A context diagram is a graphical representation of a system which must only use one process to represent the entire system and deliberately does not go into defining all the processes so as to prevent people getting bogged down in complex details at an early stage. There are only three symbols used in a context diagram:
- A circle to represent the system in terms of a single process. There will never be more than a single process in any context diagram,
- arrows to represent data flow,
- and a rectangle to represent any external entities affecting the system. There can be numerous external entities.
Many students find Context Diagrams confusing because they are so much like a DFD. This is is easy to explain because a context diagram is a level 0 DFD. This is the most basic type of DFD where all the processes and storage is represented by a single process. Context diagrams do not have storage. DFDs will always have storage.
Systems analysts draw data flow diagrams in several nested layers. A single process Context diagram can be expanded to show a more detailed data flow diagram. In the next level DFD the single process of the Context Diagram is broken down into several main processes and must include storage which previously was lumped inside the single process of the Context Diagram. Each successive layer is systematically broken down into more processes. This process can continue until the required level of detail is found.
Data flow diagrams
A Data flow diagram (DFD) is a much more complex representation of a context diagram. DFD show a further level of detail not shown in the context diagram. DFD’s identify the source of data, its flow between processes and its destination. It along also shows data generated by the system.
Data flow diagrams represent an information system as a number of processes that together form the single system. As mentioned earlier, a data flow diagram is a more detailed form of a context diagram (see above). It will show processes, inputs, outputs and storage. The Data Flow Diagram (DFD) shows the data flow between the processes within a system. A DFD can become as detailed as the user requires. The DFD will become increasingly detailed as the levels go higher. However each level will tend to focus on extending one or more of the processes rather than the entire system.
The first level DFD shows the main processes within the system. Each of these processes can be broken into further processes.
The video example below shows the processes involved in sending an email.