Modern distributed systems increasingly rely on distributed storage and lookup services. Existing storage and lookup solutions, however, provide only a subset of high availability, high scalability, and strong consistency semantics. In this project, we develop Harmony, a distributed hash table (DHT) aimed at providing very large scale applications with highly scalable, consistent and available storage and lookup. We address many significant challenges in delivering on this goal. First, the Internet has unpredictable node and communication failures, and preserving consistency in this context is a difficult task. To address this issue, we provide the abstraction of a collection of self-managing groups that coordinate to ensure atomic updates to distributed state. Second, consistency often comes at the cost of reduced availability. In our system, consistency is an inviolable safety property; availability is provided through replication. Third, coordination mechanisms for consistent replication and atomic updates often result in performance penalties. A key insight in our work is that one can improve performance with coordination mechanisms for delegation and autonomous execution. Finally, by isolating most of the communication necessary to preserve consistency within a group, we both simplify our implementation and improve its scalability. Further, adaptation to changing workloads and resource constraints is easier because it can take place within a framework for the consistent update of distributed state. If successful, the resulting storage abstraction should greatly simplify the development of complex distributed applications.
