How to remove custom analyzer / filter in Elasticsearch

Once an index is created, you cannot directly delete or modify existing analyzers or filters because these configurations are defined at index creation time and are embedded in the index settings. If you need to change analyzers or filters, you have several approaches:

1. Create a new index

This is the most common method. You can create a new index and define the required analyzers or filters within it, then reindex data from the old index to the new one. The steps are as follows:

1. Define new index settings and mappings: Set up the new analyzers and filters and apply them when creating the index.

2. Use the Reindex API to migrate data: Copy data from the old index to the new index using Elasticsearch's Reindex API to maintain data integrity and consistency.

3. Validate the data: Confirm that data has been correctly migrated and that the new analyzers or filters function as expected.

4. Delete the old index: After data migration and validation, safely delete the old index.

2. Close the index for modification (not recommended)

This approach involves higher risks and is generally not recommended. However, in certain cases where you only need to modify other configurations besides analyzers, you might consider:

1. Close the index: Use the Close Index API to make the index unavailable for search and indexing operations.

2. Modify settings: Adjust the index settings, but note that analyzer and filter configurations are typically unmodifiable.

3. Open the index: Use the Open Index API to reopen the index after modifications.

3. Use index aliases to manage index versions

Using index aliases can abstract index versions, making the migration from an old index to a new one transparent to end users. You can switch the alias from pointing to the old index to the new index without requiring users to modify their query code.

Example

Suppose you need to migrate from an index old_index containing old analyzers to a new index new_index with updated analyzer settings. The steps are as follows:

bash
# Create new index
PUT /new_index
{
  "settings": {
    "analysis": {
      "analyzer": {
        "new_custom_analyzer": {
          "tokenizer": "standard",
          "filter": ["lowercase", "asciifolding"]
        }
      }
    }
  }
}

# Reindex data
POST /_reindex
{
  "source": {
    "index": "old_index"
  },
  "dest": {
    "index": "new_index"
  }
}

# Validate data (using appropriate queries)

# Delete old index
DELETE /old_index

By using this method, you can ensure the system's maintainability and scalability while maintaining access to historical data.