Is it safe to cache the value of a singleton view (with a global window) in a DoFn?

[Steve Niemitz]

I have a singleton view in a global window that is read from a DoFn.  I'm
curious if its "correct" to cache that value from the view, or if I need to
read it every time.

As a (simplified) example, if I were to generate the view as such:

    input.getPipeline
      .apply(Create.of(Collections.singleton[Void](null)))
      .apply(MapElements.via(new SimpleFunction[Void, JLong]() {
        override def apply(input: Void): JLong = {
          Instant.now().getMillis
        }
      })).apply(View.asSingleton[JLong]())

and then read it from a DoFn (using context.sideInput), is it guaranteed
that:
- every instance of the DoFn will read the same value?
- The value will never change?

If so it seems like it'd be safe to cache the value inside the DoFn.  It
seems like this would be the case, but I've also seen cases in dataflow
where the UI indicates that the MapElements step above produced more than
one element, so I'm curious what people have to say.

Thanks!