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Limitations of protocols and component state
It has been said that protocols are one way to express a component's state. A component is allowed to provide and require multiple interfaces at the same time. The PCM does not support protocols that cover more than one interface (neither at the required nor provided side of a component). To explain the state concept of the PCM one has to look into the state of such a component: At every moment a component that provides multiple interfaces has one complex state. This complex state can be experienced only through the set of provided interfaces. For every interface (including its protocol) the component seems to have an own state. In fact this ``per-interface-state'' results from the complex state of the component. One can consider a ``per-interface-state'' as a view of the complex component state.
This limits the expressiveness of protocol use. It is not possible to model a behaviour, where calls of different provided interfaces of a component influence each other. For example this behaviour would be required to model blocking calls due to a critical common resource. The PCM assumes that calls of different provided interfaces of a component are independent from each other - a blocking call as introduced above cannot be discovered with the used protocol / state model.
TODO: DA Klaus, Kapitel 2.21.1
Next: Limitations of protocol state
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Snowball
2007-03-16