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ASF GitHub Bot commented on FLINK-5968: --------------------------------------- Github user bowenli86 commented on a diff in the pull request: https://github.com/apache/flink/pull/4833#discussion_r146134163 --- Diff: docs/dev/stream/operators/windows.md --- @@ -427,6 +427,93 @@ input The above example sums up the second fields of the tuples for all elements in a window. +### AggregateFunction + +An `AggregateFunction` is a generalized version of a `ReduceFunction` that has three types: an +input type (`IN`), accumulator type (`ACC`), and an output type (`OUT`). The input type is the type +of elements in the input stream and the `AggregateFunction` has a method for adding one input +element to an accumulator. The interface also has methods for creating an initial accumulator, +for merging two accumulators into one accumulator and for extracting an output (of type `OUT`) from +an accumulator. We will see how this works in the example below. + +Same as with `ReduceFunction`, Flink will incrementally aggregate input elements of a window as they +arrive. + +A `AggregateFunction` can be defined and used like this: --- End diff -- An > Document WindowedStream.aggregate() > ----------------------------------- > > Key: FLINK-5968 > URL: https://issues.apache.org/jira/browse/FLINK-5968 > Project: Flink > Issue Type: Sub-task > Components: DataStream API, Documentation > Reporter: Aljoscha Krettek > Assignee: Aljoscha Krettek > Priority: Blocker > Fix For: 1.4.0 > > -- This message was sent by Atlassian JIRA (v6.4.14#64029)