Elasticsearch's Query DSL and Rest API - unix1998/technical_notes GitHub Wiki

Elasticsearch's Query DSL (Domain Specific Language) is directly executed through the REST API. When you use the Query DSL, you are essentially crafting a JSON request body that defines the query you want to perform, and this JSON body is sent as part of an HTTP request to the Elasticsearch REST API.

How Query DSL and REST API Work Together

  1. Crafting the Query:

    • You write a query in JSON format using the Query DSL. This query describes what data you want to retrieve and how you want it to be filtered or aggregated.

  2. Sending the Request:

    • The JSON query is included in an HTTP request. Typically, you use a POST or GET method to send this request to a specific endpoint in the Elasticsearch REST API.

  3. Executing the Query:

    • Elasticsearch receives the HTTP request with the JSON query.
    • The Elasticsearch engine parses the JSON and executes the query against the data stored in the cluster.
    • The result is then returned as a JSON response.

Example Process

Writing a Query DSL:

{
  "query": {
    "match": {
      "field1": "value1"
    }
  }
}

Sending the Query via REST API:

Using curl to send the request:

curl -X POST "http://localhost:9200/my_index/_search" -H 'Content-Type: application/json' -d'
{
  "query": {
    "match": {
      "field1": "value1"
    }
  }
}
'

How Elasticsearch Processes This:

  1. HTTP Request:

    • Your curl command sends an HTTP POST request to the http://localhost:9200/my_index/_search endpoint with the JSON query as the request body.

  2. Parsing and Execution:

    • Elasticsearch's REST API endpoint receives the request.
    • The Elasticsearch engine parses the JSON body, interprets the Query DSL, and executes the query against the relevant index (my_index in this case).

  3. Returning Results:

    • Elasticsearch processes the query and retrieves the matching documents.
    • The results are formatted into a JSON response and sent back to the client (your curl command).

Key Points

  • Direct Execution: The Query DSL is directly executed by Elasticsearch when it receives the JSON query via the REST API.
  • No Compilation: There is no intermediate compilation step where the Query DSL is converted into REST API calls. The Query DSL itself is the payload of the REST API request.
  • REST API Integration: The entire interaction with Elasticsearch, including the execution of Query DSL queries, happens through its RESTful API endpoints.

This direct execution model ensures that queries are efficiently processed and results are promptly returned, leveraging Elasticsearch's powerful search and indexing capabilities.