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

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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 Error 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. It’s important to understand the issues related to the log, so to get started, read the general overview on common issues and tips related to the 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 which analyses ES to pinpoint the cause of many errors and provides suitable actionable recommendations how to resolve them (free tool that requires no installation).

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);


Run the Check-Up to get a customized report like this:

Analyze your cluster