Short answer: make event time a dependency, flush batches with an explicit watermark, and assert presence as a state transition rather than sleeping for an arbitrary number of milliseconds. That gives a stock-trading watchlist repeatable tests while still exercising the same batching rules used in production. A watchlist is a presence problem disguised as a delivery problem. The user cares whether AAPL is currently visible, whether a symbol disappeared after a disconnect, and whether the last...