Add overview of extensibility part 1 (#34694)

* Add overview of extensibility part 1

* Add overview of extensibility part 1

* Add overview of extensibility part 1

* Add overview of extensibility part 1

* Add overview of extensibility part 1

Author: Rick-Anderson

aspnetcore/fundamentals/servers/yarp/extensibility.md

@@ -0,0 +1,36 @@
+---
+uid: fundamentals/servers/yarp/extensibility
+title: Overview of extensibility
+description: Overview of extensibility
+author: samsp-msft
+ms.author: samsp
+ms.date: 2/10/2025
+ms.topic: article
+content_well_notification: AI-contribution
+ai-usage: ai-assisted
+---
+
+# Overview of YARP extensibility
+
+* Middleware Pipeline
+* Http Forwarder
+
+## Middleware pipeline
+
+YARP uses the concept of [Routes](config-files.md#routes), [Clusters](config-files.md#clusters) and Destinations. These can be supplied through [configuration files](config-files.md) or [directly through code](config-providers.md). Based on the routing rules, YARP picks a cluster and enumerates the possible destinations. It then uses the middleware pipeline to select the destination based on destination health, session affinity, load balancing etc.
+
+![Image](https://github.com/user-attachments/assets/ff17a04f-0c3c-46c7-8ec2-a1ed3dbc948c)
+
+Most of the pre-built pipeline modules can be customized through code. You can also change the pipeline definition to replace modules with your own implementation(s) or add additional modules as needed.
+
+For more information see [Middleware](xref:fundamentals/servers/yarp/middleware).
+
+## Http Forwarder
+
+If the YARP pipeline is too rigid for your needs, or the scale of routing rules and destinations is not suitable for loading into memory, then you can implement your own routing logic and use the HTTP Forwarder to direct requests to your chosen destination. The HttpForwarder component takes the HTTP context and forwards the request to the supplied destination.
+
+![Image](https://github.com/user-attachments/assets/1a060a7e-fa43-49a4-bfad-f95d7d35be63)
+
+The transform component can still be used with the forwarder is needed.
+
+For more information see [Direct forwarding](xref:fundamentals/servers/yarp/direct-forwarding).

aspnetcore/toc.yml

@@ -1177,7 +1177,7 @@ items:
         items:
           - name: Get started with YARP
             uid: fundamentals/servers/yarp/getting-started
-          - name: YARP configuration
+          - name: configuration
             items:
             - name: Configuration Files
               uid: fundamentals/servers/yarp/config-files
@@ -1187,7 +1187,7 @@ items:
               uid: fundamentals/servers/yarp/config-filters
             - name: HTTP client configuration
               uid: fundamentals/servers/yarp/http-client-config
-          - name: YARP proxy Features
+          - name: proxy Features
             items:
               - name: Header Routing
                 uid: fundamentals/servers/yarp/header-routing
@@ -1213,7 +1213,7 @@ items:
                 uid: fundamentals/servers/yarp/distributed-tracing
               - name: A/B Testing
                 uid: fundamentals/servers/yarp/ab-testing
-          - name: YARP protocols and guidance
+          - name: protocols and guidance
             items:
               - name: Header Guidelines
                 uid: fundamentals/servers/yarp/header-guidelines
@@ -1229,21 +1229,23 @@ items:
                 uid: fundamentals/servers/yarp/lets-encrypt
               - name: WebSockets and SPDY
                 uid: fundamentals/servers/yarp/websockets
-          - name: YARP extensibility
-            items:
+          - name: extensibility
+            items:            
+            - name: Extensibility
+              uid: fundamentals/servers/yarp/extensibility
             - name: Middleware
               uid: fundamentals/servers/yarp/middleware
             - name: Direct Forwarding
               uid: fundamentals/servers/yarp/direct-forwarding
             - name: Destination Resolvers
               uid: fundamentals/servers/yarp/destination-resolvers
-          - name: YARP deployment options
+          - name: deployment options
             items:
             - name: Http.sys Delegation
               uid: fundamentals/servers/yarp/httpsys-delegation
             - name: Service Fabric Integration
               uid: fundamentals/servers/yarp/service-fabric-int
-          - name: YARP Troubleshooting
+          - name: Troubleshooting
             items:
             - name: Diagnosing proxy issues
               uid: fundamentals/servers/yarp/diagnosing-yarp-issues