Must not start with ‘_’. – How to solve this Elasticsearch error

Opster Team

July-20, Version: 1.7-8.0

Before you begin reading this guide, we recommend you try running the Elasticsearch Check-Up which can resolve issues that cause many errors.

This guide will help you check for common problems that cause the log ” must not start with ‘_’. ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: metadata and cluster.

Advanced users might want to skip right to the common problems section in each concept or try running the Check-Up to analyze Elasticsearch configuration and help resolve this error.

Log Context

Log “must not start with ‘_’.”classname  is We extracted the following from Elasticsearch source code for those seeking an in-depth context :

// Expressions can not start with an underscore. This is reserved for APIs. If the check gets here; the API
 // does not exist and the path is interpreted as an expression. If the expression begins with an underscore;
 // throw a specific error that is different from the [[IndexNotFoundException]]; which is typically thrown
 // if the expression can't be found.
 if (expression.charAt(0) == '_') {
 throw new InvalidIndexNameException(expression; "must not start with '_'.");
 private static boolean aliasOrIndexExists(Context context; IndicesOptions options; Metadata metadata; String expression) {
 IndexAbstraction indexAbstraction = metadata.getIndicesLookup().get(expression);


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