Magento and FORBIDDEN/12/index read-only / allow delete (api)

If you are unlucky enough to be reading this article it means that you are seeing that error when trying to reindex Magento.

ElasticSuite Category Indexing index process unknown error:
{"error":{"root_cause":[{"type":"cluster_block_exception","reason":"blocked by: [FORBIDDEN/12/index read-only / allow delete (api)];"}],"type":"cluster_block_exception","reason":"blocked by: [FORBIDDEN/12/index read-only / allow delete (api)];"},"status":403}
ElasticSuite Thesaurus Indexing index process unknown error:
{"error":{"root_cause":[{"type":"cluster_block_exception","reason":"blocked by: [FORBIDDEN/12/index read-only / allow delete (api)];"}],"type":"cluster_block_exception","reason":"blocked by: [FORBIDDEN/12/index read-only / allow delete (api)];"},"status":403}

This happens because you are probably running out of disk space and Elasticsearch turns itself into read only mode because, well, there's no more space to do stuff.

In order to get out of this situation, you need to first disable that Elasticsearch feature of switching to read only mode (otherwise it will happen again) by running the following...

curl -XPUT -H "Content-Type: application/json" http://localhost:9200/_cluster/settings -d '{"persistent":{"cluster.routing.allocation.disk.threshold_enabled":false}}'

...and then get Elasticsearch out of the read only mode with the next command:

curl -XPUT -H "Content-Type: application/json" http://localhost:9200/_all/_settings -d '{"index.blocks.read_only_allow_delete": null}'

Besides this, you'll need to do some disk clean up to recover some space.