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<p><b>Abstract</b>—As most current query processing architectures are already pipelined, it seems logical to apply them to data streams. However, two classes of query operators are impractical for processing long or infinite data streams. Unbounded stateful operators maintain state with no upper bound in size and, so, run out of memory. Blocking operators read an entire input before emitting a single output and, so, might never produce a result. We believe that a priori knowledge of a data stream can permit the use of such operators in some cases. We discuss a kind of stream semantics called punctuated streams. Punctuations in a stream mark the end of substreams allowing us to view an infinite stream as a mixture of finite streams. We introduce three kinds of invariants to specify the proper behavior of operators in the presence of punctuation. <it>Pass invariants</it> define when results can be passed on. <it>Keep invariants</it> define what must be kept in local state to continue successful operation. <it>Propagation invariants</it> define when punctuation can be passed on. We report on our initial implementation and show a strategy for proving implementations of these invariants are faithful to their relational counterparts.</p>
Continuous queries, stream semantics, continuous data streams, query operators, stream iterators.

P. A. Tucker, T. Sheard, D. Maier and L. Fegaras, "Exploiting Punctuation Semantics in Continuous Data Streams," in IEEE Transactions on Knowledge & Data Engineering, vol. 15, no. , pp. 555-568, 2003.
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