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 | For more information, see [index-context]({{site.url}}{{site.baseurl}}/opensearch/index-context/). Amongst all settings and mappings declared for an index template, the ones coming from context will hold highest priority. As of OpenSearch 2.17, this is an experimental feature. Optional
 
 ### 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/) (experimental as of OpenSearch 2.17) for your newly created index. 
 
 ## Example request
 

_im-plugin/index-context.md

@@ -0,0 +1,170 @@
+---
+layout: default
+title: Index context
+nav_order: 14
+redirect_from:
+  - /opensearch/index-context/
+---
+
+# Index context
+
+Context can be used to declare the use case for which you are building your index. 
+Using the context information, OpenSearch can apply a pre-determined set of settings, and mappings to do the heavy-lifting around ensuring:
+
+1. Available performance optimizations are applied for the index.
+2. Existing settings are tuned according to the use-case.
+3. Mappings/Aliases based on [OpenSearch Integrations]({{site.url}}{{site.baseurl}}/integrations/)  are applied on the index if applicable.
+
+These metadata parameters applied are backed by component templates which are automatically loaded when your cluster comes up. These component templates are prefixed with `@abc_template@`, and can only be used based on the usage pattern outlined in this document.
+{: .important}
+
+The `context` parameter is available on indices and index templates as an experimental feature starting OpenSearch 2.17
+{: .warning}
+
+## Usage
+
+### Installation
+
+In order to use the `context` field while creating an index or index-template:
+
+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. Enable setting `cluster.application_templates.enabled` to `true`. See [configuring settings]({{site.url}}{{site.baseurl}}/install-and-configure/configuring-opensearch/index/#static-settings) for steps on how to do this.
+
+
+### Create Index
+
+For example, if you are using your indices to store metrics data, you can declare `metrics` as a context on your index:
+
+````
+```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 few settings applied:
+
+
+````
+```json
+GET /my-metrics-index
+```
+{% include copy-curl.html %}
+````
+
+````
+```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"
+        }
+    }
+}
+```
+````
+
+#### Considerations
+
+There are few restrictions which are applied when you decide to use `context` param when creating your indices:
+
+1. When you create an index using `context` field, any settings which are declared in context cannot be provided as part of index creation, or dynamic settings updates.
+2. Once set on an index/index-template, context cannot be removed from it.
+
+The above points ensure that at any point, the recommended settings/mappings are always applied on the indices.
+
+
+
+### Create Index Templates
+
+You can also provide the `context` parameter while creating index templates:
+
+````
+```json
+PUT _index_template/my-logs
+{
+    "context": {
+        "name": "logs",
+        "version": "1"
+    },
+    "index_patterns": [
+        "my-logs-*"
+    ]
+}
+```
+{% include copy-curl.html %}
+````
+
+````
+```json
+GET _index_template/my-logs
+```
+{% include copy-curl.html %}
+````
+
+````
+```json
+{
+    "index_templates": [
+        {
+            "name": "my-logs2",
+            "index_template": {
+                "index_patterns": [
+                    "my-logs1-*"
+                ],
+                "context": {
+                    "name": "logs",
+                    "version": "1"
+                }
+            }
+        }
+    ]
+}
+```
+````
+
+
+This will create an index template with the context information as `logs`, and all indices created via this index template will get the metadata provided by the associated component template.
+If there is any overlap between settings/mappings/aliases declared by your template directly and the same coming through the backing component template for the context, the latter gets higher priority during index creation.
+
+## Available Templates for Context
+
+Following templates are available to be used through the `context` parameter as of today:
+
+1. logs
+2. metrics
+3. nginx-logs
+4. amazon-cloudtrail-logs
+5. amazon-elb-logs
+6. amazon-s3-logs
+7. apache-web-logs
+8. k8s-logs
+
+To see more details on what these templates provide, please take a look at the [repository](https://github.com/opensearch-project/opensearch-system-templates/tree/d71e12588e3144ad0fd26745d084ba49e9e08349/src/main/resources/org/opensearch/system/applicationtemplates/v1).
+You can also check the current version of these templates installed on your cluster through `GET /_component_template`