Elasticsearch Highlighting Tutorial

Introduction

Highlighting is a powerful feature in Elasticsearch that allows you to emphasize certain parts of your search results. It is particularly useful when you want to show users where their search terms appear within the search results, making it easier for them to understand why a particular result was returned.

Basic Highlighting

At its core, highlighting in Elasticsearch is performed by specifying a highlight section in your search query. The following example demonstrates a basic highlighting setup:

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

In this example, we are performing a match query on the content field and asking Elasticsearch to highlight the occurrences of the term "Elasticsearch" in that field.

Customizing Highlighting

You can customize the appearance of highlighted text using pre-tags and post-tags. These tags wrap around the highlighted terms and allow you to style them as needed. Here's an example:

{
  "query": {
    "match": {
      "content": "Elasticsearch"
    }
  },
  "highlight": {
    "fields": {
      "content": {
        "pre_tags": [""],
        "post_tags": [""]
      }
    }
  }
}
                

In this example, the highlighted terms will be wrapped in <strong> and </strong> tags, making them bold in the resulting output.

Highlighting Multiple Fields

Elasticsearch allows you to highlight terms in multiple fields simultaneously. Here is an example of how to highlight terms in both the title and content fields:

{
  "query": {
    "multi_match": {
      "query": "Elasticsearch",
      "fields": ["title", "content"]
    }
  },
  "highlight": {
    "fields": {
      "title": {},
      "content": {}
    }
  }
}
                

This query will highlight occurrences of the term "Elasticsearch" in both the title and content fields.

Using Different Highlighters

Elasticsearch supports different types of highlighters. The default highlighter is the unified highlighter. You can also use the plain or fvh (Fast Vector Highlighter) highlighters. Here's an example using the fvh highlighter:

{
  "query": {
    "match": {
      "content": "Elasticsearch"
    }
  },
  "highlight": {
    "type": "fvh",
    "fields": {
      "content": {}
    }
  }
}
                

The fvh highlighter is useful for highlighting large documents and can be more efficient in certain scenarios.

Handling Large Documents

When dealing with large documents, it may be necessary to limit the size of the highlighted text fragments. You can do this by setting the fragment_size parameter:

{
  "query": {
    "match": {
      "content": "Elasticsearch"
    }
  },
  "highlight": {
    "fields": {
      "content": {
        "fragment_size": 150,
        "number_of_fragments": 3
      }
    }
  }
}
                

In this example, the highlighted text fragments will be limited to 150 characters, and up to 3 fragments will be shown for each matching document.

Conclusion

Highlighting in Elasticsearch is a powerful feature that allows you to emphasize search terms within your search results. By customizing the highlighting settings, you can improve the readability and usability of your search results, making it easier for users to find the information they need.