Updating number of replicas to for indices – How to solve this OpenSearch error

Opster Team

Aug-23, Version: 1-2.9

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Briefly, this error occurs when there’s an attempt to update the number of replicas for certain indices, but the specified number is invalid or the system is unable to process the request due to resource constraints. To resolve this, ensure that the number of replicas is valid and within the capacity of your cluster. Also, check the health of your cluster and ensure it has enough resources to handle the additional replicas. If the issue persists, consider reindexing or scaling your cluster to accommodate the desired number of replicas.

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This guide will help you check for common problems that cause the log ” updating number_of_replicas to [{}] for indices {} ” to appear. To understand the issues related to this log, read the explanation below about the following OpenSearch concepts: routing, cluster, indices, allocation.

Log Context

Log “updating number_of_replicas to [{}] for indices {}” classname is AllocationService.java.
We extracted the following from OpenSearch source code for those seeking an in-depth context :

                    final IndexMetadata.Builder indexMetadataBuilder = new IndexMetadata.Builder(indexMetadata).settingsVersion(
                        1 + indexMetadata.getSettingsVersion()
                logger.info("updating number_of_replicas to [{}] for indices {}"; numberOfReplicas; indices);
            final ClusterState fixedState = ClusterState.builder(clusterState)


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