How to use synonyms

OpenSearchServer offers considerable flexibility when working with synonyms:

  • OpenSearchServer handles multiple lists of synonyms.
  • Synonyms can be a single word, or a sequence of words.
  • A given term can have any number of synonyms.
  • Synonyms only get used when querying, which is particularly handy in practice.

In OpenSearchServer, lists of synonyms are simply created using the Schema / Synonyms tab. Just write one series of synonyms per line. For instance :

pc,computer,personal computer

Configuring schemas to work with synonyms

An index's schema must be configured to make use of these lists of synonyms. This is done with Analyzers - by adding a filter to an existing analyzer. Here is an example :

Creation of a synonyms analyzer

Here are the tokens created by this modified analyzer when querying with the keywords "a great PC":

Test results

Here is what is happening, step by step:

  1. The tokenizer splits the sentence into tokens.
  2. The filter LowerCaseFilter transforms every word to lower case.
  3. The filter Shingler filter creates several groups of tokens. In this example it was configured to produce groups of 1 to 3 words, as you can see in the Max shingle size and Min shingle size settings in the screenshot. This is because Max shingle size must match the number of words in the longest synonym of the list. If the longest synonym in the list is "personal computer", a Max shingle size of 2 is thus sufficient.
  4. The filter SynonymFilter detects available synonyms by comparing each token to the list of synonyms. The applicable synonyms are then added to the list of tokens.
  5. The filter RemoveTokenTypeFilter removes the extra shingles.

The last 3 filters are only applied at query time. This allows for considerable flexibility: documents can be indexed first and list of synonyms can be written, modified or expanded later, without having to index the data again.

Configuring queries to make use of synonyms

Queries must belong to the Search (field) type. Searched fields must use the Term & Phrase mode to properly handle synonyms. Here is an example of a search for "cheap pc" on a field called title, using the analyzer from the previous example:

Query using synonyms

The final query pattern used by OpenSearchServer is (title:cheap (title:pc title:computer title:"personal computer")) title:"cheap pc"~10. This is functionally equivalent to running a cluster of queries in one shot. The automatic use of the Boolean OR operator in this pattern means that the query can be expressed as :

search for cheap pc OR cheap computer OR cheap personal computer OR "cheap pc".

And thus, a document titled "A cheap computer" will be found when searching for "cheap pc".

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