OpenSearch/docs/reference/indices/simulate-multi-component-templates.asciidoc

[[simulate-multi-component-templates]]
== 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:

////
[source,console]
--------------------------------------------------
PUT /_index_template/template_1
{
  "index_patterns" : ["my*"],
  "priority" : 1,
  "template": {
    "settings" : {
      "number_of_shards" : 2
    }
  }
}
--------------------------------------------------
// TESTSETUP

[source,console]
--------------------------------------------------
DELETE /_index_template/template_1
--------------------------------------------------
// TEARDOWN

////

[source,console]
--------------------------------------------------
POST /_index_template/_simulate_index/my-index-000001
--------------------------------------------------

To simulate the settings that would be applied from an existing template:

[source,console]
--------------------------------------------------
POST /_index_template/_simulate/template_1
--------------------------------------------------

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.

[source,console]
--------------------------------------------------
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"]
}
--------------------------------------------------


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and any overlapping templates whose configuration would be superseded by the simulated template body 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.

[source,console-result]
---------------------------------------------------------
{
  "template" : {
    "settings" : {
      "index" : {
        "number_of_shards" : "3",   <1>
        "number_of_replicas" : "0"
      }
    },
    "mappings" : {
      "properties" : {
        "@timestamp" : {
          "type" : "date"           <2>
        }
      }
    },
    "aliases" : { }
  },
  "overlapping" : [
    {
      "name" : "template_1",        <3>
      "index_patterns" : [
        "my*"
      ]
    }
  ]
}
---------------------------------------------------------
<1> The number of shards from the simulated template body
<2> The `@timestamp` field inherited from the `ct2` component template
<3> Any overlapping templates that would have matched, but have lower priority