The effect of statically and dynamically replicated components on system reliability

The reliability of general systems using dynamic and static redundancy schemes is derived, and communication protocols are considered as a representative example. The system reliability for three broadcast protocols using various redundancy-allocation policies is studied. The analytic and simulation results show that, in some cases, static redundancy yields a more reliable system than dynamic redundancy. This is essential for distributed system applications. In some cases, the failure detection time is substantial, so that the hardware reliability and hence the system reliability are adversely affected when using dynamic redundancy. This can be a critical factor for distributed system applications, because a large overhead of communication can be required for error detection. In these cases, unreliable protocols can provide better system reliability than reliable protocols, especially when the communication network is highly reliable and when the machine failure rate is relatively large. Since unreliable protocols generate less load and less resource contention, they are preferable in such cases. The reliability should be analyzed to determine the optimal balance between reliable and unreliable protocols. Static redundancy can be more reliable than dynamic redundancy if the failure-detection time is large.<
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