The key advantage Storm brought to the table was, as briefly mentioned in the article, the at-least once processing guarantee Storm offers thanks to its efficie

The key advantage Storm brought to the table was, as briefly mentioned in the article, the at-least once processing guarantee Storm offers thanks to its efficient tracking of tuples and their descendants (see https://storm.incubator.apache.org/documentation/Guaranteeing-message-processing.html https://storm.incubator.apache.org/documentation/Guaranteein...). To my knowledge S4 offers no such guarantees. I think this made Storm more attractive for many use-cases, it did for me.