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d14e79e849
We currently convert pipeline aggregators to their corresponding InternalAggregation instance as part of the final reduction phase. They arrive to the coordinating node as part of QuerySearchResult objects fom the shards and, despite we may incrementally reduce aggs (hence we may have some non-final reduce and the final one later) all the reduction phases happen on the same node. With CCS minimizing roundtrips though, each cluster performs its own non-final reduction, and then serializes the results back to the CCS coordinating node which will perform the final coordination. This breaks the assumptions made up until now around reductions happening all on the same node. With #40101 we have made sure that top-level pipeline aggs are not reduced as part of the non-final reduction. The next step is to make sure that they don't get lost, meaning that each coordinating node needs to send them back to the CCS coordinating node as part of the top-level `InternalAggregations` object. Closes #40059