Aggregation of a Term Vocabulary for P2P-IR: A DHT Stress Test

There has been an increasing research interest in developing full-text retrieval based on peer-to-peer (P2P) technology. So far, these research efforts have largely concentrated on efficiently distributing an index. However, ranking of the results retrieved from the index is a crucial part in information retrieval. To determine the relevance of a document to a query, ranking algorithms use collection-wide statistics. Term frequency inverse document frequency (TF-IDF), for example, is based on frequencies of documents containing a given term in the whole collection. Such global frequencies are not readily available in a distributed system. In this paper, we study the feasibility of aggregating global frequencies for a large term vocabulary in a P2P setting. We use a distributed hash table (DHT) for our analysis. Traditional applications of DHTs, such as file sharing, index keys in the order of tens of thousands. Aggregation of a vocabulary consisting of millions of terms poses extreme requirements to a DHT implementation. We study different aggregation strategies and propose optimizations to DHTs to efficiently process large numbers of keys.