Querying Unstructured and Structured Peer-to-Peer Networks: Models, Issues, Algorithms

Peer-to-Peer (P2P) networks are defined as a collection of peers that expose client/server functionalities simultaneously. P2P systems, built on top of P2P networks, support information sharing primitives and lookup mechanisms of data objects located on peers. It has been demonstrated that the P2P paradigm is able to efficiently capture models and, above all, dynamics of modern applications, beyond limitations of conventional produce/consumer paradigms. Traditionally, P2P primitives and mechanisms have been implemented by means of keyword-based search and matching operations. Modern P2P systems require more complex query functionalities, beyond capabilities of DBMS-inspired models and algorithms. As a consequence, the issue of efficiently querying the underling P2P network is gaining momentum in the research community. Indeed, querying P2P networks, which can be in the unstructured and structure modes, plays a critical role in next-generation P2P systems, as its performance heavily affect the efficiency of the overall system. Starting from these considerations, in this paper we propose a survey on models, issues and algorithms for querying unstructured and structured P2P systems. We also put in evidence similarities and differences of state-of-the-art proposals appearing in literature, with critical discussion, and we provide a rigorous taxonomy of P2P query strategies. Finally, we complete our analytical contribution via highlighting future directions in this research field.