Flatten graph token filter

Flattens a token graph produced by a graph token filter, such as synonym_graph or word_delimiter_graph.

Flattening a token graph containing multi-position tokens makes the graph suitable for indexing. Otherwise, indexing does not support token graphs containing multi-position tokens.

The flatten_graph filter uses Lucene’s FlattenGraphFilter.

To see how the flatten_graph filter works, you first need to produce a token graph containing multi-position tokens.

The following analyze API request uses the synonym_graph filter to add internet phonebook as a multi-position synonym for domain name system in the text domain name system is fragile:

 GET /_analyze {
  "tokenizer": "standard",
  "filter": [
    {
      "type": "synonym_graph",
      "synonyms": [ "internet phonebook, domain name system" ]
    }
  ],
  "text": "domain name system is fragile"
}

The filter produces the following token graph with internet phonebook as a multi-position token.

Indexing does not support token graphs containing multi-position tokens. To make this token graph suitable for indexing, it needs to be flattened.

To flatten the token graph, add the flatten_graph filter after the synonym_graph filter in the previous analyze API request.

 GET /_analyze {
  "tokenizer": "standard",
  "filter": [
    {
      "type": "synonym_graph",
      "synonyms": [ "internet phonebook, domain name system" ]
    },
    "flatten_graph"
  ],
  "text": "domain name system is fragile"
}

The filter produces the following flattened token graph, which is suitable for indexing.

The following create index API request uses the flatten_graph token filter to configure a new custom analyzer.

In this analyzer, a custom word_delimiter_graph filter produces token graphs containing catenated, multi-position tokens. The flatten_graph filter flattens these token graphs, making them suitable for indexing.

 PUT /my-index-000001 {
  "settings": {
    "analysis": {
      "analyzer": {
        "my_custom_index_analyzer": {
          "type": "custom",
          "tokenizer": "standard",
          "filter": [
            "my_custom_word_delimiter_graph_filter",
            "flatten_graph"
          ]
        }
      },
      "filter": {
        "my_custom_word_delimiter_graph_filter": {
          "type": "word_delimiter_graph",
          "catenate_all": true
        }
      }
    }
  }
}