How to undo setting Elasticsearch Index to readonly?

When an Elasticsearch index is set to read-only, any write operation to the index is rejected. This is typically used during maintenance or to protect data from accidental modification. To revert this setting and make the index writable again, update the index settings.

To revert the read-only setting of an Elasticsearch index, follow these steps:

1. Use Elasticsearch's REST API: Change the index settings by sending an HTTP request to the Elasticsearch cluster. Specifically, use a PUT request to update the index settings.

2. Modify Index Settings: In the request body, set index.blocks.read_only or index.blocks.read_only_allow_delete to false. Both settings control the read-only state of the index, but index.blocks.read_only_allow_delete is automatically triggered when disk space is insufficient.

3. Example: Assume we have an index named my_index that we wish to revert from read-only state. Use the following curl command:

   bash
   curl -X PUT "localhost:9200/my_index/_settings" -H "Content-Type: application/json" -d'
   {
     "index.blocks.read_only": false
   }
   '

   If the index was set to read-only due to index.blocks.read_only_allow_delete being true, use:

   bash
   curl -X PUT "localhost:9200/my_index/_settings" -H "Content-Type: application/json" -d'
   {
     "index.blocks.read_only_allow_delete": false
   }
   '

4. Verify the Changes: After updating the settings, verify the change by checking the current index settings using the index settings API:

   bash
   curl -X GET "localhost:9200/my_index/_settings?pretty"

By following these steps, you can effectively revert the read-only setting of an Elasticsearch index, making it writable again. In practice, ensure you have sufficient permissions to modify the index settings, and exercise caution when performing such operations in production environments.