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Các nhà nghiên cứu khác (bao gồm cả tác giả) có gắn các điều kiện chính xác (trong các mô hình khác nhau) theo đó các giao thức đồng thuận thành viên năng động được đảm bảo để làm cho tiến độ [BDM95, FKMBD95, GS96, Nei96], | 308 Kenneth P. Birman - Building Secure and Reliable Network Applications membership and hence prevented from making progress. Other researchers including the author have pinned down precise conditions in various models under which dynamic membership consensus protocols are guaranteed to make progress BDM95 FKMBD95 GS96 Nei96 and the good news is that for most practical settings the answer is that such protocols make progress with overwhelmingly high probability if the probability of failures and message loss are uniform and independent over the processes and messages sent in the system. In effect only partitioning failures or a very intelligent adversary one that in practice could never be implemented can prevent these systems from making progress. Thus we know that all of these models face conditions under which progress is not possible. Research is still underway on pinning down the precise conditions when progress is possible in each approach the maximum rates of failures that dynamic systems can sustain. But as a practical matter the evidence is that all of these models are perfectly reasonable for building reliable distributed systems. The theoretical impossibility results do not appear to represent practical impediments to implementing reliable distributed software they simply tell us that there will be conditions that these reliability approaches cannot overcome. The choice in a practical sense is to match the performance and consistency properties of the solution to the performance and consistency requirements of the application. The weaker the requirements the better the performance we can achieve. Our study also revealed two other issues that deserve comment the need or lack thereof for a primary component in a partitioned membership model and the broader but related question of how consistency is tied to ordering properties in distributed environments. The question of a primary component is readily understood in terms of the air-traffic control example
