The Viewpoints

The ODP reference model work has identified 5 viewpoints which may be used to examine a distributed system. These viewpoints are tools to help with the analysis of a distributed system. Different groups of people from different cultures are involved in specifying, building, using and running a distributed system. These viewpoints represent the important perspectives of the distributed system to different groups of people involved in the system. Each group sees some aspects in great detail, but other parts are described in the vaguest way as they are not important to that group. For instance, the users of the system will want to know a lot about what the system does in terms of using information and producing useful results, they will not be interested in exactly which CPU model, or disc drive, or even disc sector, they are using to achieve their work. On the other hand the operations manager will be very concerned about CPU utilization, disc blocking, and sharing hardware resources, he will not be very interested in what the distributed system is actually doing in terms of the input and the results. The five viewpoints are not meant to represent 5 layers, however they do have some relationship, in that we can see how refining one viewpoint may well lead to another. Viewpoints are not in and of themselves refinements or part of any particular development methodology. All view points are always valid. The viewpoints will overlap, there is no distinct boundary. The viewpoints are just perspectives on an underlying model, or actual system. If there were no underlying model then the perspectives become disjoint. When used in the context of an underlying model then each perspective (viewpoint description) is different in the concepts that it brings into focus as important and the concepts that are hidden or ignored. Each perspective will represent a different level of abstraction of the system. These are illustrated in table#tbvp#49>. A number of projects have contributed to the the way these ideas may be projected onto distributed systems in five ways:

Table: Framework of Abstractions document

Who uses which viewpoint is show in figure {ref<#58#>tb:vpu<#58#>.

Table: Viewpoint users

<#229#>#tex2html_wrap3794#<#229#> The Open Distributed Processing model is still undergoing evolution. What is presented in this book is not meant to reflect that communities model at all, but this author's extraction of those points that were found useful, and reasonably complete at the time of writing. <#230#>#tex2html_wrap3796#<#230#>