trigger an alert.&rdquo;</em></p>
<p>In other words, given a stream of transactions partitioned by a key that
combines the payer and the beneficiary fields, we would like to look
back in time and determine, for each incoming transaction, if the sum of
all previous payments between the two specific participants exceeds the
defined threshold. In effect, the computation window is always moved
along to the position of the last observed event for a particular data
partitioning key.</p>
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