Highlighting matched text in Elasticsearch - Mini Project: Build & Apply

Highlighting Matched Text in Elasticsearch

📖 Scenario: You are building a search feature for a small online bookstore. When users search for a word, you want to show the book titles with the searched word highlighted so it is easy to spot.

🎯 Goal: Create an Elasticsearch query that searches for the word magic in book titles and highlights the matched word in the results.

📋 What You'll Learn

Create an Elasticsearch index called books with a title field.

Add a document with the title 'The Magic of Thinking Big'.

Write a search query that looks for the word magic in the title field.

Add highlighting to the query so the matched word magic is wrapped in <em> tags in the results.

💡 Why This Matters

🌍 Real World

Highlighting matched text helps users quickly see why a search result was returned, improving user experience in search applications.

💼 Career

Understanding Elasticsearch highlighting is useful for roles involving search engine development, backend engineering, and data retrieval optimization.

1

Create the Elasticsearch index and add a book document

Create an Elasticsearch index called books with a title field. Then add a document with title set to 'The Magic of Thinking Big'.

Elasticsearch

# Create the index and add the document below

Hint

Use PUT to create the index with mappings, then POST to add the document.

2

Set up the search query to find the word 'magic'

Write a search query that looks for the word magic in the title field of the books index.

Elasticsearch

# Add a search query to find 'magic' in the title field below

Hint

Use the match query on the title field with the word magic.

3

Add highlighting to the search query

Add a highlight section to the search query that highlights matches in the title field using <em> tags.

Elasticsearch

# Add the highlight section to the search query below

Hint

Use highlight with fields and specify pre_tags and post_tags for the title field.

4

Display the search results with highlighted matched text

Run the search query and print the highlight section from the first hit to show the matched word magic wrapped in <em> tags.

Elasticsearch

# Print the highlight from the first search hit below

Hint

Extract the highlight field from the first hit in the search results and print it.

Practice

1. What is the main purpose of the highlight section in an Elasticsearch query?

easy

A. To filter documents before searching

B. To sort the search results by relevance

C. To update documents in the index

D. To show matched words clearly in search results

Solution

1. Step 1: Understand the role of highlighting

   Highlighting is used to emphasize matched words in search results so users can see why a document matched.

2. Step 2: Compare with other query parts

   Sorting, filtering, and updating are different functions not related to highlighting matched text.

3. Final Answer:

   To show matched words clearly in search results -> Option D

4. Quick Check:

   Highlight = show matched words [OK]

Hint: Highlight section shows matched words in results [OK]

Common Mistakes:

• Confusing highlight with sorting or filtering
• Thinking highlight updates documents
• Assuming highlight changes search relevance

2. Which of the following is the correct syntax to add highlighting for the field content in an Elasticsearch query?

easy

A. "highlight": {"fields": {"content": {}}}

B. "highlight": {"field": "content"}

C. "highlight": {"content": {"fields": {}}}

D. "highlight": {"fields": ["content"]}

Solution

1. Step 1: Recall correct highlight syntax

   The correct syntax uses "highlight" with "fields" as an object mapping field names to options, like {"content": {}}.

2. Step 2: Check each option

   "highlight": {"fields": {"content": {}}} matches the correct structure. Options B, C, and D use incorrect keys or data types.

3. Final Answer:

   "highlight": {"fields": {"content": {}}} -> Option A

4. Quick Check:

   Highlight fields syntax = "highlight": {"fields": {"content": {}}} [OK]

Hint: Use "highlight": {"fields": {"fieldname": {}}} [OK]

Common Mistakes:

• Using "field" instead of "fields"
• Putting fields inside an array instead of object
• Misplacing keys inside highlight

3. Given this Elasticsearch query snippet, what will the highlight section return for a match in the title field?

{
  "query": {"match": {"title": "quick fox"}},
  "highlight": {"fields": {"title": {}}}
}

medium

A. An error because highlight requires custom tags

B. Only the word "quick" highlighted, "fox" ignored

C. The matched words "quick" and "fox" wrapped in default highlight tags

D. No highlighting because tags are missing

Solution

1. Step 1: Understand default highlight behavior

   When highlight is enabled with empty options, Elasticsearch wraps matched words with default tags like <em> and </em>.

2. Step 2: Analyze the query

   The query matches both "quick" and "fox" in the title, so both words will be highlighted.

3. Final Answer:

   The matched words "quick" and "fox" wrapped in default highlight tags -> Option C

4. Quick Check:

   Default highlight tags wrap matched words [OK]

Hint: Empty highlight options use default tags to wrap matches [OK]

Common Mistakes:

• Thinking highlight needs custom tags to work
• Assuming only first matched word is highlighted
• Believing missing tags cause errors

4. Identify the error in this Elasticsearch highlight query snippet:

{
  "query": {"match": {"description": "fast car"}},
  "highlight": {"fields": ["description"]}
}

medium

A. "match" query is invalid for highlighting

B. "fields" should be an object, not an array

C. "highlight" must be inside "query"

D. "description" field cannot be highlighted

Solution

1. Step 1: Check highlight fields syntax

   The "fields" key must be an object mapping field names to options, not an array.

2. Step 2: Verify other parts

   "match" query is valid, "highlight" is correctly placed, and any field can be highlighted if indexed.

3. Final Answer:

   "fields" should be an object, not an array -> Option B

4. Quick Check:

   Highlight fields = object, not array [OK]

Hint: Highlight fields must be an object, not a list [OK]

Common Mistakes:

• Using array instead of object for fields
• Placing highlight inside query incorrectly
• Assuming some fields can't be highlighted

5. You want to highlight matched text in both title and content fields with custom tags <strong> and </strong>. Which is the correct highlight section to achieve this?

hard

A. "highlight": {"fields": {"title": {}, "content": {}}, "pre_tags": [""], "post_tags": [""]}

B. "highlight": {"fields": ["title", "content"], "pre_tags": "", "post_tags": ""}

C. "highlight": {"fields": {"title": {"tags": ["", ""]}, "content": {"tags": ["", ""]}}}

D. "highlight": {"fields": {"title": {}, "content": {}}, "tags": ["", ""]}

Solution

1. Step 1: Understand how to set custom tags

   Custom tags are set globally using "pre_tags" and "post_tags" arrays outside the fields object.

2. Step 2: Check each option's structure

   "highlight": {"fields": {"title": {}, "content": {}}, "pre_tags": [""], "post_tags": [""]} correctly sets "fields" as an object and uses "pre_tags" and "post_tags" arrays. Options B, C, and D misuse keys or data types.

3. Final Answer:

   "highlight": {"fields": {"title": {}, "content": {}}, "pre_tags": [""], "post_tags": [""]} -> Option A

4. Quick Check:

   Custom tags use pre_tags/post_tags arrays [OK]

Hint: Use pre_tags/post_tags arrays for custom highlight tags [OK]

Common Mistakes:

• Using arrays for fields instead of objects
• Setting tags inside each field incorrectly
• Using single strings instead of arrays for tags