Source Filtering and Highlighting
The bookshop's result list shows only a title, a price and a snippet with the shopper's words in bold. Yet every hit comes back with the whole book — blurb, reviews, attributes and all. Two features shape the response to fit the screen: source filtering trims what comes back, and highlighting marks why each hit matched.
The rule
_source picks the fields you get back; highlight marks the words that matched.
Each hit normally includes _source, the original JSON you indexed. Source filtering trims it:
GET /books/_search
{
"_source": [ "title", "price" ],
"query": { "match": { "title": "garden" } }
}Each hit's _source now holds only title and price. Other forms:
"_source": false— no source at all (you still get_idand_score)."_source": { "includes": [ "title", "reviews.*" ], "excludes": [ "reviews.user" ] }— wildcards, plus exclusions applied after the includes.
This changes only what is returned. The query still searches every field, and the stored document is untouched. Smaller responses mean less data over the network and less JSON for the app to parse.
Highlighting returns fragments of text with the matching words wrapped in tags:
GET /books/_search
{
"query": { "match": { "blurb": "slow travel" } },
"highlight": {
"fields": { "blurb": {} },
"pre_tags": [ "<mark>" ],
"post_tags": [ "</mark>" ]
}
}Each hit gains a highlight section:
"highlight": {
"blurb": [ "An overnight journey that makes the case for <mark>slow</mark> <mark>travel</mark> across India." ]
}Without pre_tags and post_tags, the default wrapper is <em>. Highlighting works on the terms the query matched, so a field only gets highlights if the query actually searched it. Long fields are cut into fragments around the matches; fragment_size and number_of_fragments control them.
The shortcut
Design the response from the screen backwards: list what the screen displays, put those fields in _source, and highlight only the fields the query searched.
What the card shows
Title, price, and a blurb snippet with the search words marked.
Drills
Answer fast — trust the rule.
What does "_source": false do?
With "_source": ["title"], can the query still match on blurb?
Which tag wraps highlighted words by default?
The query matches on title only, but you ask to highlight blurb. What appears for blurb?
Remember
- _source picks the fields you get back; highlight marks the words that matched.
- Source filtering trims the response, never the search or the stored data.
- Default highlight tag is
<em>; highlight only fields the query searched.