Computing hierarchical summary of the data streams

Data stream processing is an important function in many online applications such as network traffic analysis, web applications, and financial data analysis. Computing summaries of data stream is challenging since streaming data is generally unbounded, and cannot be permanently stored or accessed more than once. In this paper, we have proposed two counter based hierarchical (CHS) ε-approximation algorithms to create hierarchical summaries of one dimensional data. CHS maintains a data structure, where each entry contains the incoming data item and an associated counter to store its frequency. Since every item in streaming data cannot be stored, CHS only maintains frequent items (known as hierarchical heavy hitters) at various levels of generalization hierarchy by exploiting the natural hierarchy of the data. The algorithm guarantees accuracy of count within an ε bound. Furthermore, using aperiodic (CHS-A) and periodic (CHS-P) compression strategy the proposed technique offers improved space complexities of O(η/ε) and O(η/ε log εN), respectively. We provide theoretical proofs for both space and time requirements of CHS algorithm. We have also experimentally compared the proposed algorithm with the existing benchmark techniques. Experimental results show that the proposed algorithm requires fewer updates per element of data, and uses a moderate amount of bounded memory. Moreover, precision-recall analysis demonstrates that CHS algorithm provides a high quality output compared to existing benchmark techniques. For the experimental validation, we have used both synthetic data derived from an open source generator, and real benchmark data sets from an international Internet Service Provider.