[[patterns]]
== Patterns
[[scoped-services]]
=== Scoped services
Whenever Kibana needs to get access to data saved in Elasticsearch, it
should perform a check whether an end-user has access to the data. 
The Kibana Platform introduced a handler interface on the server-side to perform that association
internally. Core services, that require impersonation with an incoming
request, are exposed via `context` argument of
{kib-repo}/blob/8.9/src/core/packages/http/server/src/router/request_handler.ts[the request handler interface].

[source,js]
----
async function handler(context, req, res) {
  const data = await context.core.elasticsearch.client.asCurrentUser('ping');
}
----

The {kib-repo}/blob/8.9/src/core/packages/http/server/src/router/request_handler.ts[request handler context] exposes the following scoped *core* services:

* {kib-repo}/blob/8.9/src/core/packages/saved-objects/api-server/src/saved_objects_client.ts[`context.savedObjects.client`]
* {kib-repo}/blob/8.9/src/core/packages/elasticsearch/server/src/client/scoped_cluster_client.ts[`context.elasticsearch.client`]
* {kib-repo}/blob/8.9/packages/core/ui-settings/core-ui-settings-server/src/ui_settings_client.ts[`context.uiSettings.client`]

==== Declare a custom scoped service

Plugins can extend the handler context with a custom API that will be
available to the plugin itself and all dependent plugins. For example,
the plugin creates a custom Elasticsearch client and wants to use it via
the request handler context:

[source,typescript]
----
import type { CoreSetup, RequestHandlerContext, IScopedClusterClient } from '@kbn/core/server';

interface MyRequestHandlerContext extends RequestHandlerContext {
 myPlugin: {
   client: IScopedClusterClient;
 };
}

class MyPlugin {
  setup(core: CoreSetup) {
    const client = core.elasticsearch.createClient('myClient');
    core.http.registerRouteHandlerContext<MyRequestHandlerContext, 'myPlugin'>('myPlugin', (context, req, res) => {
      return { client: client.asScoped(req) };
    });
    const router = core.http.createRouter<MyRequestHandlerContext>();
    router.get(
      { path: '/api/my-plugin/', validate: … },
      async (context, req, res) => {
        // context type is inferred as MyPluginContext
        const data = await context.myPlugin.client.asCurrentUser('endpoint');
      }
    );
  }
----
