Highlighting matched text in Elasticsearch - Step-by-Step Execution

Concept Flow - Highlighting matched text

Send Search Query with Highlight

↓

Elasticsearch Matches Documents

↓

Highlight Matched Text in Fields

↓

Return Results with Highlighted Snippets

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Client Displays Highlighted Text

The search query is sent with highlight instructions, Elasticsearch finds matches, highlights matched text, and returns results with highlighted snippets.

Execution Sample

Elasticsearch

{
  "query": {
    "match": {"content": "apple"}
  },
  "highlight": {
    "fields": {"content": {}}
  }
}

This query searches for 'apple' in the 'content' field and highlights matched text in the results.

Execution Table

Step	Action	Query Part	Result/Effect
1	Send search query	{"match":{"content":"apple"},"highlight":{"fields":{"content":{}}}}	Query received by Elasticsearch
2	Match documents	match content: apple	Documents containing 'apple' found
3	Highlight matched text	highlight fields: content	Matched 'apple' text wrapped with highlight tags
4	Return results	results with highlight	Results include original fields plus highlighted snippets
5	Client displays	highlighted snippets	User sees 'apple' highlighted in search results

💡 All matching documents processed and highlighted text returned to client

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
query	{}	{"match":{"content":"apple"}}	{"match":{"content":"apple"},"highlight":{"fields":{"content":{}}}}	Same query throughout
matched_docs	[]	[doc1, doc2]	[doc1, doc2]	[doc1, doc2] with highlights
highlighted_text	N/A	N/A	["<em>apple</em>"]	["<em>apple</em>"]

Key Moments - 3 Insights

Why does the highlight only appear in the 'content' field?

Because the highlight section specifies only the 'content' field for highlighting, as shown in execution_table step 3.

What happens if no documents contain the word 'apple'?

No documents match in step 2, so no highlights are created or returned, ending the process early.

Does the original text change when highlighted?

No, the original text stays the same; highlight adds tags around matched words in the returned snippet only (step 3).

Visual Quiz - 3 Questions

Look at the execution_table, at which step does Elasticsearch find documents containing 'apple'?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Check the 'Action' and 'Result/Effect' columns in execution_table row for Step 2

According to variable_tracker, what is the value of 'highlighted_text' after Step 3?

A []

B ["apple"]

C ["<em>apple</em>"]

D N/A

💡 Hint

Look at the 'highlighted_text' row under 'After Step 3' in variable_tracker

If the highlight field was changed to 'title' instead of 'content', what would happen?

A Highlighting would appear in the 'title' field instead of 'content'

B Highlighting would appear in both 'title' and 'content'

C No highlighting would appear

D Query would fail

💡 Hint

Refer to execution_table step 3 where highlight fields are specified

Concept Snapshot

Highlighting matched text in Elasticsearch:
- Add 'highlight' section in query
- Specify fields to highlight
- Elasticsearch wraps matched terms with tags
- Results include highlighted snippets
- Client shows highlighted text to user

Full Transcript

This visual execution shows how Elasticsearch highlights matched text. First, a search query with a highlight section is sent. Elasticsearch finds documents matching the query term 'apple' in the 'content' field. Then it wraps the matched text with highlight tags like <em>. The results returned include these highlighted snippets. Finally, the client displays the highlighted text to the user. Variables like matched_docs and highlighted_text change as the query runs. Key points include that only specified fields are highlighted and original text is unchanged. The quiz checks understanding of steps and variable states.