POSD is a notation for representing the structure of systems. Existing notations fulfill this role adequately for small homogeneous systems. However engineers now need to design business (or engineering) processes and design the IT systems that support these processes, so their notations need to be capable of representing the structure of large heterogeneous systems encompassing processes and IT systems (software and hardware). Existing notations are proving inadequate for this task and there is a very good reason for their failure. Existing notations view systems as very complex compositions of interacting component systems. However they treat the interactions between the component systems as comparatively primitive entities (such as function calls or message passing or document flow). This restriction does not work for a large heterogeneous system, in which the nature of the interactions varies markedly (from one part of the system to another).
POSD removes this deficiency by recognising interactions as shared components between the interacting systems with the same compositional power as other component systems.
Thus, for instance, when POSD is used for a retail supply problem, the supply process is viewed as a complex interaction between the manufacturing process and the retail process. The high level representation of this problem treats the supply process as one of perhaps many interactions between manufacturing, retail and other processes. Lower level representations display the detailed structure of the supply process and its supporting IT systems. POSD makes it possible for the different levels of representation to be correlated with each other. The ability of POSD to represent the structure of systems in this way has been tested in a number of business system domains including retail, construction, local government and news propagation. For full descriptions see full examples.
A stepwise refinement approach to the development of POSD models has been established and has been demonstrated in a construction supply process example. The approach starts with a high level model of a system such as the triple of manufacturing process, supply process and retail process described above. The system is then developed in a series of refinement steps with each step replacing one component in the system with an implementation in terms of lower level component systems. A tool to support this and other approaches has been developed. A zipped-up version of the tool can be downloaded by clicking here. You should unzip this file and extract its contents into a POSDRELEASE directory. Then you should click on the start.html page in that directory to find out how to finish installing the tool.
The systems engineering community needs to draw at one end on broad but relatively unrigorous work on business structure and business re-engineering (such as the work of Beer) and at the other end on more restricted but much more rigorous work on concurrent system structure (such as the work of Milner). Work on POSD relates well to both ends of this spectrum. Some of the POSD examples have demonstrated the use of POSD in conjunction with other notations (such as IDEF0, popular in the workflow community). POSD is a graphical notation for representing structure but many of its ideas are applicable to other notations (such as Milner's notation).
The development and application of POSD has benefited from the help of ICL staff and others. POSD® is a registered trademark of ICL in the UK.
The POSD html has been validated by browsing under Netscape Communicator and Internet Explorer 3.0. It can be browsed on the web or downloaded (zip or self extracting exe) and viewed locally starting at webposd/posd.htm.
Author of these pages is Graham Pratten