[DOC] Document the usage clearly about update operation with ingest pipeline

[gaobinlong]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request.

The update document API doesn't support pipeline parameter, which indicates that update operation doesn't support ingest pipeline, but if there's a default ingest pipeline or final ingest pipeline in the index, then that ingest pipeline can be executed sometimes, we need to document the usage clearly, do not confuse users.

Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.

2.19.0

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.

opensearch-project/OpenSearch#16663

opensearch-project/OpenSearch#16712

[gaobinlong]

Please assign this to me, thanks!

[Naarcha-AWS]

@gaobinlong: Feel free to ping me once you have a PR open for this.

[RiwEZ]

@gaobinlong I tried updating a document in an index that has the default ingest pipeline enabled, and it still works in v3.5.0. Did we decide to keep this behavior as-is, or is there a change planned? Just wanted to double-check—I’m a bit confused.

minimal working example on opensearch dashboard console

GET /
{
  "name": "a77c1fb423a1",
  "cluster_name": "docker-cluster",
  "cluster_uuid": "DnHiW9jzR9mxJ8IuTjKdqw",
  "version": {
    "distribution": "opensearch",
    "number": "3.5.0",
    "build_type": "tar",
    "build_hash": "bbc94f0bdc3a759011e6529ecfe52840856f91a3",
    "build_date": "2026-02-07T07:54:31.169913465Z",
    "build_snapshot": false,
    "lucene_version": "10.3.2",
    "minimum_wire_compatibility_version": "2.19.0",
    "minimum_index_compatibility_version": "2.0.0"
  },
  "tagline": "The OpenSearch Project: https://opensearch.org/"
}

PUT _ingest/pipeline/my_default_pipeline
{
  "description": "Default pipeline that adds timestamp and uppercase a field",
  "processors": [
    {
      "set": {
        "field": "ingested_at",
        "value": "{{_ingest.timestamp}}"
      }
    },
  ]
}

PUT my_index
{
  "settings": {
    "index": {
      "default_pipeline": "my_default_pipeline",
      "number_of_shards": 1,
      "number_of_replicas": 0
    }
  },
  "mappings": {
    "properties": {
      "name": { "type": "text" },
      "status": { "type": "keyword" },
      "age": { "type": "integer" },
      "description": { "type": "text" },
      "ingested_at": { "type": "date" }
    }
  }
}

POST my_index/_doc/1
{
  "name": "John Doe",
  "status": "active",
  "age": 30,
  "description": "Software engineer"
}


POST my_index/_update/1
{
  "doc": {
    "age": 33,
    "description": "Senior software engineer"
  }
}

GET my_index/_search

when we update the doc, ingested_at is still being updated.

[gaobinlong]

Hi @RiwEZ , haven't had plan to change the behavior, will keep the existing behavior for a long time, thanks!