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Aggregator State in Keyed Windowed Stream

Since the aggregate() function on a keyed window stream does not allow
using rich functions, I can only use an AggregateFunction. Is the
accumulator state of the AggregateFunction backed by RocksDB and
persisted in checkpoints if I use the RocksDB backend. My job looks
like the following,

    .aggregate(new MyAggFunc());

Since the stream has high cardinality keys, each window could have
millions of them. If the accumulator state is not backed by RocksDB,
there might be a lot of data stored on the heap.

And why is rich functions not allowed here?