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.
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}}/opensearch/rest-api/create-index/) request override any settings or mappings specified in an index template and its component templates.
OpenSearch applies the component templates in the order in which you specify them within the index template. The settings, mappings, and aliases that you specify inside the index template are applied last.
