After we do any interesting operations (e.g. reduce) on KeyedStream, the
result becomes DataStream. In a lot of cases, the output still has the same
or compatible keys with the KeyedStream (logically). But to do further
operations on these keys, we are forced to use keyby again. This works
semantically, but is costly in two aspects. First, it destroys the
possibility of chaining, which is one of the most important optimization
technique. Second, keyby will greatly expand the connected components of
tasks, which has implications in failover optimization.
To address this shortcoming, we propose a new operator partitionedKeyBy.
DataStream {
public <K> KeyedStream<T, K> partitionedKeyBy(KeySelector<T, K> key)
}
Semantically, DataStream.partitionedKeyBy(key) is equivalent to
DataStream.keyBy(partitionedKey) where partitionedKey is key plus the
taskid as an extra field. This guarantees that records from different tasks
will never produce the same keys.
With this, it's possible to do
ds.keyBy(key1).reduce(func1)
.partitionedKeyBy(key1).reduce(func2)
.partitionedKeyBy(key2).reduce(func3);
Most importantly, in certain cases, we will be able to chains these into a
single vertex.
Please share your thoughts. The JIRA is at https://issues.apache.org/j
ira/browse/FLINK-4855
Xiaowei
