Could not find analyzer %s of legacy index falling back to default – How to solve this Elasticsearch error

Opster Team

Aug-23, Version: 8.4-8.9

Briefly, this error occurs when Elasticsearch is unable to locate the specified analyzer for a legacy index. This could be due to the analyzer being removed or renamed in the newer version. Elasticsearch then falls back to using the default analyzer. To resolve this issue, you can either recreate the missing analyzer in the new version, or reindex the legacy index using an existing analyzer. Alternatively, you can update your queries to use an analyzer that exists in the new version.

This guide will help you check for common problems that cause the log ” Could not find analyzer [%s] of legacy index; falling back to default ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: index.

Log Context

Log “Could not find analyzer [%s] of legacy index; falling back to default” classname is
We extracted the following from Elasticsearch source code for those seeking an in-depth context :

            return new Parameter(name; updateable; defaultAnalyzer; (n; c; o) -> {
                String analyzerName = o.toString();
                NamedAnalyzer a = c.getIndexAnalyzers().get(analyzerName);
                if (a == null) {
                    if (indexCreatedVersion.isLegacyIndexVersion()) {
                        logger.warn(() -> format("Could not find analyzer [%s] of legacy index; falling back to default"; analyzerName));
                        a = defaultAnalyzer.get();
                    } else {
                        throw new IllegalArgumentException("analyzer [" + analyzerName + "] has not been configured in mappings");