Jackson System Development
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JSD (Jackson System Development) is a software development method developed by Michael A. Jackson and John Cameron.
JSD was described in Jackson's book System Development (1983) and in the collection of articles JSP and JSD, edited by John Cameron (1989).
Arguably, JSD was the first "object-oriented" system design method, as its basic conception of a computer system was as a network of communicating sequential processes (objects).
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Overview
Jackson System Development evolved from ideas in Jackson's earlier, programming design, methodolgy, JSP. Between 1980 and 1983, Jackson and John Cameron evolved the core concepts of JSD. After 1983, Jackson was less involved in the development of JSD, and refinements in JSD were made based on experiences of JSD consultants on a variety of consulting projects. Updated versions of the method were described in LBMS Jackson system development method manual (Wiley, 1992) and Practical program development using JSP (Blackwell, 1987) by Ralph Storer.
When it was originally presented by Jackson in 1983, the method consisted of six steps.
- Entity/Action Step
- Initial Model Step
- Interactive Function Step
- Information Function Step
- System Timing Step
- System Implementation Step
Later, some steps were combined to create a method with only three steps.
- Modelling Stage
- Network Stage
- Implementation Stage
Diagrams
JSD uses several types of diagrams to express the specifications for a system:
- entity structure diagram
- system specification diagram (SSD), also known as a network diagram
- system implementation diagram (SID)
Steps in the Method
Modelling Stage
In the modelling stage the designer creates a collection of entity structure diagrams and identifies the entities in the system, the actions they perform, the time-ordering of the actions in the life of the entities, and the attributes of the actions and entities. Entity structure diagrams use the diagramming notation of Jackson Structured Programming structure diagrams.
Network Stage
In the network stage a model of the system as a whole is developed and represented as a system specification diagram (SSD) (also known as a network diagram). Network diagrams show process (rectangles) and how they they communicate with each other, either via state vector connections (diamonds) or via datastream connections (circles).
Implementation Stage
In the implementation stage the abstract network model of the solution is converted into a physical system, represented as a system implementation diagram (SID). The SID shows the system as a scheduler process that calls modules that implement the processes. Datastreams are represented as calls to inverted processes. Database symbols represent collections of entity state-vectors, and there are special symbols for file buffers (which must be implemented when processes are scheduled to run at different time intervals).