I would like to propose an enhancement to the Sink API, the ability to receive upstream watermarks. I'm aware that the sink context provides the current watermark for a given record. I'd like to be able to write a sink function that is invoked whenever the watermark changes. Out of scope would be event-time timers (since sinks aren't keyed).
For context, imagine that a stream storage system had the ability to persist watermarks in addition to ordinary elements, e.g. to serve as source watermarks in a downstream processor. Ideally one could compose a multi-stage, event-driven application, with watermarks flowing end-to-end without need for a heuristics-based watermark at each stage. The specific proposal would be a new method on `SinkFunction` and/or on `SinkWriter`, called 'processWatermark' or 'writeWatermark', with a default implementation that does nothing. Thoughts? Thanks! Eron Wright StreamNative
