Unknown map reducer mapReducerName – How to solve this Elasticsearch exception

Opster Team

August-23, Version: 8.4-8.9

Briefly, this error occurs when Elasticsearch attempts to execute a map reduce operation with an undefined or incorrect map reducer name. The map reducer is a function used to process and generate results from data. To resolve this issue, ensure that the map reducer name is correctly defined in your code. Also, check if the map reducer is properly installed and configured in your Elasticsearch environment. Lastly, ensure that the version of Elasticsearch you are using supports the map reducer you are trying to use.

This guide will help you check for common problems that cause the log ” Unknown map reducer [” + mapReducerName + “] ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: plugin.

Log Context

Log “Unknown map reducer [” + mapReducerName + “]” class name is FrequentItemSetsAggregatorFactory.java. We extracted the following from Elasticsearch source code for those seeking an in-depth context :

 return (in -> new InternalItemSetMapReduceAggregation<>(in; (mapReducerReader) -> {
 String mapReducerName = in.readString();
 if (EclatMapReducer.NAME.equals(mapReducerName)) {
 return new EclatMapReducer(FrequentItemSetsAggregationBuilder.NAME; in);
 throw new AggregationExecutionException("Unknown map reducer [" + mapReducerName + "]");
 }  private final List fields;
 private final double minimumSupport;


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