---
title: Retrieve a runtime field
description: Use the fields parameter on the _search API to retrieve the values of runtime fields. Runtime fields won’t display in _source, but the fields API works...
url: https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/4608/manage-data/data-store/mapping/retrieve-runtime-field
products:
  - Elasticsearch
applies_to:
  - Elastic Cloud Serverless: Generally available
  - Elastic Stack: Generally available in 9.0+
---

# Retrieve a runtime field
Use the [`fields`](https://docs-v3-preview.elastic.dev/elastic/elasticsearch/tree/main/reference/elasticsearch/rest-apis/retrieve-selected-fields) parameter on the `_search` API to retrieve the values of runtime fields. Runtime fields won’t display in `_source`, but the `fields` API works for all fields, even those that were not sent as part of the original `_source`.

## Define a runtime field to calculate the day of week

For example, the following request adds a runtime field called `day_of_week`. The runtime field includes a script that calculates the day of the week based on the value of the `@timestamp` field. We’ll include `"dynamic":"runtime"` in the request so that new fields are added to the mapping as runtime fields.
```json

{
  "mappings": {
    "dynamic": "runtime",
    "runtime": {
      "day_of_week": {
        "type": "keyword",
        "script": {
          "source": "emit(doc['@timestamp'].value.dayOfWeekEnum.getDisplayName(TextStyle.FULL, Locale.ENGLISH))"
        }
      }
    },
    "properties": {
      "@timestamp": {"type": "date"}
    }
  }
}
```


## Ingest some data

Let’s ingest some sample data, which will result in two indexed fields: `@timestamp` and `message`.
```json

{ "index": {}}
{ "@timestamp": "2020-06-21T15:00:01-05:00", "message" : "211.11.9.0 - - [2020-06-21T15:00:01-05:00] \"GET /english/index.html HTTP/1.0\" 304 0"}
{ "index": {}}
{ "@timestamp": "2020-06-21T15:00:01-05:00", "message" : "211.11.9.0 - - [2020-06-21T15:00:01-05:00] \"GET /english/index.html HTTP/1.0\" 304 0"}
{ "index": {}}
{ "@timestamp": "2020-04-30T14:30:17-05:00", "message" : "40.135.0.0 - - [2020-04-30T14:30:17-05:00] \"GET /images/hm_bg.jpg HTTP/1.0\" 200 24736"}
{ "index": {}}
{ "@timestamp": "2020-04-30T14:30:53-05:00", "message" : "232.0.0.0 - - [2020-04-30T14:30:53-05:00] \"GET /images/hm_bg.jpg HTTP/1.0\" 200 24736"}
{ "index": {}}
{ "@timestamp": "2020-04-30T14:31:12-05:00", "message" : "26.1.0.0 - - [2020-04-30T14:31:12-05:00] \"GET /images/hm_bg.jpg HTTP/1.0\" 200 24736"}
{ "index": {}}
{ "@timestamp": "2020-04-30T14:31:19-05:00", "message" : "247.37.0.0 - - [2020-04-30T14:31:19-05:00] \"GET /french/splash_inet.html HTTP/1.0\" 200 3781"}
{ "index": {}}
{ "@timestamp": "2020-04-30T14:31:27-05:00", "message" : "252.0.0.0 - - [2020-04-30T14:31:27-05:00] \"GET /images/hm_bg.jpg HTTP/1.0\" 200 24736"}
{ "index": {}}
{ "@timestamp": "2020-04-30T14:31:29-05:00", "message" : "247.37.0.0 - - [2020-04-30T14:31:29-05:00] \"GET /images/hm_brdl.gif HTTP/1.0\" 304 0"}
{ "index": {}}
{ "@timestamp": "2020-04-30T14:31:29-05:00", "message" : "247.37.0.0 - - [2020-04-30T14:31:29-05:00] \"GET /images/hm_arw.gif HTTP/1.0\" 304 0"}
{ "index": {}}
{ "@timestamp": "2020-04-30T14:31:32-05:00", "message" : "247.37.0.0 - - [2020-04-30T14:31:32-05:00] \"GET /images/nav_bg_top.gif HTTP/1.0\" 200 929"}
{ "index": {}}
{ "@timestamp": "2020-04-30T14:31:43-05:00", "message" : "247.37.0.0 - - [2020-04-30T14:31:43-05:00] \"GET /french/images/nav_venue_off.gif HTTP/1.0\" 304 0"}
```


## Search for the calculated day of week

The following request uses the search API to retrieve the `day_of_week` field that the original request defined as a runtime field in the mapping. The value for this field is calculated dynamically at query time without reindexing documents or indexing the `day_of_week` field. This flexibility allows you to modify the mapping without changing any field values.
```json

{
  "fields": [
    "@timestamp",
    "day_of_week"
  ],
  "_source": false
}
```

The previous request returns the `day_of_week` field for all matching documents. We can define another runtime field called `client_ip` that also operates on the `message` field and will further refine the query:
```json

{
  "runtime": {
    "client_ip": {
      "type": "ip",
      "script" : {
      "source" : "String m = doc[\"message\"].value; int end = m.indexOf(\" \"); emit(m.substring(0, end));"
      }
    }
  }
}
```

Run another query, but search for a specific IP address using the `client_ip` runtime field:
```json

{
  "size": 1,
  "query": {
    "match": {
      "client_ip": "211.11.9.0"
    }
  },
  "fields" : ["*"]
}
```

This time, the response includes only two hits. The value for `day_of_week` (`Sunday`) was calculated at query time using the runtime script defined in the mapping, and the result includes only documents matching the `211.11.9.0` IP address.
```json
{
  ...
  "hits" : {
    "total" : {
      "value" : 2,
      "relation" : "eq"
    },
    "max_score" : 1.0,
    "hits" : [
      {
        "_index" : "my-index-000001",
        "_id" : "oWs5KXYB-XyJbifr9mrz",
        "_score" : 1.0,
        "_source" : {
          "@timestamp" : "2020-06-21T15:00:01-05:00",
          "message" : "211.11.9.0 - - [2020-06-21T15:00:01-05:00] \"GET /english/index.html HTTP/1.0\" 304 0"
        },
        "fields" : {
          "@timestamp" : [
            "2020-06-21T20:00:01.000Z"
          ],
          "client_ip" : [
            "211.11.9.0"
          ],
          "message" : [
            "211.11.9.0 - - [2020-06-21T15:00:01-05:00] \"GET /english/index.html HTTP/1.0\" 304 0"
          ],
          "day_of_week" : [
            "Sunday"
          ]
        }
      }
    ]
  }
}
```


## Retrieve fields from related indices

The [`fields`](https://docs-v3-preview.elastic.dev/elastic/elasticsearch/tree/main/reference/elasticsearch/rest-apis/retrieve-selected-fields) parameter on the `_search` API can also be used to retrieve fields from the related indices using runtime fields with a type of `lookup`.
<note>
  Fields that are retrieved by runtime fields of type `lookup` can be used to enrich the hits in a search response. It’s not possible to query or aggregate on these fields.
</note>

```json

{
  "ip": "192.168.1.1",
  "country": "Canada",
  "city": "Montreal"
}


{
  "host": "192.168.1.1",
  "message": "the first message"
}


{
  "host": "192.168.1.2",
  "message": "the second message"
}


{
  "runtime_mappings": {
    "location": {
        "type": "lookup", <1>
        "target_index": "ip_location", <2>
        "input_field": "host", <3>
        "target_field": "ip", <4>
        "fetch_fields": ["country", "city"] <5>
    }
  },
  "fields": [
    "host",
    "message",
    "location"
  ],
  "_source": false
}
```

The above search returns the country and city from the `ip_location` index for each ip address of the returned search hits.
```json
{
  "took": 3,
  "timed_out": false,
  "_shards": {
    "total": 1,
    "successful": 1,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": {
      "value": 2,
      "relation": "eq"
    },
    "max_score": 1.0,
    "hits": [
      {
        "_index": "logs",
        "_id": "1",
        "_score": 1.0,
        "fields": {
          "host": [ "192.168.1.1" ],
          "location": [
            {
              "city": [ "Montreal" ],
              "country": [ "Canada" ]
            }
          ],
          "message": [ "the first message" ]
        }
      },
      {
        "_index": "logs",
        "_id": "2",
        "_score": 1.0,
        "fields": {
          "host": [ "192.168.1.2" ],
          "message": [ "the second message" ]
        }
      }
    ]
  }
}
```

The response of lookup fields are grouped to maintain the independence of each document from the lookup index. The lookup query for each input value is expected to match at most one document on the lookup index. If the lookup query matches more than one documents, then a random document will be selected.