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Elastic Docs ›Elasticsearch Guide [5.2] ›Analysis ›Token Filters

Synonym Graph Token Filter

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Synonym Graph Token Filter

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This functionality is in technical preview and may be changed or removed in a future release. Elastic will work to fix any issues, but features in technical preview are not subject to the support SLA of official GA features.

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.

This token filter is designed to be used as part of a search analyzer only. If you want to apply synonyms during indexing please use the standard synonym token filter.

The graph token stream created by this token filter requires special query handling. Currently only the Match and Multi Match queries can do this. Using it with any other type of analyzed query will potentially result in incorrect search results.

Synonyms are configured using a configuration file. Here is an example:

{
    "index" : {
        "analysis" : {
            "analyzer" : {
                "search_synonyms" : {
                    "tokenizer" : "whitespace",
                    "filter" : ["graph_synonyms"]
                }
            },
            "filter" : {
                "graph_synonyms" : {
                    "type" : "synonym_graph",
                    "synonyms_path" : "analysis/synonym.txt"
                }
            }
        }
    }
}

The above configures a search_synonyms filter, with a path of analysis/synonym.txt (relative to the config location). The search_synonyms analyzer is then configured with the filter. Additional settings are: ignore_case (defaults to false), and expand (defaults to true).

The tokenizer parameter controls the tokenizers that will be used to tokenize the synonym, and defaults to the whitespace tokenizer.

Two synonym formats are supported: Solr, WordNet.

Solr synonyms

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The following is a sample format of the file:

# Blank lines and lines starting with pound are comments.

# Explicit mappings match any token sequence on the LHS of "=>"
# and replace with all alternatives on the RHS.  These types of mappings
# ignore the expand parameter in the schema.
# Examples:
i-pod, i pod => ipod,
sea biscuit, sea biscit => seabiscuit

# Equivalent synonyms may be separated with commas and give
# no explicit mapping.  In this case the mapping behavior will
# be taken from the expand parameter in the schema.  This allows
# the same synonym file to be used in different synonym handling strategies.
# Examples:
ipod, i-pod, i pod
foozball , foosball
universe , cosmos
lol, laughing out loud

# If expand==true, "ipod, i-pod, i pod" is equivalent
# to the explicit mapping:
ipod, i-pod, i pod => ipod, i-pod, i pod
# If expand==false, "ipod, i-pod, i pod" is equivalent
# to the explicit mapping:
ipod, i-pod, i pod => ipod

# Multiple synonym mapping entries are merged.
foo => foo bar
foo => baz
# is equivalent to
foo => foo bar, baz

You can also define synonyms for the filter directly in the configuration file (note use of synonyms instead of synonyms_path):

{
    "filter" : {
        "synonym" : {
            "type" : "synonym_graph",
            "synonyms" : [
                "lol, laughing out loud",
                "universe, cosmos"
            ]
        }
    }
}

However, it is recommended to define large synonyms set in a file using synonyms_path, because specifying them inline increases cluster size unnecessarily.

WordNet synonyms

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Synonyms based on WordNet format can be declared using format:

{
    "filter" : {
        "synonym" : {
            "type" : "synonym_graph",
            "format" : "wordnet",
            "synonyms" : [
                "s(100000001,1,'abstain',v,1,0).",
                "s(100000001,2,'refrain',v,1,0).",
                "s(100000001,3,'desist',v,1,0)."
            ]
        }
    }
}

Using synonyms_path to define WordNet synonyms in a file is supported as well.

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