XML stream querying problem involves evaluating a given, potentially large, set of query expressions on a continuous stream of XML messages. Since the messages arrive continuously, it is essential that the query processing rate matches the data arrival rate. Therefore, it is necessary to index the given set of query expressions appropriately to enable real-time processing of the streaming XML data. In this paper we propose a simple and scalable system for the XML stream querying problem. The

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... tem indexes the queries compactly using a query guide and uses simple integer stacks to efficiently process the stream. Our experiments demonstrate that the new system outperforms the classical stream query processor YFilter by sizeable margins without asking for more index space. Also, the system shows good time and space scalability with respect to query workload and stream size.