Scaling Out Schema-free Stream Joins

In this work, we consider computing natural joins over massive streams of JSON documents that do not adhere to a specific schema. We first propose an efficient and scalable partitioning algorithm that uses the main principles of association analysis to identify patterns of co-occurrence of the attribute-value pairs within the documents. Data is then accordingly forwarded to compute nodes and locally joined using a novel FP-tree–based join algorithm. By compactly storing the documents and efficiently traversing the FP-tree structure, the proposed join algorithm can operate on large input sizes and provide results in real-time. We discuss data-dependent scalability limitations that are inherent to natural joins over schema-free data and show how to practically circumvent them by artificially expanding the space of possible attribute-value pairs. The proposed algorithms are realized in the Apache Storm stream processing framework. Through extensive experiments with real-world as well as synthetic data, we evaluate the proposed algorithms and show that they outperform competing approaches.