Simulate multi-component templates
Since templates can be composed not only of multiple component templates, but also the index template itself, there are two simulation APIs to determine what the resulting index settings will be.

To simulate the settings that would be applied to a particular index name:

POST /_index_template/_simulate_index/my-index-000001
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To simulate the settings that would be applied from an existing template:

POST /_index_template/_simulate/template_1
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You can also specify a template definition in the simulate request. This enables you to verify that settings will be applied as expected before you add a new template.

PUT /_component_template/ct1
{
  "template": {
    "settings": {
      "index.number_of_shards": 2
    }
  }
}

PUT /_component_template/ct2
{
  "template": {
    "settings": {
      "index.number_of_replicas": 0
    },
    "mappings": {
      "properties": {
        "@timestamp": {
          "type": "date"
        }
      }
    }
  }
}

POST /_index_template/_simulate
{
  "index_patterns": ["my*"],
  "template": {
    "settings" : {
        "index.number_of_shards" : 3
    }
  },
  "composed_of": ["ct1", "ct2"]
}
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The response shows the settings, mappings, and aliases that would be applied to matching indices, and any overlapping templates whose configuration would be superseded by the simulated template body or higher-priority templates.

{
  "template" : {
    "settings" : {
      "index" : {
        "number_of_shards" : "3",   
        "number_of_replicas" : "0"
      }
    },
    "mappings" : {
      "properties" : {
        "@timestamp" : {
          "type" : "date"           
        }
      }
    },
    "aliases" : { }
  },
  "overlapping" : [
    {
      "name" : "template_1",        
      "index_patterns" : [
        "my*"
      ]
    }
  ]
}

The number of shards from the simulated template body


The @timestamp field inherited from the ct2 component template


Any overlapping templates that would have matched, but have lower priority

# Creates a component template for mappings
PUT _component_template/my-mappings
{
  "template": {
    "mappings": {
      "properties": {
        "@timestamp": {
          "type": "date",
          "format": "date_optional_time||epoch_millis"
        },
        "message": {
          "type": "wildcard"
        }
      }
    }
  },
  "_meta": {
    "description": "Mappings for @timestamp and message fields",
    "my-custom-meta-field": "More arbitrary metadata"
  }
}

# Creates a component template for index settings
PUT _component_template/my-settings
{
  "template": {
    "settings": {
      "index.lifecycle.name": "my-lifecycle-policy"
    }
  },
  "_meta": {
    "description": "Settings for ILM",
    "my-custom-meta-field": "More arbitrary metadata"
  }
}