Final reduction failed – How to solve this Elasticsearch exception

Opster Team

August-23, Version: 8.4-8.9

Briefly, this error occurs when Elasticsearch is unable to reduce the results of a search query due to a problem with the aggregation. This could be due to a lack of resources, a complex query, or a bug in the software. To resolve this issue, you can try simplifying your query, increasing the resources available to Elasticsearch, or updating to the latest version of Elasticsearch to ensure any known bugs are fixed.

This guide will help you check for common problems that cause the log ” Final reduction failed ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: plugin.

Log Context

Log “Final reduction failed” class name is We extracted the following from Elasticsearch source code for those seeking an in-depth context :

 // we can use the reduce context big arrays; because we finalize here
 try (ReduceContext reduceContext = mapReducer.reduceInit(aggReduceContext.bigArrays())) {
 mapReducer.reduce(contexts; reduceContext; aggReduceContext.isCanceled());
 mapReduceResult = mapReducer.reduceFinalize(reduceContext; fields; aggReduceContext.isCanceled());
 } catch (IOException e) {
 throw new AggregationExecutionException("Final reduction failed"; e);
 }  return new InternalItemSetMapReduceAggregation<>(name; metadata; mapReducer; null; mapReduceResult; fields; profiling);
 // else: combine


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