[[analysis-synonym-tokenfilter]]
=== Synonym Token Filter

The `synonym` token filter allows to easily handle synonyms during the
analysis process. Synonyms are configured using a configuration file.
Here is an example:

[source,js]
--------------------------------------------------
{
    "index" : {
        "analysis" : {
            "analyzer" : {
                "synonym" : {
                    "tokenizer" : "whitespace",
                    "filter" : ["synonym"]
                }
            },
            "filter" : {
                "synonym" : {
                    "type" : "synonym",
                    "synonyms_path" : "analysis/synonym.txt"
                }
            }
        }
    }
}
--------------------------------------------------

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

[float]
==== Solr synonyms

The following is a sample format of the file:

[source,js]
--------------------------------------------------
# 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

# 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`):

[source,js]
--------------------------------------------------
{
    "filter" : {
        "synonym" : {
            "type" : "synonym",
            "synonyms" : [
                "i-pod, i pod => ipod",
                "universe, cosmos"
            ] 
        }
    }
}
--------------------------------------------------

However, it is recommended to define large synonyms set in a file using
`synonyms_path`.

[float]
==== WordNet synonyms

Synonyms based on http://wordnet.princeton.edu/[WordNet] format can be
declared using `format`:

[source,js]
--------------------------------------------------
{
    "filter" : {
        "synonym" : {
            "type" : "synonym",
            "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.