From the paper, "The design of Heron allows supporting exactly once semantics, but the first version of Heron does not have this implementation. One reason for

From the paper, "The design of Heron allows supporting exactly once semantics, but the first version of Heron does not have this implementation. One reason for tolerating the lack of exactly once semantics is that Summingbird [8] simultaneously generates both a Heron query and an equivalent Hadoop job, and in our infrastructure the answers from both these parts are eventually merged". The industry term for this approach is lambda architecture (http://lambda-architecture.net/ http://lambda-architecture.net/)