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Append processor

Appends one or more values to an existing array if the field already exists and it is an array. Converts a scalar to an array and appends one or more values to it if the field exists and it is a scalar. Creates an array containing the provided values if the field doesn’t exist. Accepts a single value or an array of values.

Table 3. Append Options

Name	Required	Default	Description
`field`	yes	-	The field to be appended to. Supports template snippets.
`value`	yes	-	The value to be appended. Supports template snippets.
`allow_duplicates`	no	true	If `false`, the processor does not append values already present in the field.
`media_type`	no	`application/json`	The media type for encoding `value`. Applies only when `value` is a template snippet. Must be one of `application/json`, `text/plain`, or `application/x-www-form-urlencoded`.
`description`	no	-	Description of the processor. Useful for describing the purpose of the processor or its configuration.
`if`	no	-	Conditionally execute the processor. See Conditionally run a processor.
`ignore_failure`	no	`false`	Ignore failures for the processor. See Handling pipeline failures.
`on_failure`	no	-	Handle failures for the processor. See Handling pipeline failures.
`tag`	no	-	Identifier for the processor. Useful for debugging and metrics.

{
  "append": {
    "field": "tags",
    "value": ["production", "{{{app}}}", "{{{owner}}}"]
  }
}

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https://www.elastic.co/guide/en/elasticsearch/reference/7.17/append-processor.html