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 Error Check-Up which analyzes 2 JSON files to detect many configuration errors.

Briefly, this error occurs when a field name is specified in uppercase or mixed case in Elasticsearch. Elasticsearch requires field names to be in lowercase and may return this error if the field name is not in lowercase. To resolve the issue, ensure that the field name is in lowercase.

To easily locate the root cause and resolve this issue try AutoOps for Elasticsearch & OpenSearch. It diagnoses problems by analyzing hundreds of metrics collected by a lightweight agent and offers guidance for resolving them.

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.

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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