Semantic text field type

The semantic_text field type automatically generates embeddings for text content using an inference endpoint. Long passages are automatically chunked to smaller sections to enable the processing of larger corpuses of text.

The semantic_text field type specifies an inference endpoint identifier that will be used to generate embeddings. You can create the inference endpoint by using the Create inference API. This field type and the semantic query type make it simpler to perform semantic search on your data.

If you don’t specify an inference endpoint, the inference_id field defaults to .elser-2-elasticsearch, a preconfigured endpoint for the elasticsearch service.

Using semantic_text, you won’t need to specify how to generate embeddings for your data, or how to index it. The inference endpoint automatically determines the embedding generation, indexing, and query to use.

If you use the preconfigured .elser-2-elasticsearch endpoint, you can set up semantic_text with the following API request:

const response = await client.indices.create({
  index: "my-index-000001",
  mappings: {
    properties: {
      inference_field: {
        type: "semantic_text",
      },
    },
  },
});
console.log(response);

PUT my-index-000001
{
  "mappings": {
    "properties": {
      "inference_field": {
        "type": "semantic_text"
      }
    }
  }
}

To use a custom inference endpoint instead of the default .elser-2-elasticsearch, you must Create inference API and specify its inference_id when setting up the semantic_text field type.

const response = await client.indices.create({
  index: "my-index-000002",
  mappings: {
    properties: {
      inference_field: {
        type: "semantic_text",
        inference_id: "my-openai-endpoint",
      },
    },
  },
});
console.log(response);

PUT my-index-000002
{
  "mappings": {
    "properties": {
      "inference_field": {
        "type": "semantic_text",
        "inference_id": "my-openai-endpoint" 
      }
    }
  }
}

The recommended way to use semantic_text is by having dedicated inference endpoints for ingestion and search. This ensures that search speed remains unaffected by ingestion workloads, and vice versa. After creating dedicated inference endpoints for both, you can reference them using the inference_id and search_inference_id parameters when setting up the index mapping for an index that uses the semantic_text field.

const response = await client.indices.create({
  index: "my-index-000003",
  mappings: {
    properties: {
      inference_field: {
        type: "semantic_text",
        inference_id: "my-elser-endpoint-for-ingest",
        search_inference_id: "my-elser-endpoint-for-search",
      },
    },
  },
});
console.log(response);

PUT my-index-000003
{
  "mappings": {
    "properties": {
      "inference_field": {
        "type": "semantic_text",
        "inference_id": "my-elser-endpoint-for-ingest",
        "search_inference_id": "my-elser-endpoint-for-search"
      }
    }
  }
}

The inference_id will not be validated when the mapping is created, but when documents are ingested into the index. When the first document is indexed, the inference_id will be used to generate underlying indexing structures for the field.

Inference endpoints have a limit on the amount of text they can process. To allow for large amounts of text to be used in semantic search, semantic_text automatically generates smaller passages if needed, called chunks.

Each chunk refers to a passage of the text and the corresponding embedding generated from it. When querying, the individual passages will be automatically searched for each document, and the most relevant passage will be used to compute a score.

For more details on chunking and how to configure chunking settings, see Configuring chunking in the Inference API documentation.

Refer to this tutorial to learn more about semantic search using semantic_text and the semantic query.

You can extract the most relevant fragments from a semantic text field by using the highlight parameter in the Search API.

const response = await client.indices.create({
  index: "test-index",
  query: {
    semantic: {
      field: "my_semantic_field",
    },
  },
  highlight: {
    fields: {
      my_semantic_field: {
        type: "semantic",
        number_of_fragments: 2,
        order: "score",
      },
    },
  },
});
console.log(response);

PUT test-index
{
    "query": {
        "semantic": {
            "field": "my_semantic_field"
        }
    },
    "highlight": {
        "fields": {
            "my_semantic_field": {
                "type": "semantic",
                "number_of_fragments": 2,  
                "order": "score"           
            }
        }
    }
}

semantic_text uses defaults for indexing data based on the inference endpoint specified. It enables you to quickstart your semantic search by providing automatic inference and a dedicated query so you don’t need to provide further details.

In case you want to customize data indexing, use the sparse_vector or dense_vector field types and create an ingest pipeline with an inference processor to generate the embeddings. This tutorial walks you through the process. In these cases - when you use sparse_vector or dense_vector field types instead of the semantic_text field type to customize indexing - using the semantic_query is not supported for querying the field data.

Updates that use scripts are not supported for an index contains a semantic_text field. Even if the script targets non-semantic_text fields, the update will fail when the index contains a semantic_text field.

The semantic_text field type can be the target of copy_to fields. This means you can use a single semantic_text field to collect the values of other fields for semantic search. Each value has its embeddings calculated separately; each field value is a separate set of chunk(s) in the resulting embeddings.

This imposes a restriction on bulk requests and ingestion pipelines that update documents with semantic_text fields. In these cases, all fields that are copied to a semantic_text field, including the semantic_text field value, must have a value to ensure every embedding is calculated correctly.

For example, the following mapping:

const response = await client.indices.create({
  index: "test-index",
  mappings: {
    properties: {
      infer_field: {
        type: "semantic_text",
        inference_id: ".elser-2-elasticsearch",
      },
      source_field: {
        type: "text",
        copy_to: "infer_field",
      },
    },
  },
});
console.log(response);

PUT test-index
{
    "mappings": {
        "properties": {
            "infer_field": {
                "type": "semantic_text",
                "inference_id": ".elser-2-elasticsearch"
            },
            "source_field": {
                "type": "text",
                "copy_to": "infer_field"
            }
        }
    }
}

Will need the following bulk update request to ensure that infer_field is updated correctly:

resp = client.bulk(
    index="test-index",
    operations=[
        {
            "update": {
                "_id": "1"
            }
        },
        {
            "doc": {
                "infer_field": "updated inference field",
                "source_field": "updated source field"
            }
        }
    ],
)
print(resp)

const response = await client.bulk({
  index: "test-index",
  operations: [
    {
      update: {
        _id: "1",
      },
    },
    {
      doc: {
        infer_field: "updated inference field",
        source_field: "updated source field",
      },
    },
  ],
});
console.log(response);

PUT test-index/_bulk
{"update": {"_id": "1"}}
{"doc": {"infer_field": "updated inference field", "source_field": "updated source field"}}

Notice that both the semantic_text field and the source field are updated in the bulk request.

semantic_text field types have the following limitations: