Understanding Membership

A membership service is used in a distributed system to maintain information about which sites are functioning and which have failed at any given time. Such services have proven to be fundamental for constructing distributed applications, with many example services and algorithms defined in the literature. Despite these efforts, however, little has been done on examining the abstract properties commonly guaranteed by membership services independent of a given implementation. Here, a number of these properties are identified and defined. These properties range from agreement among sites on membership changes, consistent ordering of change notifications, and timing properties to various ways for dealing with recoveries and partitions. Message ordering graphs, which are an abstract representation of the set of messages at each site in the system and their potential delivery order, are used to define the properties. Dependency graphs, which are a graphical representation expressing when a property is defined assuming the existance of another property or when a stronger property includes a weaker property, are used to illustrate the relationships between properties. These graphs help differentiate existing services, as well as facilitate the design of new configurable services in which only those properties actually required by an application are included. Finally, a number of existing membership services are characterized in terms of the properties identified.