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So if I get you, it's for append-only data - probably no updates, definitely no no deletions? Still don't get how you don't need logical clocks to pick out the delta(s), but thanks for your prompt answer.

Edit: your example gives replaceExisting() so that's supporting an update of some kind.

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No it is explicitly not for append-only data. It works with inserts, updates and deletes. I think I have problems understanding what exactly you mean by the need for a logical clock. The algorithm is feeded with the old query results plus one event, and then returns the new query results. Since there is only one event at each point of time, it does not have to order or maintain them.

I had the same question as #2. Basically, it has to be the front-end to any event that reads/writes the data, in strict order of occurrence?

Not exactly. To use EventReduce you must have a changestream out of all writes to your data int the correct order. You can do that by wrapping a frontend over your database.

But easier you do that by using a database that already provides a changestream like couchdb, Postgres, mongodb and so on.

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