Index templates let you initialize new indices with predefined mappings and settings. For example, if you continuously index log data, you can define an index template so that all of these indices have the same number of shards and replicas.
This command creates a template named `daily_logs` and applies it to any new index whose name matches the regular expression `logs-2020-01-*` and also adds it to the `my_logs` alias:
You can create multiple index templates for your indices. If the index name matches more than one template, OpenSearch merges all mappings and settings from all matching templates and applies them to the index.
The settings from the more recently created index templates override the settings of older index templates. So, you can first define a few common settings in a generic template that can act as a catch-all and then add more specialized settings as required.
An even better approach is to explicitly specify template priority using the `order` parameter. OpenSearch applies templates with lower priority numbers first and then overrides them with templates with higher priority numbers.
For example, say you have the following two templates that both match the `logs-2020-01-02` index and there’s a conflict in the `number_of_shards` field:
#### Template 1
```json
PUT _index_template/template-01
{
"index_patterns": [
"logs*"
],
"priority": 0,
"template": {
"settings": {
"number_of_shards": 2
}
}
}
```
#### Template 2
```json
PUT _index_template/template-02
{
"index_patterns": [
"logs-2020-01-*"
],
"priority": 1,
"template": {
"settings": {
"number_of_shards": 3
}
}
}
```
Because `template-02` has a higher `priority` value, it takes precedence over `template-01` . The `logs-2020-01-02` index would have the `number_of_shards` value as 3.
Managing multiple index templates has the following challenges:
- If you have duplication between index templates, storing these index templates results in a bigger cluster state.
- If you want to make a change across all your index templates, you have to manually make the change for each template.
- If an index matches multiple templates, Opensearch might merge the templates in an unexpected way that you discover only after an index is created.
You can use composable index templates to overcome these challenges. Composable index templates let you abstract common settings, mappings, and aliases into a reusable building block called a component template.
You can combine component templates to compose an index template.
Settings and mappings that you specify directly in the [create index]({{site.url}}{{site.baseurl}}/rest-api/create-index/) request overrides any settings or mappings specified in an index template and its component templates.
{: .note }
### Create a component template
Let's define two component templates—`component_template_1` and `component_template_2`:
#### Component template 1
```json
PUT _component_template/component_template_1
{
"template": {
"mappings": {
"properties": {
"@timestamp": {
"type": "date"
}
}
}
}
}
```
#### Component template 2
```json
PUT _component_template/component_template_2
{
"template": {
"mappings": {
"properties": {
"ip_address": {
"type": "ip"
}
}
}
}
}
```
### Use component templates to create an index template
When creating index templates, you need to include the component templates in a `composed_of` list.
Opensearch applies the component templates in the order in which you specify them within the index template. The setting, mappings, and aliases that you specify inside the index template are applied last.
For index templates composed of multiple component templates, you can simulate applying a new template to verify whether the settings are applied as you expect.
To simulate the settings that would be applied to a specific index name:
```json
POST _index_template/_simulate_index/<index_name>
```
To simulate the settings that would be applied from an existing template:
```json
POST _index_template/_simulate/<index_template>
```
You can also specify a template definition in the simulate request:
The `_simulate` API returns the final settings, mappings, and aliases that will be applied to indices that match the index pattern. You can also see any overlapping templates whose configuration is superseded by the simulated template body or higher-priority templates:
