Depending on where a script is used, it will have access to certain special variables and document fields.

Update scriptsedit

A script used in the update, update-by-query, or reindex API will have access to the ctx variable which exposes:

Search and aggregation scriptsedit

With the exception of script fields which are executed once per search hit, scripts used in search and aggregations will be executed once for every document which might match a query or an aggregation. Depending on how many documents you have, this could mean millions or billions of executions: these scripts need to be fast!

Field values can be accessed from a script using doc-values, the _source field, or stored fields, each of which is explained below.

Accessing the score of a document within a scriptedit

Scripts used in the function_score query, in script-based sorting, or in aggregations have access to the _score variable which represents the current relevance score of a document.

Here’s an example of using a script in a function_score query to alter the relevance _score of each document:

PUT my_index/_doc/1?refresh
{
  "text": "quick brown fox",
  "popularity": 1
}

PUT my_index/_doc/2?refresh
{
  "text": "quick fox",
  "popularity": 5
}

GET my_index/_search
{
  "query": {
    "function_score": {
      "query": {
        "match": {
          "text": "quick brown fox"
        }
      },
      "script_score": {
        "script": {
          "lang": "expression",
          "source": "_score * doc['popularity']"
        }
      }
    }
  }
}

Doc valuesedit

By far the fastest most efficient way to access a field value from a script is to use the doc['field_name'] syntax, which retrieves the field value from doc values. Doc values are a columnar field value store, enabled by default on all fields except for analyzed text fields.

PUT my_index/_doc/1?refresh
{
  "cost_price": 100
}

GET my_index/_search
{
  "script_fields": {
    "sales_price": {
      "script": {
        "lang":   "expression",
        "source": "doc['cost_price'] * markup",
        "params": {
          "markup": 0.2
        }
      }
    }
  }
}

Doc-values can only return "simple" field values like numbers, dates, geo- points, terms, etc, or arrays of these values if the field is multi-valued. It cannot return JSON objects.

The document _sourceedit

The document _source can be accessed using the _source.field_name syntax. The _source is loaded as a map-of-maps, so properties within object fields can be accessed as, for example, _source.name.first.

For instance:

PUT my_index
{
  "mappings": {
    "properties": {
      "first_name": {
        "type": "text"
      },
      "last_name": {
        "type": "text"
      }
    }
  }
}

PUT my_index/_doc/1?refresh
{
  "first_name": "Barry",
  "last_name": "White"
}

GET my_index/_search
{
  "script_fields": {
    "full_name": {
      "script": {
        "lang": "painless",
        "source": "params._source.first_name + ' ' + params._source.last_name"
      }
    }
  }
}

Stored fieldsedit

Stored fields — fields explicitly marked as "store": true in the mapping — can be accessed using the _fields['field_name'].value or _fields['field_name'] syntax:

PUT my_index
{
  "mappings": {
    "properties": {
      "full_name": {
        "type": "text",
        "store": true
      },
      "title": {
        "type": "text",
        "store": true
      }
    }
  }
}

PUT my_index/_doc/1?refresh
{
  "full_name": "Alice Ball",
  "title": "Professor"
}

GET my_index/_search
{
  "script_fields": {
    "name_with_title": {
      "script": {
        "lang": "painless",
        "source": "params._fields['title'].value + ' ' + params._fields['full_name'].value"
      }
    }
  }
}