How do I search Elasticsearch by document ID?

Searching for a document in Elasticsearch by its ID is a fundamental operation. Here's how you can do it:

1. Using the GET API:

- The most straightforward way to retrieve a document by its ID is using the GET API. You'll need to specify the index, the document ID, and the type (if using Elasticsearch versions before 7.x). The basic structure of the request is:

GET /<index_name>/_doc/<document_id>

- Replace <index_name> with the name of your index and <document_id> with the ID of the document you want to retrieve.

- For example, if your index is named "products" and the document ID is "123", the request would be:

GET /products/_doc/123

2. Response:

- If the document exists, Elasticsearch will return a JSON response containing the document's source data, along with metadata like the index, type, and version.

- If the document does not exist, Elasticsearch will return a 404 Not Found error.

3. Using the Search API (Less Efficient for ID-Based Retrieval):

- While the GET API is the most efficient method for retrieving documents by ID, you can also use the Search API with a term query. However, this is generally less efficient because it involves a search operation rather than a direct lookup.

- The request structure would look like this:

GET /<index_name>/_search
{
  "query": {
    "term": {
      "_id": "<document_id>"
    }
  }
}

- Replace <index_name> and <document_id> with your index name and document ID, respectively.

- For example:

GET /products/_search
{
  "query": {
    "term": {
      "_id": "123"
    }
  }
}

4. Best Practice:

- Always prefer the GET API for retrieving documents by ID due to its efficiency and direct lookup mechanism.

In summary, using the GET API with the correct index name and document ID is the most efficient and recommended way to search for a document by its ID in Elasticsearch.

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