A Scalable Conflict-Free Replicated Set Data Type

Replication of state is the fundamental approach to achieve scalability and availability. In order to maintain or restore replica consistency under updates, some form of synchronization is needed. Conflict-free Replicated Data Types (CRDTs) ensure eventual consistency, such that replicas converge to a common state, equivalent to a correct sequential execution without foreground synchronization. A particular CRDT is the set data type, which is a pervasive abstraction for storing collections of unique elements and constitutes an important building block for other, more complex data structures. Since the original specification is not scalable, we improve it by introducing an efficient algorithm for sending deltas of updates between replicas and by partitioning a set replica into disjunctive subsets. We further add support for limited-lifetime elements, which, in turn, enable simple garbage collection strategies to address the problem of unbounded database growth. Lastly, implementation details and evaluation results of a client library for this data structure are presented.