Hi Saiph,
In Flink, the key for keyBy() can be provided in different ways:
https://ci.apache.org/projects/flink/flink-docs-master/apis/programming_guide.html#specifying-keys
(the doc is for DataSet API, but specifying keys is basically the same for
DataStream and DataSet).
As described in the documentation, calls like keyBy(0) are meant for Tuples,
so it only works for DataStream[Tuple]. Other key definition types like
keyBy(new KeySelector() {...}) can basically take any DataStream of
arbitrary data type. Flink finds out whether or not there is a conflict
between the type of the data in the DataStream and the way the key is
defined at runtime.
Hope this helps!
Cheers,
Gordon
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