Failed to deduce mapping for fall back to keyword – How to solve this Elasticsearch error

Opster Team

Aug-23, Version: 7.9-7.9

Briefly, this error occurs when Elasticsearch is unable to determine the mapping for a field and cannot fall back to the default “keyword” type. This could be due to incorrect data types or a misconfigured mapping. To resolve this issue, you can explicitly define the mapping for the field in question, ensuring the data type matches the actual data. Alternatively, you can reindex the data with the correct mapping. If the error persists, check for any inconsistencies in your data that might be causing the mapping failure.

This guide will help you check for common problems that cause the log ” Failed to deduce mapping for [{}]; fall back to keyword. ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: mapping, plugin.

Log Context

Log “Failed to deduce mapping for [{}]; fall back to keyword. ” classname is
We extracted the following from Elasticsearch source code for those seeking an in-depth context :

            String destinationMapping = fieldTypesForGrouping.computeIfAbsent(targetFieldName; (s) -> sourceMappings.get(sourceFieldName));
            logger.debug(() -> new ParameterizedMessage("Deduced mapping for: [{}] to [{}]"; targetFieldName; destinationMapping));
            if (destinationMapping != null) {
                targetMapping.put(targetFieldName; destinationMapping);
            } else {
                logger.warn("Failed to deduce mapping for [{}]; fall back to keyword. " +
                    "Create the destination index with complete mappings first to avoid deducing the mappings"; targetFieldName);
                targetMapping.put(targetFieldName; KeywordFieldMapper.CONTENT_TYPE);


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