Hi all, While I was going over The Beam Model [model evolution] <https://docs.google.com/presentation/d/1SHie3nwe-pqmjGum_QDznPr-B_zXCjJ2VBDGdafZme8/edit#slide=id.g12846a6162_0_5> to learn the basics of the model, I found the explanation of watermark (in slide 27), "No timestamp earlier than the watermark will be seen" and "It declares that no event times earlier than this point are expected to appear in the future", hard to understand.
>From the graph in the slide, the watermark seems to be a two-dimensional concept, whereas timestamp (regardless of event or processing time) is one-dimensional. Hence, my confusion around the explanation. It seems to me that we can only talk about watermark in the context of processing time. My own interpretation of water ,based on the graph, is Given a point (e, p)on the the watermark curve, at processing time p, the system is confident (since watermark is just heuristics) that no events happened earlier than e are expected to be seen. Is the understanding roughly correct? I plan to read the Dataflow paper to get a more precise understanding, but would also like to hear explanations in a less formal terms. Any help is greatly appreciated. Best, Haibo Chen
