Sub-millisecond Stateful Stream Querying over Fast-evolving Linked Data.

Applications like social networking, urban monitoring and market feed processing require stateful stream query: a query consults not only streaming data but also stored data to extract timely information; useful information from streaming data also needs to be continuously and consistently integrated into stored data to serve inflight and future queries. However, prior streaming systems either focus on stream computation, or are not stateful, or cannot provide low latency and high throughput to handle the fast-evolving linked data and increasing concurrency of queries. This paper presents Wukong+S, a distributed stream querying engine that provides sub-millisecond stateful query at millions of queries per-second over fast-evolving linked data. Wukong+S uses an integrated design that combines the stream processor and the persistent store with efficient state sharing, which avoids the cross-system cost and sub-optimal query plan in conventional composite designs (e.g., Storm/Heron+Wukong). Wukong+S uses a hybrid store to differentially manage timeless data and timing data accordingly and provides an efficient stream index with locality-aware partitioning to facilitate fast access to streaming data. Wukong+S further provides decentralized vector timestamps with bounded snapshot scalarization to scale with nodes and massive queries at efficient memory usage. We have designed Wukong+S conforming to the RDF data model and Continuous SPARQL (C-SPARQL) query interface and have implemented Wukong+S by extending a state-of-the-art static RDF store (namely Wukong). Evaluation on an 8-node RDMA-capable cluster using LSBench and CityBench shows that Wukong+S significantly outperforms existing system designs (e.g., CSPARQL-engine, Storm/Heron+Wukong, and Spark Streaming/Structured Streaming) for both latency and throughput, usually at the scale of orders of magnitude.