---
title: Append processor
description: Use the Append processor to add a value to an existing array field, or create the field as an array if it doesn't exist. To use an append processor: Select...
url: https://www.elastic.co/elastic/docs-builder/docs/3028/solutions/observability/streams/management/extract/append
products:
  - Elastic Cloud Enterprise
  - Elastic Cloud Hosted
  - Elastic Cloud Serverless
  - Elastic Cloud on Kubernetes
  - Elastic Observability
  - Elastic Stack
  - Elasticsearch
  - Kibana
applies_to:
  - Elastic Cloud Serverless: Generally available
  - Elastic Stack: Generally available since 9.2, Preview in 9.1
---

# Append processor
Use the **Append** processor to add a value to an existing array field, or create the field as an array if it doesn't exist.
To use an append processor:
1. Select **Create** → **Create processor**.
2. Select **Append** from the **Processor** menu.
3. Set **Source Field** to the field you want append values to.
4. Set **Target field** to the values you want to append to the **Source Field**.

This functionality uses the Elasticsearch [append processor](https://docs-v3-preview.elastic.dev/elastic/docs-builder/docs/3028/reference/enrich-processor/append-processor) internally, but you configure it in Streamlang. Streamlang doesn't always have 1:1 parity with the ingest processor options and behavior. Refer to [Processor limitations and inconsistencies](/elastic/docs-builder/docs/3028/solutions/observability/streams/management/extract#streams-processor-inconsistencies).

## YAML reference

In [YAML mode](/elastic/docs-builder/docs/3028/solutions/observability/streams/management/extract#streams-editing-yaml-mode), configure the append processor using the following parameters. For the complete Streamlang syntax, refer to the [Streamlang reference](https://www.elastic.co/elastic/docs-builder/docs/3028/solutions/observability/streams/management/streamlang).

| Parameter          | Type    | Required | Description                                      |
|--------------------|---------|----------|--------------------------------------------------|
| `to`               | string  | Yes      | Array field to append to.                        |
| `value`            | array   | Yes      | Values to append.                                |
| `allow_duplicates` | boolean | No       | When `false`, duplicate values are not appended. |

```yaml
- action: append
  to: attributes.tags
  value:
    - processed
    - reviewed
```