gianm commented on issue #3956: Thread safe reads for aggregators in IncrementalIndex URL: https://github.com/apache/incubator-druid/pull/3956#issuecomment-406384627 I was just looking at this issue again after the conversations on the mailing list about sketch synchronization: https://lists.apache.org/thread.html/9899aa790a7eb561ab66f47b35c8f66ffe695432719251351339521a@%3Cdev.druid.apache.org%3E I was wondering, does it make more sense for thread-safety here to be handled systematically (at the IncrementalIndex) or for each aggregator to be thread safe? Currently we do different approaches: the sketch aggregators endeavor to be thread-safe on their own. The primitive aggregators don't bother to even try, and they're probably fine, since they're primitives. HyperLogLogAggregator tries a little bit -- it at least makes sure the different calls use different buffer objects -- but I bet it has a bug where "get" could potentially read something weird and corrupt in some rare situations. (Like if the offset is being updated while a "get" is going on.)
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