Traditional search engines rely on word-to-word matching (referred to as lexical search) to find results for queries. Although this works well for specific queries such as television model numbers, it struggles with more abstract searches. For example, when searching for “shoes for the beach,” a lexical search merely matches individual words “shoes,” “beach,” “for,” and “the” in catalog items, potentially missing relevant products like “water-resistant sandals” or “surf footwear” that don’t contain the exact search terms.
Large language models (LLMs) create dense vector embeddings for text that expand retrieval beyond individual word boundaries to include the context in which words are used. Dense vector embeddings capture the relationship between shoes and beaches by learning how often they occur together, enabling better retrieval for more abstract queries through what is called semantic search.
Sparse vectors combine the benefits of lexical and semantic search. The process starts with a WordPiece tokenizer to create a limited set of tokens from text. A transformer model then assigns weights to these tokens. During search, the system calculates the dot-product of the weights on the tokens (from the reduced set) from the query with tokens from the target document. You get a blended score from the terms (tokens) whose weights are high for both the query and the target. Sparse vectors encode semantic information, like dense vectors, and supply word-to-word matching through the dot-product, giving you a hybrid lexical-semantic match. For a detailed understanding of sparse and dense vector embeddings, visit Improving document retrieval with sparse semantic encoders in the OpenSearch blog.
Automatic semantic enrichment for Amazon OpenSearch Serverless makes implementing semantic search with sparse vectors effortless. You can now experiment with search relevance improvements and deploy to production with only a few clicks, requiring no long-term commitment or upfront investment. In this post, we show how automatic semantic enrichment removes friction and makes the implementation of semantic search for text data seamless, with step-by-step instructions to enhance your search functionality.
Automatic semantic enrichment
You could already enhance search relevance scoring beyond OpenSearch’s default lexical scoring with the Okapi BM25 algorithm, integrating dense vector and sparse vector models for semantic search using OpenSearch’s connector framework. However, implementing semantic search in OpenSearch Serverless has been complex and costly, requiring model selection, hosting, and integration with an OpenSearch Serverless collection.
Automatic semantic enrichment lets you automatically encode your text fields in your OpenSearch Serverless collections as sparse vectors by just setting the field type. During ingestion, OpenSearch Serverless automatically processes the data through a service-managed machine learning (ML) model, converting text to sparse vectors in native Lucene format.
Automatic semantic enrichment supports both English-only and multilingual options. The multilingual variant supports the following languages: Arabic, Bengali, Chinese, English, Finnish, French, Hindi, Indonesian, Japanese, Korean, Persian, Russian, Spanish, Swahili, and Telugu.
Model details and performance
Automatic semantic enrichment uses a service-managed, pre-trained sparse model that works effectively without requiring custom fine-tuning. The model analyzes the fields you specify, expanding them into sparse vectors based on learned associations from diverse training data. The expanded terms and their significance weights are stored in native Lucene index format for efficient retrieval. We’ve optimized this process using document-only mode, where encoding happens only during data ingestion. Search queries are merely tokenized rather than processed through the sparse model, making the solution both cost-effective and performant.
Our performance validation during feature development used the MS MARCO passage retrieval dataset, featuring passages averaging 334 characters. For relevance scoring, we measured average Normalized discounted cumulative gain (NDCG) for the first 10 search results (ndcg@10) on the BEIR benchmark for English content and average ndcg@10 on MIRACL for multilingual content. We assessed latency through client-side, 90th-percentile (p90) measurements and search response p90 took values. These benchmarks provide baseline performance indicators for both search relevance and response times.
The following table shows the automatic semantic enrichment benchmark.
| Language | Relevance improvement | P90 search latency |
| English | 20.0% over lexical search | 7.7% lower latency over lexical search (bm25 is 26 ms, and automatic semantic enrichment is 24 ms) |
| Multilingual | 105.1% over lexical search | 38.4% higher latency over lexical search (bm25 is 26 ms, and automatic semantic enrichment is 36 ms) |
Given the unique nature of each workload, we encourage you to evaluate this feature in your development environment using your own benchmarking criteria before making implementation decisions.
Pricing
OpenSearch Serverless bills automatic semantic enrichment based on OpenSearch Compute Units (OCUs) consumed during sparse vector generation at indexing time. You’re charged only for actual usage during indexing. You can monitor this consumption using the Amazon CloudWatch metric SemanticSearchOCU. For specific details about model token limits and volume throughput per OCU, visit Amazon OpenSearch Service Pricing.
Prerequisites
Before you create an automatic semantic enrichment index, verify that you’ve been granted the necessary permissions for the task. Contact an account administrator for assistance if required. To work with automatic semantic enrichment in OpenSearch Serverless, you need the account-level AWS Identity and Access Management (IAM) permissions shown in the following policy. The permissions serve the following purposes:
- The
aoss:*IndexIAM permissions is used to create and manage indices. - The
aoss:APIAccessAllIAM permission is used to perform OpenSearch API operations.
You also need an OpenSearch Serverless data access policy to create and manage Indices and associated resources in the collection. For more information, visit Data access control for Amazon OpenSearch Serverless in the OpenSearch Serverless Developer Guide. Use the following policy:
To access private collections, set up the following network policy:
Set up an automatic semantic enrichment index
To set up an automatic semantic enrichment index, follow these steps:
- To create an automatic semantic enrichment index using the AWS Command Line Interface (AWS CLI), use the create-index command:
- To describe the created index, use the following command:
You can also use AWS CloudFormation templates (Type: AWS::OpenSearchServerless::CollectionIndex) or the AWS Management Console to create semantic search during collection provisioning as well as after the collection is created.
Example: Index setup for product catalog search
This section shows how to set up a product catalog search index. You’ll implement semantic search on the title_semantic field (using an English model). For the product_id field, you’ll maintain default lexical search functionality.
In the following index-schema, the title_semantic field has a field type set to text and has parameter semantic_enrichment set to status ENABLED. Setting the semantic_enrichment parameter enables automatic semantic enrichment on the title_semantic field. You can use the language_options field to specify either english or multi-lingual. For this post, we generate a nonsemantic title field named title_non_semantic. Use the following code:
Data ingestion
After the index is created, you can ingest data through standard OpenSearch mechanisms, including client libraries, REST APIs, or directly through OpenSearch Dashboards. Here’s an example of how to add multiple documents using bulk API in OpenSearch Dashboards Dev Tools:
Search against automatic semantic enrichment index
After the data is ingested, you can query the index:
The following is the response:
The search successfully matched the document with Red shoes despite the query using crimson footwear, demonstrating the power of semantic search. The system automatically generated semantic embeddings for the document (truncated here for brevity) which enable these intelligent matches based on meaning rather than exact keywords.
Comparing search results
By running a similar query against the nonsemantic index title_non_semantic, you can confirm that nonsemantic fields can’t search based on context:
The following is the search response:
Limitations of automatic semantic enrichment
Automatic semantic search is most effective when applied to small-to-medium sized fields containing natural language content, such as movie titles, product descriptions, reviews, and summaries. Although semantic search enhances relevance for most use cases, it might not be optimal for certain scenarios:
- Very long documents – The current sparse model processes only the first 8,192 tokens of each document for English. For multilingual documents, it’s 512 tokens. For lengthy articles, consider implementing document chunking to ensure complete content processing.
- Log analysis workloads – Semantic enrichment significantly increases index size, which might be unnecessary for log analysis where exact matching typically suffices. The additional semantic context rarely improves log search effectiveness enough to justify the increased storage requirements.
Consider these limitations when deciding whether to implement automatic semantic enrichment for your specific use case.
Conclusion
Automatic semantic enrichment marks a significant advancement in making sophisticated search capabilities accessible to all OpenSearch Serverless users. By eliminating the traditional complexities of implementing semantic search, search developers can now enhance their search functionality with minimal effort and cost. Our feature supports multiple languages and collection types, with a pay-as-you-use pricing model that makes it economically viable for various use cases. Benchmark results are promising, particularly for English language searches, showing both improved relevance and reduced latency. However, although semantic search enhances most scenarios, certain use cases such as processing extremely long articles or log analysis might benefit from alternative approaches.
We encourage you to experiment with this feature and discover how it can optimize your search implementation so you can deliver better search experiences without the overhead of managing ML infrastructure. Check out the video and tech documentation for additional details.
About the Authors
Jon Handler is Director of Solutions Architecture for Search Services at Amazon Web Services, based in Palo Alto, CA. Jon works closely with OpenSearch and Amazon OpenSearch Service, providing help and guidance to a broad range of customers who have generative AI, search, and log analytics workloads for OpenSearch. Prior to joining AWS, Jon’s career as a software developer included four years of coding a large-scale, eCommerce search engine. Jon holds a Bachelor of the Arts from the University of Pennsylvania, and a Master of Science and a Ph. D. in Computer Science and Artificial Intelligence from Northwestern University.
Arjun Kumar Giri is a Principal Engineer at AWS working on the OpenSearch Project. He primarily works on OpenSearch’s artificial intelligence and machine learning (AI/ML) and semantic search features. He is passionate about AI, ML, and building scalable systems.
Siddhant Gupta is a Senior Product Manager (Technical) at AWS, spearheading AI innovation within the OpenSearch Project from Hyderabad, India. With a deep understanding of artificial intelligence and machine learning, Siddhant architects features that democratize advanced AI capabilities, enabling customers to harness the full potential of AI without requiring extensive technical expertise. His work seamlessly integrates cutting-edge AI technologies into scalable systems, bridging the gap between complex AI models and practical, user-friendly applications.
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