apache kafka streams – How do I access the actual messages comprising a kstream sliding window aggregation?

I have a stream of messages. When emitting any satisfied sliding windows, I want to know the list that the window matched.

I want to avoid taking on the accumulation job myself within my .aggregate() update lambda by appending each payload value to an internal List that my aggregation state store would keep. So, every state store record would end up with its own list of contributing messages embedded. I guess it makes sense if I indeed must do this, since it is just another form of accumulation, albeit not numeric or mathematical in nature.

My understanding is that:

.groupByKey().windowedBy().aggregate() uses a WindowedKey-oriented store which is the grouping K augmented with the unique sliding window range stamps

.groupByKey().aggregate uses the grouping K

but other than the finer-grain of the WindowedKey that adds the timestamp range to distinguish the sliding windows for you, neither pipeline has any innate way to remember the messages that comprise a closed aggregation. In both cases and I need to accumulate myself as mentioned above.

1. Is there any better pattern to use that doesn’t need this sort of internal list accumulation, when what you want are the matched messages when aggregation windows close as satisfied?

2. When any given message V is processed in the WindowedKey aggregation, does my updater get called possibly multiple times (once for the sliding window that V actually starts, and N more times for any other ongoing sliding windows that V is in range of)?

3. Is message contribution dedupe (i.e. a message can only contribute to at most one satisfied sliding window) also up to my own tracking?

4. Is minimum frequency matching (i.e. “find 7+ messages within 5 minutes” instead of “find 7 messages within 5 minutes”) supported, since I don’t see any window-end temporal triggering so you can only emit when something proactively streams through to trigger. There is no “at end of temporal window+grace, trigger a final check to emit” right?