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Highlighting API

The highlighting API allows you to have only the relevant portions of document matching a search query returned as a result. This allows users to quickly see how a document relates to their query, with the search terms highlighted, usually in bold letters.

RediSearch implements high performance highlighting and summarization algorithms, with the following API:

Command syntax

FT.SEARCH ...
    SUMMARIZE [FIELDS {num} {field}] [FRAGS {numFrags}] [LEN {fragLen}] [SEPARATOR {sepstr}]
    HIGHLIGHT [FIELDS {num} {field}] [TAGS {openTag} {closeTag}]

There are two sub-commands commands used for highlighting. One is HIGHLIGHT which surrounds matching text with an open and/or close tag, and the other is SUMMARIZE which splits a field into contextual fragments surrounding the found terms. It is possible to summarize a field, highlight a field, or perform both actions in the same query.

Summarization

FT.SEARCH ...
    SUMMARIZE [FIELDS {num} {field}] [FRAGS {numFrags}] [LEN {fragLen}] [SEPARATOR {sepStr}]

Summarization will fragment the text into smaller sized snippets; each snippet will contain the found term(s) and some additional surrounding context.

RediSearch can perform summarization using the SUMMARIZE keyword. If no additional arguments are passed, all returned fields are summarized using built-in defaults.

The SUMMARIZE keyword accepts the following arguments:

  • FIELDS: If present, must be the first argument. This should be followed by the number of fields to summarize, which itself is followed by a list of fields. Each field present is summarized. If no FIELDS directive is passed, then all fields returned are summarized.

  • FRAGS: How many fragments should be returned. If not specified, a default of 3 is used.

  • LEN The number of context words each fragment should contain. Context words surround the found term. A higher value will return a larger block of text.

  • SEPARATOR The string used to divide between individual summary snippets. The default is ... which is common among search engines; but you may override this with any other string if you desire to programmatically divide them later on. You may use a newline sequence, as newlines are stripped from the result body anyway (thus, it will not be conflated with an embedded newline in the text)

Highlighting

FT.SEARCH ... HIGHLIGHT [FIELDS {num} {field}] [TAGS {openTag} {closeTag}]

Highlighting will highlight the found term (and its variants) with a user-defined tag. This may be used to display the matched text in a different typeface using a markup language, or to otherwise make the text appear differently.

RediSearch can perform highlighting using the HIGHLIGHT keyword. If no additional arguments are passed, all returned fields are highlighted using built-in defaults.

The HIGHLIGHT keyword accepts the following arguments:

  • FIELDS If present, must be the first argument. This should be followed by the number of fields to highlight, which itself is followed by a list of fields. Each field present is highlighted. If no FIELDS directive is passed, then all fields returned are highlighted.

  • TAGS If present, must be followed by two strings; the first is prepended to each term match, and the second is appended to it. If no TAGS are specified, a built-in tag value is appended and prepended.

Field selection

If no specific fields are passed to the RETURN, SUMMARIZE, or HIGHLIGHT keywords, then all of a document's fields are returned. However, if any of these keywords contain a FIELD directive, then the SEARCH command will only return the sum total of all fields enumerated in any of those directives.

The RETURN keyword is treated specially, as it overrides any fields specified in SUMMARIZE or HIGHLIGHT.

In the command RETURN 1 foo SUMMARIZE FIELDS 1 bar HIGHLIGHT FIELDS 1 baz, the fields foo is returned as-is, while bar and baz are not returned, because RETURN was specified, but did not include those fields.

In the command SUMMARIZE FIELDS 1 bar HIGHLIGHT FIELDS 1 baz, bar is returned summarized and baz is returned highlighted.