Hey! Now that we are implementing more and more applications for streaming that use iterations we realized a huge shortcoming of the current iteration api. Currently it only allows to feedback data of the same type to the iteration head.
This makes sense because the operators are typed but makes it awkward if we indeed want to use a different feedback (such as a model syncing for machine learning applications). To do this developers need to use wrapper types and flags to distinguish the inputs. I propose to add the possibility to tread the original input of the iteration head operator and the feedback stream as a ConnectedDataStream so we can apply operators such as CoMap, CoFlatMap etc. This helps distinguishing the inputs and also allows different feedback types to be used. I believe this change is inevitable if we want to write elegant applications without unnecessary wrapper types. I made a PR <https://github.com/apache/flink/pull/870> already that introduces this functionality (it is a very small change in fact). Cheers, Gyula
