Metadata in Elasticsearch

What it is

Metadata is information about the data. In Elasticsearch, each document has associated metadata such as _id and _index meta fields.

Examples

• Routing meta field:
  • _routing, a routing value that places a document in a particular shard.
• Other meta field
  • _meta, not used by Elasticsearch but can be used to store application-specific metadata.

PUT index_01
{
  "mappings": {
    "_meta": { 
      "class": "App01::User01",
      "version": "01"
    }
  }
}

• Identity meta fields: 
  _id , the id of the document, for example querying based on _id field

PUT index01/_doc/1
{
  "text": "Document with ID 1"
}
 
PUT index01/_doc/2
{
  "text": "Document with ID 2"
}
 
GET index01/_search
{
  "query": {
    "terms": {
      "_id": [ "1", "2" ] 
    }
  }
}

Template in Elasticsearch

What it is

A template in Elasticsearch falls into  one of the two following categories and is  indexed inside Elasticsearch using its dedicated endpoint: 

1. Index Templates, which are a way to define a set of rules including index settings, mappings and an index pattern. The template is applied automatically whenever a new index is created with the matching pattern. Templates are also used to dynamically apply custom mapping for the fields which are not predefined inside existing mapping.

1. Search Templates, which help in defining templates for search queries using mustache scripting language. These templates act as a placeholder for variables defined inside the search queries.

Examples

Create a dynamic index template

PUT /_template/template_1?pretty
{
  "index_patterns": [
    "logs*",
    "api*"
  ],
  "settings": {
    "number_of_shards": 2
  },
  "mappings": {
    "dynamic_templates": [
      {
        "strings": {
          "match_mapping_type": "string",
          "mapping": {
            "type": "keyword"
          }
        }
      }
    ],
    "properties": {
      "host_name": {
        "type": "keyword"
      },
      "created_at": {
        "type": "date"
      }
    }
  }
}

Create a search template

POST /_scripts/search_template_1?pretty
{
    "script": {
        "lang": "mustache",
        "source": {
            "query": {
                "match": {
                    "description": "{{query_string}}"
                }
            }
        }
    }
}

Executing a search query using search template

GET /_search/template?pretty
{
    "id": "search_template_1", 
    "params": {
        "query_string": "hello world"
    }
}

The search request will be executed by default on all the indices available in the cluster and can be limited to particular indices using an index parameter.

Notes

A dynamic index template is always useful when you do not know the field names in advance and want to control their mapping as per the business use case.