[[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]
--------------------------------------------------
PUT /test_index
{
    "settings": {
        "index" : {
            "analysis" : {
                "analyzer" : {
                    "synonym" : {
                        "tokenizer" : "whitespace",
                        "filter" : ["synonym"]
                    }
                },
                "filter" : {
                    "synonym" : {
                        "type" : "synonym",
                        "synonyms_path" : "analysis/synonym.txt"
                    }
                }
            }
        }
    }
}
--------------------------------------------------
// CONSOLE

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.

This filter tokenize synonyms with whatever tokenizer and token filters
appear before it in the chain.

Additional settings are:

* `expand` (defaults to `true`).
* `lenient` (defaults to `false`). If `true` ignores exceptions while parsing the synonym configuration. It is important
to note that only those synonym rules which cannot get parsed are ignored. For instance consider the following request:

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


[float]
==== `tokenizer` and `ignore_case` are deprecated

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.

Two synonym formats are supported: Solr, WordNet.

[float]
==== Solr synonyms

The following is a sample format of the file:

[source,synonyms]
--------------------------------------------------
include::{es-test-dir}/cluster/config/analysis/synonym.txt[]
--------------------------------------------------

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

[source,js]
--------------------------------------------------
PUT /test_index
{
    "settings": {
        "index" : {
            "analysis" : {
                "filter" : {
                    "synonym" : {
                        "type" : "synonym",
                        "synonyms" : [
                            "i-pod, i pod => ipod",
                            "universe, cosmos"
                        ]
                    }
                }
            }
        }
    }
}
--------------------------------------------------
// CONSOLE

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

[float]
==== WordNet synonyms

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

[source,js]
--------------------------------------------------
PUT /test_index
{
    "settings": {
        "index" : {
            "analysis" : {
                "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)."
                        ]
                    }
                }
            }
        }
    }
}
--------------------------------------------------
// CONSOLE

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