A core component of any robust search application is the ability to retrieve relevant results even when users employ different terminology. This is where Service Synonyms come into play. In Azure AI Search, synonym maps provide a powerful mechanism to associate equivalent terms, effectively broadening the scope of a query without requiring users to input every possible variation. This article delves into the functionality and implementation of synonym maps in Azure AI Search.
Understanding Service Synonyms in Azure AI Search
Synonym maps in Azure AI Search function as a lookup table, linking words or phrases with their semantic equivalents. Imagine a user searching for “car repair.” A well-configured synonym map could expand this query to include related terms like “auto repair,” “vehicle maintenance,” or “automotive service,” significantly increasing the likelihood of retrieving all relevant documents.
Key features of synonym maps include:
- Centralized Resource: A single synonym map can be applied across multiple indexes, ensuring consistency and simplifying management.
- String Field Applicability: Synonym maps operate on string fields within your search index.
- Dynamic Application: Create and assign synonym maps at any point without disrupting ongoing indexing or query operations.
- Scalability: The number of synonym maps you can create is determined by your service tier.
- Singular Assignment: While multiple synonym maps can exist within a service, each field within an index can only be associated with one synonym map.
Creating and Implementing Service Synonym Maps
Synonym maps are defined programmatically using REST APIs or Azure SDKs. The structure consists of a name, format (solr is currently the only supported format), and a set of rules outlining the synonym relationships.
Defining Synonym Rules:
Rules adhere to the Apache Solr synonym filter specification and come in two flavors:
- Equivalency: Comma-separated terms within a rule are treated as equal substitutes. For instance, “USA, United States, United States of America” would ensure a query for any of these terms matches documents containing any of the others. Note: Phrase matching requires a quoted phrase query.
- Explicit Mapping: The
=>symbol designates an explicit mapping, where terms on the left are rewritten to the term on the right at query time. For example, “Washington, Wash., WA => WA” directs the engine to search only for “WA” when any of the left-hand terms are queried. This mapping is unidirectional.
Escaping Special Characters:
Standard analyzer rules for special characters apply to synonyms. To preserve characters typically discarded, consider using analyzers like Microsoft natural language analyzers or custom analyzers. Escaping with backslashes () might be necessary for characters like commas.
Managing and Assigning Synonym Maps
Synonym maps can be updated dynamically without impacting service availability. However, deleting a map associated with a field will result in query failures. Management operations (create, update, delete) apply to the entire map; partial modifications are not supported.
Assigning to Fields:
Assign synonym maps to specific string fields in your index programmatically. The field must be searchable ("searchable":true). A field can have only one synonym map assigned. No reindexing is required after assigning a synonym map; it takes effect on subsequent queries.
Querying and Execution with Service Synonyms
Querying remains unchanged after implementing synonyms. The search engine automatically expands or rewrites queries based on the defined rules.
Query Execution Behavior:
- Scope: Synonyms only apply to fields with synonym map assignments and free-form text queries. They do not affect filters, facets, autocomplete, or suggestions.
- Analysis: Synonym terms undergo the same text analysis as the associated field.
- Internal Rewriting: The engine internally rewrites queries using the OR operator for expanded terms, impacting hit highlighting and scoring.
- Wildcard Limitations: Synonym expansion does not apply to wildcard, prefix, fuzzy, or regex search terms. Combine these searches with synonym expansion using the OR operator if needed.
By leveraging service synonyms in Azure AI Search, you can significantly enhance the precision and recall of your search application, ensuring a more comprehensive and satisfying user experience. Experiment with synonym maps in a development environment to fine-tune their impact on your specific search scenarios.
