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1. All the iteration types should support multiple inputs and multiple outputs. 
2. All the iteration types require some kind of back edges that transfer the data back to the iteration head. Since Flink does not support cycles in scheduler and network stack, the back edges should not be visible in the StreamGraph and JobGraph.
3. All the iteration should support checkpoints mechanism in Stream execution mode.
4. All the iteration types should support 

Different types of iterations differ in their requirements for Progress tracking. Progress tracking is analogous to the watermark outside the iteration and it tracks the “progress” inside the iteration:

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