Single node AWS Opensearch domain is yellow due to ".plugins-ml-stop-words" index

This is a known issue with single-node Amazon OpenSearch Service domains, particularly when upgrading from earlier versions. The ".plugins-ml-stop-words" index is automatically created with a default setting of one replica, which causes the cluster to remain in a yellow state since there's no second node to host the replica.

Unfortunately, as a master user, you typically cannot directly manipulate the settings of system indices like ".plugins-ml-stop-words", even with fine-grained access control enabled. This is a protective measure to prevent accidental changes to critical system indices.

However, there are a few potential remedies you can try:

1. Enable Auto-Tune: Auto-Tune is a feature in Amazon OpenSearch Service that can automatically optimize your domain's performance. It might be able to adjust the replica count for system indices. To enable Auto-Tune:

• Go to the OpenSearch Service console
• Select your domain
• Choose "Turn on" for Auto-Tune if it's not already enabled
• Optionally, you can schedule optimizations during an off-peak window

2. Restart the OpenSearch process: Sometimes, restarting the OpenSearch process on the node can help resolve cluster state issues. To do this:

• Go to the OpenSearch Service console
• Select your domain
• Navigate to the "Instance health" tab
• Select the node
• Choose "Restart OpenSearch/Elasticsearch process" from the Actions dropdown

3. Contact AWS Support: If the above steps don't resolve the issue, it's recommended to contact AWS Support. They may be able to assist you directly or provide a patch or workaround for this specific situation.

4. Consider upgrading to a multi-node cluster: While this may not be ideal for all use cases, moving to a multi-node cluster would allow for proper replication and resolve the yellow state caused by unassigned replicas.

Remember, while the yellow state isn't ideal, it doesn't necessarily mean your cluster isn't functioning. It's still operational, but some replicas are unassigned.
Sources
Enabling or disabling Auto-Tune - Amazon OpenSearch Service
Restart the OpenSearch process on a node - Amazon OpenSearch Service
Amazon OpenSearch Service now supports OpenSearch version 2.13 - AWS