[[search-request-highlighting]]
=== Highlighting

Allows to highlight search results on one or more fields. The
implementation uses either the lucene `highlighter`, `fast-vector-highlighter`
or `postings-highlighter`. The following is an example of the  search request
body:

[source,js]
--------------------------------------------------
{
    "query" : {...},
    "highlight" : {
        "fields" : {
            "content" : {}
        }
    }
}
--------------------------------------------------

In the above case, the `content` field will be highlighted for each
search hit (there will be another element in each search hit, called
`highlight`, which includes the highlighted fields and the highlighted
fragments).

In order to perform highlighting, the actual content of the field is
required. If the field in question is stored (has `store` set to `yes`
in the mapping) it will be used, otherwise, the actual `_source` will
be loaded and the relevant field will be extracted from it.

The field name supports wildcard notation. For example, using `comment_*`
will cause all fields that match the expression to be highlighted.

==== Postings highlighter

If `index_options` is set to `offsets` in the mapping the postings highlighter
will be used instead of the plain highlighter. The postings highlighter:

* Is faster since it doesn't require to reanalyze the text to be highlighted:
the larger the documents the better the performance gain should be
* Requires less disk space than term_vectors, needed for the fast vector
highlighter
* Breaks the text into sentences and highlights them. Plays really well with
natural languages, not as well with fields containing for instance html markup
* Treats the document as the whole corpus, and scores individual sentences as
if they were documents in this corpus, using the  BM25 algorithm

Here is an example of setting the `content` field to allow for
highlighting using the postings highlighter on it:

[source,js]
--------------------------------------------------
{
    "type_name" : {
        "content" : {"index_options" : "offsets"}
    }
}
--------------------------------------------------

Note that the postings highlighter is meant to perform simple query terms
highlighting, regardless of their positions. That means that when used for
instance in combination with a phrase query, it will highlight all the terms
that the query is composed of, regardless of whether they are actually part of
a query match, effectively ignoring their positions.


==== Fast vector highlighter

If `term_vector` information is provided by setting `term_vector` to 
`with_positions_offsets` in the mapping then the fast vector highlighter
will be used instead of the plain highlighter.  The fast vector highlighter:

* Is faster especially for large fields (> `1MB`)
* Can be customized with `boundary_chars`, `boundary_max_scan`, and
 `fragment_offset` (see <<boundary-characters,below>>)
* Requires setting `term_vector` to `with_positions_offsets` which
  increases the size of the index

Here is an example of setting the `content` field to allow for
highlighting using the fast vector highlighter on it (this will cause
the index to be bigger):

[source,js]
--------------------------------------------------
{
    "type_name" : {
        "content" : {"term_vector" : "with_positions_offsets"}
    }
}
--------------------------------------------------

==== Force highlighter type

The `type` field allows to force a specific highlighter type. This is useful
for instance when needing to use the plain highlighter on a field that has
`term_vectors` enabled. The allowed values are: `plain`, `postings` and `fvh`.
The following is an example that forces the use of the plain highlighter:

[source,js]
--------------------------------------------------
{
    "query" : {...},
    "highlight" : {
        "fields" : {
            "content" : { "type" : "plain"}
        }
    }
}
--------------------------------------------------


[[tags]]
==== Highlighting Tags

By default, the highlighting will wrap highlighted text in `<em>` and
`</em>`. This can be controlled by setting `pre_tags` and `post_tags`,
for example:

[source,js]
--------------------------------------------------
{
    "query" : {...},
    "highlight" : {
        "pre_tags" : ["<tag1>"],
        "post_tags" : ["</tag1>"],
        "fields" : {
            "_all" : {}
        }
    }
}
--------------------------------------------------

Using the fast vector highlighter there can be more tags, and the "importance"
is ordered.

[source,js]
--------------------------------------------------
{
    "query" : {...},
    "highlight" : {
        "pre_tags" : ["<tag1>", "<tag2>"],
        "post_tags" : ["</tag1>", "</tag2>"],
        "fields" : {
            "_all" : {}
        }
    }
}
--------------------------------------------------

There are also built in "tag" schemas, with currently a single schema
called `styled` with the following `pre_tags`:

[source,js]
--------------------------------------------------
<em class="hlt1">, <em class="hlt2">, <em class="hlt3">,
<em class="hlt4">, <em class="hlt5">, <em class="hlt6">,
<em class="hlt7">, <em class="hlt8">, <em class="hlt9">,
<em class="hlt10">
--------------------------------------------------

and `</em>` as `post_tags`. If you think of more nice to have built in tag
schemas, just send an email to the mailing list or open an issue. Here
is an example of switching tag schemas:

[source,js]
--------------------------------------------------
{
    "query" : {...},
    "highlight" : {
        "tags_schema" : "styled",
        "fields" : {
            "content" : {}
        }
    }
}
--------------------------------------------------


==== Encoder

An `encoder` parameter can be used to define how highlighted text will
be encoded. It can be either `default` (no encoding) or `html` (will
escape html, if you use html highlighting tags).

==== Highlighted Fragments

Each field highlighted can control the size of the highlighted fragment
in characters (defaults to `100`), and the maximum number of fragments
to return (defaults to `5`).
For example:

[source,js]
--------------------------------------------------
{
    "query" : {...},
    "highlight" : {
        "fields" : {
            "content" : {"fragment_size" : 150, "number_of_fragments" : 3}
        }
    }
}
--------------------------------------------------

The `fragment_size` is ignored when using the postings highlighter, as it
outputs sentences regardless of their length.

On top of this it is possible to specify that highlighted fragments need
to be sorted by score:

[source,js]
--------------------------------------------------
{
    "query" : {...},
    "highlight" : {
        "order" : "score",
        "fields" : {
            "content" : {"fragment_size" : 150, "number_of_fragments" : 3}
        }
    }
}
--------------------------------------------------

If the `number_of_fragments` value is set to `0` then no fragments are
produced, instead the whole content of the field is returned, and of
course it is highlighted. This can be very handy if short texts (like
document title or address) need to be highlighted but no fragmentation
is required. Note that `fragment_size` is ignored in this case.

[source,js]
--------------------------------------------------
{
    "query" : {...},
    "highlight" : {
        "fields" : {
            "_all" : {},
            "bio.title" : {"number_of_fragments" : 0}
        }
    }
}
--------------------------------------------------

When using `fast-vector-highlighter` one can use `fragment_offset`
parameter to control the margin to start highlighting from.

In the case where there is no matching fragment to highlight, the default is
to not return anything. Instead, we can return a snippet of text from the
beginning of the field by setting `no_match_size` (default `0`) to the length
of the text that you want returned. The actual length may be shorter than
specified as it tries to break on a word boundary. When using the postings
highlighter it is not possible to control the actual size of the snippet,
therefore the first sentence gets returned whenever `no_match_size` is
greater than `0`.

[source,js]
--------------------------------------------------
{
    "query" : {...},
    "highlight" : {
        "fields" : {
            "content" : {
                "fragment_size" : 150,
                "number_of_fragments" : 3,
                "no_match_size": 150
            }
        }
    }
}
--------------------------------------------------


==== Highlight query

It is also possible to highlight against a query other than the search
query by setting `highlight_query`.  This is especially useful if you
use a rescore query because those are not taken into account by
highlighting by default.  Elasticsearch does not validate that
`highlight_query` contains the search query in any way so it is possible
to define it so legitimate query results aren't highlighted at all.
Generally it is better to include the search query in the
`highlight_query`.  Here is an example of including both the search
query and the rescore query in `highlight_query`.
[source,js]
--------------------------------------------------
{
    "fields": [ "_id" ],
    "query" : {
        "match": {
            "content": {
                "query": "foo bar"
            }
        }
    },
    "rescore": {
        "window_size": 50,
        "query": {
            "rescore_query" : {
                "match_phrase": {
                    "content": {
                        "query": "foo bar",
                        "phrase_slop": 1
                    }
                }
            },
            "rescore_query_weight" : 10
        }
    },
    "highlight" : {
        "order" : "score",
        "fields" : {
            "content" : {
                "fragment_size" : 150,
                "number_of_fragments" : 3,
                "highlight_query": {
                    "bool": {
                        "must": {
                            "match": {
                                "content": {
                                    "query": "foo bar"
                                }
                            }
                        },
                        "should": {
                            "match_phrase": {
                                "content": {
                                    "query": "foo bar",
                                    "phrase_slop": 1,
                                    "boost": 10.0
                                }
                            }
                        },
                        "minimum_should_match": 0
                    }
                }
            }
        }
    }
}
--------------------------------------------------

Note that the score of text fragment in this case is calculated by the Lucene
highlighting framework. For implementation details you can check the
`ScoreOrderFragmentsBuilder.java` class. On the other hand when using the
postings highlighter the fragments are scored using, as mentioned above,
the BM25 algorithm.

[[highlighting-settings]]
==== Global Settings

Highlighting settings can be set on a global level and then overridden
at the field level.

[source,js]
--------------------------------------------------
{
    "query" : {...},
    "highlight" : {
        "number_of_fragments" : 3,
        "fragment_size" : 150,
        "tag_schema" : "styled",
        "fields" : {
            "_all" : { "pre_tags" : ["<em>"], "post_tags" : ["</em>"] },
            "bio.title" : { "number_of_fragments" : 0 },
            "bio.author" : { "number_of_fragments" : 0 },
            "bio.content" : { "number_of_fragments" : 5, "order" : "score" }
        }
    }
}
--------------------------------------------------

[[field-match]]
==== Require Field Match

`require_field_match` can be set to `true` which will cause a field to
be highlighted only if a query matched that field. `false` means that
terms are highlighted on all requested fields regardless if the query
matches specifically on them.

[[boundary-characters]]
==== Boundary Characters

When highlighting a field using the fast vector highlighter,
`boundary_chars` can be configured to define what constitutes a boundary
for highlighting. It's a single string with each boundary character
defined in it. It defaults to `.,!? \t\n`.

The `boundary_max_scan` allows to control how far to look for boundary
characters, and defaults to `20`.
