The Cost Of Order In Asynchronous Systems

We consider the Group Membership Problem (GMP) in asynchronous systems. This problem consists of maintaining a list of processes belonging to the system, and updating it as processes join (are started) and leave (terminate or fail). Our investigations led to four independent properties that characterize instances of this problem. We closely examine three membership services, comparing the message cost to implement them, as well as their fault-tolerance and ability to adapt to environmental changes. We also examine their relative merits by comparing the cost to a distributed application that employs each of the membership services. We show that in typical system executions Strong GMP is less expensive to implement, is always more responsive to dynamic aspects in the environment, and allows applications to accomplish more work with less effort. As Strong GMP is the sole instance providing a linear order on membership changes, these results emphasize the benefits of providing Order as well as the cost of not providing it when it is available so cheaply.