A Comparison of Peer-to-peer Query Response Modes

In a large distributed system spanning many administrative domains such as a Grid, it is desirable to maintain and query dynamic and timely information about active participants such as services, resources and user communities. However, in such a database system, the set of information tuples in the universe is partitioned over one or more distributed nodes, for reasons including autonomy, scalability, availability, performance and security. This suggests the use of Peer-to-Peer (P2P) query technology. A variety of query response modes can be used to return matching query results from P2P nodes to an originator. Although from the functional perspective all response modes are equivalent, no mode is optimal under all circumstances. Which query response modes allow to express suitable trade-offs for a wide range of P2P applications? In this paper, we answer this question by systematically describing and characterizing four query response modes for the Unified Peer-to-Peer Database Framework (UPDF) proposed in our prior studies, namely Routed Response, Direct Response, Routed Metadata Response, and Direct Metadata Response. The response models are compared with respect to distribution and location transparency, efficiency of query support, economics, number of TCP connections at originator and agent, latency, caching and trust delegation to unknown parties. We discuss to what extent a given P2P network must mandate the use of any particular response mode throughout the system. As a result, we propose that response modes can be mixed by switches and shifts, in arbitrary permutations.