COMBINE: An Improved Directory-Based Consistency Protocol

This paper presents COMBINE, a directory-based consistency protocol for shared objects, designed for large-scale distributed systems with unreliable links. Directory-based consistency protocols support move requests, allowing to write the object locally, as well as lookup requests, providing a read-only copy of the object. They have been used in distributed shared memory implementations and are a key element of data-flow implementations of distributed software transactional memory in large-scale systems. The protocol runs on an overlay tree, whose leaves are the nodes of the system, and its main novelty is in combining requests that overtake each other as they pass through the same node. Combining requests on a simple tree structure allows the protocol to tolerate non-fifo links and handle concurrent requests. Combining also avoids race conditions and ensures that the cost of serving a request is proportional to the cost of the shortest path between the requesting node and the serving node, in the overlay tree. Using an overlay tree with a good stretch factor yields an efficient protocol.