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1. The reason why the connection between Global Stream and Non-Global Stream is not supported is that the number of partitions of GlobalStream is forced to be 1, but it is generally not 1 for Non-Global Stream, which will cause conflicts when determining the number of partitions of the output stream. If necessary, they should be transformed into mutually compatible streams and then connected.
2. Connecting two broadcast streams doesn't really make sense, because each parallelism would have exactly same input data from both streams and any process would be duplicated. 
3. The reason why the output of two keyed partition streams can be keyed or non-keyed is the same as we mentioned above in the case of single input.
4. When we connect two KeyedPartitioinStream, they must have the same key type, otherwise we can't decide how to merge the partitions of the two streams. At the same time, things like access state and register timer are also restricted to the partition itself, cross-partition interaction is not meaningful

   When we connect two KeyedPartitionStream, they must have the same key type, otherwise we can't decide how to merge the partitions of the two streams. At the same time, things like access state and register timer are also restricted to the partition itself, cross-partition interaction is not meaningful.

5. The reasons why the connection between KeyedPartitionStream and NonKeyedPartitionStream is not supported are as follows:
   1. The data on KeyedStream is deterministic, but on NonKeyed is not. It is difficult to think of a scenario where the two need to be connected.
   2. This will complicate the state declaration and access rules. A more detailed discussion can be seen in the subsequent state-related sub-FLIP.
   3. If we see that most people have clear demands for this, we can support it in the future.

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