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    一种基于数据流环境感知的共享过滤算法

    An Adaptive Shared Filter Ordering Algorithm for Data Stream Systems

    • 摘要: 为了有效过滤数据流中的有害信息,通常在数据流上注册大量查询,同时构建过滤器来计算这些查询.在多媒体流环境中,查询和过滤器常常是一种“多对多”的连接,也就是说,对于单个过滤器的计算可能会同时给出多个查询的结果.在这种情况下,如何排序所有的过滤器来获得最小的过滤代价变得非常重要.对于过滤器的排序一般依赖于3个指标:过滤器本身的执行代价c、过滤器连接的查询数目p以及过滤器对于随机样本判断为真的概率s.目前基于贪心的排序算法虽然在一定程度上给出了近似最优的结果,但是还存在以下两个问题:1) 指标s只是简单依据经验值设定,不能随着流的变化而自适应变化;2)将3个指标融合成一个代价函数进行排序,而没有深入分析各个指标之间的关系.考虑到以上方法存在的不足,提出一个层次排序算法(adaptive hierarchal ordering, AHO)来高效地过滤多媒体数据流.该算法首先依据过滤器的指标c和p进行分类,然后再在每个类别上按照s进行二次排序.在真实多媒体流环境中的过滤结果证明: AHO可以在不降低准确度的情况下,自适应调整过滤器顺序,其性能优于已有的贪心排序算法.

       

      Abstract: In multimedia stream filtering scenario, there usually exist many filtering queries that specify the filtering objectives and many filters that estimate the filtering queries. A filtering query may connect to several different filters and a filter may connect to several different filtering queries. An open problem in such a filtering scenario is how to order the filters in an optimal sequence so as to decrease the filtering cost. Existing methods are based on a greedy strategy which orders the filters according to three factors of the filters, i.e., the selectivity, popularity, and cost. Although all these methods reported good results, there are still some problems that havent been addressed yet. Firstly, the selectivity factor is set empirically, which can not adaptively adjust with stream passing by. Secondly, the relationships among the three factors are not considerably explored. Under these observations, in this paper, we propose an adaptive hierarchal ordering (AHO) algorithm which executes a two-stage ordering strategy. In the first ordering stage, AHO clusters all the filters according to their popularities and costs factors. In the second stage, for each cluster all the filters are then ordered according to their selectivity. Experiments on both synthetic and real life multimedia streams demonstrate that our AHO method outperforms other simple filtering methods.

       

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