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Transform string fields to text/keyword

Older Elasticsearch versions used string as the primary text field type. Modern Elasticsearch and OpenSearch use text and keyword instead. Migration Assistant automatically converts legacy string fields into modern text or keyword mappings during metadata migration.

Built-in transformation behavior

Migration Assistant chooses the target field type based on how the original string field was configured:

• Analyzed string behavior becomes text.
• Non-analyzed or exact-match string behavior becomes keyword.

The transformation also removes or normalizes incompatible legacy mapping properties where needed.

Identifying string fields

To verify whether your source uses string fields, run the following command:

console clusters curl source /_mapping

If the source mapping contains "type":"string", this built-in transformation may apply.

Post-migration validation

After metadata migration, verify the target mapping:

console clusters curl target /your-index/_mapping
workflow show

Then validate the application behavior that depends on those fields, especially:

• Term queries
• Aggregations
• Sorting
• Case-sensitive exact-match logic

Custom transformer

Use a custom field type transformer if the built-in string conversion is not enough for your application semantics, for example if:

• You need custom multi-field behavior.
• You want field renaming at the same time.
• You need special cleanup of field properties.