Months distributed approach. As this architecture experimentation exposed the design tradeoffs in distribution strategies, the SAS process design was changed back to the original number of processes, but the SAS task-level design converged on an increased number of tasks. The basic problems being examined by the architecture team were the tradeoffs in concurrency, operating system process overhead, run-time library tasking overhead, paging, context switching, and the mix of interprocess, intertask, and internode message exchange. The complexity of such run-time interactions made modeling and simulation ineffective. Only the early run-time demonstrations of multiple distribution configurations allowed the architecture team to achieve the understanding of technical trade-offs necessary to select an adequate solution. If the change in the distribution design had occurred very late in the project, the impact could have been immense. Because sockets and messages were fairly simple to change and corresponded to lower level application interfaces, changes to these numbers continued at a low level through the critical design review (CDR) milestone.
The freedom to experiment with an architecture proved to be very valuable to the achievement of an adequate architecture baseline early in the life cycle. This was enabled primarily by the flexibility of the NAS CSCI.
CCPDS-R was seriously committed to an architecture-first approach. The "architecture" description of CCPDS-R was centered on the process view, as described in Chapter 7. This was due primarily to the stringent run-time performance requirements and the risks associated with a first-generation distributed architecture.
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