The synonym_graph token filter allows to easily handle synonyms, including multi-word synonyms correctly during the analysis process.

In order to properly handle multi-word synonyms this token filter creates a graph token stream during processing. For more information on this topic and its various complexities, please read the Lucene’s TokenStreams are actually graphs blog post.

Define synonyms setsedit

Synonyms in a synonyms set are defined using synonym rules. Each synonym rule contains words that are synonyms.

You can use two formats to define synonym rules: Solr and WordNet.

Solr formatedit

This format uses two different definitions:

WordNet formatedit

WordNet defines synonyms sets spanning multiple lines. Each line contains the following information:

The following example defines a synonym set for the words "come", "advance" and "approach":

s(100000002,1,'come',v,1,0).
s(100000002,2,'advance',v,1,0).
s(100000002,3,'approach',v,1,0).""";

Configure synonyms setsedit

Synonyms can be configured using the synonyms API, a synonyms file, or directly inlined in the token filter configuration. See store your synonyms set for more details on each option.

Use synonyms_set configuration option to provide a synonym set created via Synonyms Management APIs:

  "filter": {
    "synonyms_filter": {
      "type": "synonym",
      "synonyms_set": "my-synonym-set",
      "updateable": true
    }
  }

Use synonyms_path to provide a synonym file :

  "filter": {
    "synonyms_filter": {
      "type": "synonym",
      "synonyms_path": "analysis/synonym-set.txt"
    }
  }

The above configures a synonym filter, with a path of analysis/synonym-set.txt (relative to the config location).

Use synonyms to define inline synonyms:

  "filter": {
    "synonyms_filter": {
      "type": "synonym",
      "synonyms": ["pc => personal computer", "computer, pc, laptop"]
    }
  }

Additional settings are:

PUT /test_index
{
  "settings": {
    "index": {
      "analysis": {
        "analyzer": {
          "synonym": {
            "tokenizer": "standard",
            "filter": [ "my_stop", "synonym_graph" ]
          }
        },
        "filter": {
          "my_stop": {
            "type": "stop",
            "stopwords": [ "bar" ]
          },
          "synonym_graph": {
            "type": "synonym_graph",
            "lenient": true,
            "synonyms": [ "foo, bar => baz" ]
          }
        }
      }
    }
  }
}

With the above request the word bar gets skipped but a mapping foo => baz is still added. However, if the mapping being added was foo, baz => bar nothing would get added to the synonym list. This is because the target word for the mapping is itself eliminated because it was a stop word. Similarly, if the mapping was "bar, foo, baz" and expand was set to false no mapping would get added as when expand=false the target mapping is the first word. However, if expand=true then the mappings added would be equivalent to foo, baz => foo, baz i.e, all mappings other than the stop word.

tokenizer and ignore_case are deprecatededit

The tokenizer parameter controls the tokenizers that will be used to tokenize the synonym, this parameter is for backwards compatibility for indices that created before 6.0. The ignore_case parameter works with tokenizer parameter only.

Configure analyzers with synonym graph token filtersedit

To apply synonyms, you will need to include a synonym graph token filter into an analyzer:

      "analyzer": {
        "my_analyzer": {
          "type": "custom",
          "tokenizer": "standard",
          "filter": ["lowercase", "synonym_graph"]
        }
      }

Token filters orderingedit

Order is important for your token filters. Text will be processed first through filters preceding the synonym filter before being processed by the synonym filter.

In the above example, text will be lowercased by the lowercase filter before being processed by the synonyms_filter. This means that all the synonyms defined there needs to be in lowercase, or they won’t be found by the synonyms filter.

The synonym rules should not contain words that are removed by a filter that appears later in the chain (like a stop filter). Removing a term from a synonym rule means there will be no matching for it at query time.

Because entries in the synonym map cannot have stacked positions, some token filters may cause issues here. Token filters that produce multiple versions of a token may choose which version of the token to emit when parsing synonyms. For example, asciifolding will only produce the folded version of the token. Others, like multiplexer, word_delimiter_graph or ngram will throw an error.

If you need to build analyzers that include both multi-token filters and synonym filters, consider using the multiplexer filter, with the multi-token filters in one branch and the synonym filter in the other.