# Highlighting Redis Open Source uses advanced algorithms for highlighting and summarizing, which enable only the relevant portions of a document to appear in response to a search query. This feature allows users to immediately understand the relevance of a document to their search criteria, typically highlighting the matching terms in bold text. ## 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 used for highlighting. The first is `HIGHLIGHT`, which surrounds matching text with an open and/or close tag. The second 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 of which containing the found term(s) and some additional surrounding context. Redis 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, it 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 is summarized. If no `FIELDS` directive is passed, then all returned fields are summarized. * **`FRAGS`**: The number of fragments to be returned. If not specified, a default is 3. * **`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. If not specified, the default value is 20. * **`SEPARATOR`**: The string used to divide 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 the snippets later on. You may also use a newline sequence, as newlines are stripped from the result body during processing. ### Highlighting ``` FT.SEARCH ... HIGHLIGHT [FIELDS {num} {field}] [TAGS {openTag} {closeTag}] ``` Highlighting will surround the found term (and its variants) with a user-defined pair of tags. 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. Redis performs 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, it 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 returned fields are highlighted. * **`TAGS`**: If present, it must be followed by two strings. The first string is prepended to each matched term. The second string is appended to each matched term. If no `TAGS` are specified, a built-in tag pair is prepended and appended to each matched term. #### 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.