Consumer Application State Considerations Clause Samples

Consumer Application State Considerations. A message bus communication can have different kind of implementations and features. The most primitive ones transmit messages received from producers either to all consumers or only those that have registered for the particular messages. More advanced message bus implementations include advanced routing, caching and access management. Some implementations can even store messages for a certain duration and may, for example, retain the most recent messages for new consumers to receive. This is especially beneficial for new data or events that is seldom updated. From a plant-wide monitoring and control application development point of view, the state management responsibility is transferred to the particular applications and case implementations. Implementing the event-driven approach will require either that 1) all operational data is periodically updated (even if no changes occur), 2) most recent messages are retained (as some events can happen very seldom), or 3) there are means to also query recent data and events. To enable integration with various systems, adapters are typically utilised to wrap existing systems to new interfaces. The first option of always updating values is straightforward to implement in adapters but increases the amount of data transferred unnecessarily. This also puts more pressure on consumer applications to know when and if to process new measurements. The second option is easier to implement in client applications but reduces the number of standard message buses and protocols available as many of such features are implementation specific. The third and the favourable option provides most flexibility and also enables request-response like behaviour but incur additional functional requirements for adapters realising the query capabilities.