POST _ingest/pipeline/_simulate?verbose
{
  "pipeline" :
  {
    "description": "_description",
    "processors": [
      {
        "set" : {
          "tag": "my_set1",
          "field" : "field1",
          "value" : "value1"
        }
      },
      {
        "set" : {
          "tag": "my_set2",
          "field" : "field2",
          "value" : "value1"
        }
      }
    ]
  },
  "docs": [
    {
      "_index": "index",
      "_type": "_doc",
      "_id": "id",
      "_source": {
        "a": "foo-bar",
        "b": "bar"
      }
    }
  ]
}

POST _ingest/pipeline/_simulate?verbose
{
  "pipeline": {
    "processors": [
      {
        "set": {
          "field": "field1",
          "value": "{{a}}"
        }
      }
    ]
  },
  "docs": [
    {
      "_index": "index",
      "_type": "_doc",
      "_id": "id",
      "_source": {
        "a": "foo-bar"
      }
    }
  ]
}