Optimal Broadcasting Strategies for Conjunctive Queries over Distributed Data

Abstract

In a distributed context where data is dispersed over many computing nodes, monotone queries can be evaluated in an eventually consistent and coordination-free manner through a simple but naive broadcasting strategy which makes all data available on every computing node. In this paper, we investigate more economical broadcasting strategies for full conjunctive queries without self-joins that only transmit a part of the local data necessary to evaluate the query at hand. We consider oblivious broadcasting strategies which determine which local facts to broadcast independent of the data at other computing nodes. We introduce the notion of broadcast dependency set (BDS) as a sound and complete formalism to represent locally optimal oblivious broadcasting functions. We provide algorithms to construct a BDS for a given conjunctive query and study the complexity of various decision problems related to these algorithms. ACM Subject Classification H.2.3 Query Languages, H.2.4 Distributed databases Keywords and phrases IntroductionWe assume the setting introduced in the context of declarative networking [6,14], where queries are specified on a logical level over a global schema and are evaluated by multiple computing nodes over which the input database is distributed. These nodes can perform local computations and communicate asynchronously with each other via messages. The model then operates under the assumption that messages can never be lost but can be arbitrarily delayed. It is known that every monotone query can be evaluated in an eventually consistent and coordination-free manner through a naive broadcasting strategy that makes all data available to all nodes [14].

Weaker Forms of Monotonicity for Declarative Networking

“…In particular, as an initial step, [34] investigate more economical broadcasting strategies for full conjunctive queries without self-joins that only transmit a part of the local data necessary to evaluate the query at hand. Another contribution of this paper is the identification of (semi-)connected Datalog variants which to the best of our knowledge have not been considered before.…”

Logical Aspects of Massively Parallel and Distributed Systems

“…It would be interesting to investigate efficient coordination-free algorithms. As an initial step, Ketsman and Neven [37] investigate more economical broadcasting strategies for full conjunctive queries without self-joins that only transmit a part of the local data necessary to evaluate the query at hand. In a sense, such approaches need to identify what part of the data is essential for answering a given query.…”