Must be lowercase – 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 be lowercase ” 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 be lowercase”classname  is We extracted the following from Elasticsearch source code for those seeking an in-depth context :

* Validate the name for an index against some static rules and a cluster state.
 public void validateIndexName(String index; ClusterState state) {
 validateIndexOrAliasName(index; InvalidIndexNameException::new);
 if (!index.toLowerCase(Locale.ROOT).equals(index)) {
 throw new InvalidIndexNameException(index; "must be lowercase");
 // NOTE: dot-prefixed index names are validated after template application; not here 
 if (state.routingTable().hasIndex(index)) {


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