Must not start with ‘_’. – Elasticsearch Error How To Solve Related Issues

Must not start with ‘_’. – Elasticsearch Error How To Solve Related Issues

Opster Team

July-20, Version: 1.7-8.0


Before you read this guide, we recommend you run the Elasticsearch Error Check-Up which detects issues in ES causing log errors (free tool that requires no installation). 

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, 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 discover the cause of many errors and provides suitable actionable recommendations how to resolve them (join over 700 users who use this free tool).

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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