A New Way to Handle Synonyms in a Search Engine

22 May 2014

We recently added the support for Synonyms in Algolia! It has been the most requested feature in Algolia since our launch in September. While it may seem simple, it actually took us some time to implement because we wanted to do it in a different way than classic search engines.

What's wrong with synonyms

There are two main problems with how existing search engines handle synonyms. These issues disturb the user experience and could make them think "this search engine is buggy".

Typeahead

In most search engines, synonyms are not compatible with typeahead search. For example, if you want tablet  to equal  ipad in a query, the prefix search for t , ta , tab , tabl  & table  will not trigger the expansion on iPad ; Only the tablet query will. Thus, a single new letter in the search bar could totally change the result set, catching users off-guard.

Highlighting

Highlighting matched text is a key element of the user experience, especially when the search engine tolerates typos. This is the difference between making users think "I don't understand this result" and "This engine was able to understand my errors". Synonym expansions are rarely highlighted, which breaks the trust of the users in the search results and can feel like a bug.

Our implementation

We have identified two different use cases for synonyms: equalities and placeholders. The first and most common use case is when you tell the search engine that several words must be considered equal, for example st and street in an address. The second use case, which we call a placeholder, is when you indicate that a specific token can be replaced by a set of possible words and that the token itself is not searchable. For example, the content  street could be matched by the queries 1st street or 2nd street but not the query number street.

For the first use case, we have added a support of synonyms that is compatible with prefix search and have implemented two different ways to do highlighting (controlled by thereplaceSynonymsInHighlight  query parameter):

1. A mode where the original word that matched via a synonym is highlighted. For example if you have a record that contains black ipad 64GB  and a synonym black equals dark, then the following queries will fully highlight the black word : ipad d , ipad da , ipad dar & ipad dark. The typeahead search is working and the synonym expansion is fully highlighted: **black** **ipad** 64GB .
2. A mode where the original word is replaced by the synonym, and the matched prefix is highlighted. For example ipad d  query will replace black by dark and will highlight the first letter of dark: **d**ark **ipad** 64GB. This method allows to fully explain the results when the original word can be safely replaced by the matched synonym.

For the second use case, we have added support for placeholders. You can add a specific token in your records that will be safely replaced by a set of words defined in your configuration. The highlighting mode that replaces the original word by the expansion totally makes sense here. For example if you have mission street  record with a placeholder = [ "1st", "2nd", ....] , then the query 1st missionstreet will replace by 1st  and will highlight all words: **1st mission street**.

We believe this is a better way to handle synonyms and we hope you will like it :) We would love to get your feedback and ideas for improvement on this feature! Feel free to contact us at hey(at)algolia.com.