Add documentation for context and ABC templates

_api-reference/index-apis/create-index-template.md

@@ -45,7 +45,7 @@ Parameter | Type | Description
 `priority` | Integer | A number that determines which index templates take precedence during the creation of a new index or data stream. OpenSearch chooses the template with the highest priority. When no priority is given, the template is assigned a `0`, signifying the lowest priority. Optional.
 `template` | Object | The template that includes the `aliases`, `mappings`, or `settings` for the index. For more information, see [#template]. Optional.
 `version` | Integer | The version number used to manage index templates. Version numbers are not automatically set by OpenSearch. Optional.
-
+`context` | Object | (Experimental) Context provides use case specific predefined templates which can be applied to an index. Amongst all settings and mappings declared for a template, context templates hold the highest priority. For more information, see [index-context]({{site.url}}{{site.baseurl}}/im-plugin/index-context/).
 
 ### Template
 

_api-reference/index-apis/create-index.md

@@ -50,7 +50,7 @@ timeout | Time | How long to wait for the request to return. Default is `30s`.
 
 ## Request body
 
-As part of your request, you can optionally specify [index settings]({{site.url}}{{site.baseurl}}/im-plugin/index-settings/), [mappings]({{site.url}}{{site.baseurl}}/field-types/index/), and [aliases]({{site.url}}{{site.baseurl}}/opensearch/index-alias/) for your newly created index. 
+As part of your request, you can optionally specify [index settings]({{site.url}}{{site.baseurl}}/im-plugin/index-settings/), [mappings]({{site.url}}{{site.baseurl}}/field-types/index/), [aliases]({{site.url}}{{site.baseurl}}/opensearch/index-alias/), and [context]({{site.url}}{{site.baseurl}}/opensearch/index-context/). 
 
 ## Example request
 

_im-plugin/index-context.md

@@ -0,0 +1,175 @@
+---
+layout: default
+title: Index context
+nav_order: 14
+redirect_from:
+  - /opensearch/index-context/
+---
+
+# Index context
+
+This is an experimental feature and is not recommended for use in a production environment. For updates on the progress the feature or if you want to leave feedback, join the discussion in the [OpenSearch forum](https://forum.opensearch.org/).    
+{: .warning}
+
+Index Context declares the use case for an index. Using the context information, OpenSearch applies a pre-determined set of settings and mappings, which gives the following benefits:
+
+- Optimized performance.
+- Settings tuned to your specific use case.
+- Accurate mappings and aliases based on [OpenSearch Integrations]({{site.url}}{{site.baseurl}}/integrations/).
+
+The settings and metadata configuration that are applied using component templates are automatically loaded when your cluster starts. Component templates that start with `@abc_template@` or Application Based Configuration (ABC) templates can only be used through a `context` object declaration, in order to prevent configuration issues.
+{: .warning}
+
+
+## Installation
+
+To install the index context feature:
+
+1. Install the `opensearch-system-templates` plugin on all nodes in your cluster using one of the [installation methods]({{site.url}}{{site.baseurl}}/install-and-configure/plugins/#install)
+
+2. Set the feature flag `opensearch.experimental.feature.application_templates.enabled` to `true`. For more information about enabling and disabling feature flags, see [Enabling experimental features]({{site.url}}{{site.baseurl}}/install-and-configure/configuring-opensearch/experimental/).
+
+3. Set the `cluster.application_templates.enabled` setting to `true`. For instructions on how to configure OpenSearch, see [configuring settings]({{site.url}}{{site.baseurl}}/install-and-configure/configuring-opensearch/index/#static-settings).
+
+## Using the `context` setting
+
+Use the `context` setting with the Index API to add use-case specific context.
+
+### Considerations
+
+Consider the following when using the `context` parameter during index creation:
+
+1. If you use the `context` field to create an index, you cannot include any settings declared in context during index creation or dynamic settings updates.
+2. Context becomes permanent when set on an index or index-template.
+
+When you adhere to these limitations, suggested configurations or mappings are uniformly applied on indexed data within the specified context.
+
+### Examples
+
+The following examples show how to use index context.
+
+
+#### Create Index
+
+The following example request creates an index to store metrics data by declaring a `metrics` mapping as the context:
+
+```json
+PUT /my-metrics-index
+{
+  "context": {
+    "name": "metrics"
+  }
+}
+```
+{% include copy-curl.html %}
+
+After creation, the index will have the context added to it, and also have corresponding settings applied:
+
+
+**GET request**
+
+```json
+GET /my-metrics-index
+```
+{% include copy-curl.html %}
+
+
+**Response**
+
+```json
+{
+    "my-metrics-index": {
+        "aliases": {},
+        "mappings": {},
+        "settings": {
+            "index": {
+                "codec": "zstd_no_dict",
+                "refresh_interval": "60s",
+                "number_of_shards": "1",
+                "provided_name": "my-metrics-index",
+                "merge": {
+                    "policy": "log_byte_size"
+                },
+                "context": {
+                    "created_version": "1",
+                    "current_version": "1"
+                },
+                ...
+            }
+        },
+        "context": {
+            "name": "metrics",
+            "version": "_latest"
+        }
+    }
+}
+```
+
+
+#### Create index templates
+
+You can also use the `context` parameter when creating an index template. The following example request creates an index template with the context information as `logs`.
+
+```json
+PUT _index_template/my-logs
+{
+    "context": {
+        "name": "logs",
+        "version": "1"
+    },
+    "index_patterns": [
+        "my-logs-*"
+    ]
+}
+```
+{% include copy-curl.html %}
+
+All indexes created using this index template will get the metadata provided by the associated component template. The following request and response shows how `context` is added to the template:
+
+**Get index template**
+
+```json
+GET _index_template/my-logs
+```
+{% include copy-curl.html %}
+
+**Response**
+
+```json
+{
+    "index_templates": [
+        {
+            "name": "my-logs2",
+            "index_template": {
+                "index_patterns": [
+                    "my-logs1-*"
+                ],
+                "context": {
+                    "name": "logs",
+                    "version": "1"
+                }
+            }
+        }
+    ]
+}
+```
+
+If there is any overlap between any settings, mappings, or aliases declared by your template directly and the backing component template for the context, the latter gets higher priority during index creation.
+
+
+## Available templates for context
+
+The following templates are available to be used through the `context` parameter as of OpenSearch 2.17:
+
+- `logs`
+- `metrics`
+- `nginx-logs`
+- `amazon-cloudtrail-logs`
+- `amazon-elb-logs`
+- `amazon-s3-logs`
+- `apache-web-logs`
+- `k8s-logs`
+
+For more information about these templates, see the [OpenSearch system templates repository](https://github.com/opensearch-project/opensearch-system-templates/tree/main/src/main/resources/org/opensearch/system/applicationtemplates/v1).
+
+To view the current version of these templates on your cluster, use `GET /_component_template`.