrams. Originally sourced from Newman College 6/4/2001 - Data Flow Diagrams- edited FDS for Computer Science 2009 5