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Rethinking Timers as PCollections

TLDR Perhaps we should revisit https://s.apache.org/beam-portability-timers in light of the fact that Timers are more like State than PCollections. 


While looking at implementing State and Timers in the Python SDK, I've been revisiting the ideas presented at https://s.apache.org/beam-portability-timers , and am now starting to wonder if this is actually the best way to model things (at least at the Runner level). Instead it seems Timers are more resemble, and are tightly bound to, State than PCollections. 

This is especially clear when writing timers. These timers are not a bag of emitted elements, rather one sets (and clears) timers and the set of timers that end up firing are a result of this *ordered* sequence of operations. It is also often important that the setting of timers be ordered with respect to the setting and clearing of state itself (and is more often than not collocated with such requests). 

In addition, these self-loops add complexity to the graph but provide no additional information--they are entirely redundant with the timerspecs already present on DoFns. Generally I prefer less redundancy in the spec, rather than have it be over-constrained. It's unclear what a runner that didn't introspect the DoFn's TimerSpecs would do with this these special edges, and also unclear how they would differ from possible self-loops due to more traditional iteration. 

The primary motivation to express timers in this way seems to be the desire to push them to workers using the data plan, rather than inventing another mechanism or making them pull-based like with state. I think this could be done by simply adding a Timer field to the Elements (or Data) proto. (Note that this is not the same as having an hacky ElementOrTimer elements flow through the graph.) Writes would be state requests, and perhaps it would even make sense to "read" the current value of an unfired timer over the state API, to be able to set things like {min,max}(new_timestamp,old_timestamp}.

(We could alternatively attempt to model State(s) as a PCollection(s), but this is more speculative and would likely exacerbate some of the issues above (though it could open the door for DoFns that somehow *share* state). They seem like different objects though, one is a mutable store, the other an immutable stream.) 

I realize this is a big shift, but we could probably adapt the existing Python/Java implementations fairly easily (and it would probably simplify them). And it's easier to do simplifications like this sooner rather than later. 

What do people think about this? Any obvious (or not-so-obvious) downsides that I'm missing? 

- Robert