Add details supporting existing resources (#1691)

* Add details supporting existing resources. Fixes #1577

* Apply suggestions from code review

Author: IEvangelist

docs/fundamentals/app-host-overview.md

@@ -1,31 +1,42 @@
 ---
 title: .NET Aspire orchestration overview
-description: Learn the fundamental concepts of .NET Aspire orchestration and explore the various APIs to express resource references.
-ms.date: 09/24/2024
+description: Learn the fundamental concepts of .NET Aspire orchestration and explore the various APIs for adding resources and expressing dependencies.
+ms.date: 09/25/2024
 ms.topic: overview
 uid: aspire/app-host
 ---
 
 # .NET Aspire orchestration overview
 
-.NET Aspire provides APIs for expressing resources and dependencies within your distributed application. In addition to these APIs, [there's tooling](setup-tooling.md#install-net-aspire) that enables some compelling scenarios. The orchestrator is intended for local development purposes.
+.NET Aspire provides APIs for expressing resources and dependencies within your distributed application. In addition to these APIs, [there's tooling](setup-tooling.md#install-net-aspire) that enables several compelling scenarios. The orchestrator is intended for _local development_ purposes and isn't supported in production environments.
 
 <span id="terminology"></span>
 
 Before continuing, consider some common terminology used in .NET Aspire:
 
 - **App model**: A collection of resources that make up your distributed application (<xref:Aspire.Hosting.DistributedApplication>). For a more formal definition, see [Define the app model](#define-the-app-model).
 - **App host/Orchestrator project**: The .NET project that orchestrates the _app model_, named with the _*.AppHost_ suffix (by convention).
-- **Resource**: A [resource](#built-in-resource-types) represents a part of an application whether it be a .NET project, container, or executable, or some other resource like a database, cache, or cloud service (such as a storage service).
+- **Resource**: A [resource](#built-in-resource-types) is a dependent part of an application, such as a .NET project, container, executable, database, cache, or cloud service. It represents any part of the application that can be managed or referenced.
 - **Integration**: An integration is a NuGet package for either the _app host_ that models a _resource_ or a package that configures a client for use in a consuming app. For more information, see [.NET Aspire integrations overview](integrations-overview.md).
 - **Reference**: A reference defines a connection between resources, expressed as a dependency using the <xref:Aspire.Hosting.ResourceBuilderExtensions.WithReference*> API. For more information, see [Reference resources](#reference-resources).
 
 > [!NOTE]
-> .NET Aspire's orchestration is designed to enhance your local development experience by simplifying the management of your cloud-native app's configuration and interconnections. While it's an invaluable tool for development, it's not intended to replace production environment systems like [Kubernetes](../deployment/overview.md#deploy-to-kubernetes), which are specifically designed to excel in that context.
+> .NET Aspire's orchestration is designed to enhance your _local development_ experience by simplifying the management of your cloud-native app's configuration and interconnections. While it's an invaluable tool for development, it's not intended to replace production environment systems like [Kubernetes](../deployment/overview.md#deploy-to-kubernetes), which are specifically designed to excel in that context.
 
 ## Define the app model
 
-.NET Aspire empowers you to seamlessly build, provision, deploy, configure, test, run, and observe your cloud application. This is achieved through the utilization of an _app model_ that outlines the resources in your app and their relationships. These resources encompass projects, executables, containers, as well as external services and cloud resources that your app depends on. Within every .NET Aspire project, there is a designated [App host project](#app-host-project), where the app model is precisely defined using methods available on the <xref:Aspire.Hosting.IDistributedApplicationBuilder>. This builder is obtained by invoking <xref:Aspire.Hosting.DistributedApplication.CreateBuilder%2A?displayProperty=nameWithType>.
+.NET Aspire empowers you to seamlessly build, provision, deploy, configure, test, run, and observe your distributed applications. All of these capabilities are achieved through the utilization of an _app model_ that outlines the resources in your .NET Aspire solution and their relationships. These resources encompass projects, executables, containers, and external services and cloud resources that your app depends on. Within every .NET Aspire solution, there's a designated [App host project](#app-host-project), where the app model is precisely defined using methods available on the <xref:Aspire.Hosting.IDistributedApplicationBuilder>. This builder is obtained by invoking <xref:Aspire.Hosting.DistributedApplication.CreateBuilder%2A?displayProperty=nameWithType>.
+
+```csharp
+// Create a new app model builder
+var builder = DistributedApplication.CreateBuilder(args);
+
+// TODO:
+//   Add resources to the app model
+//   Express dependencies between resources
+
+builder.Build().Run();
+```
 
 ## App host project
 
@@ -49,11 +60,11 @@ To help visualize the relationship between the app host project and the resource
 
 :::image type="content" source="../media/app-host-resource-diagram.png" lightbox="../media/app-host-resource-diagram.png" alt-text="The relationship between the projects in the .NET Aspire Starter Application template.":::
 
-Each resource must be uniquely named. This diagram shows each resource and the relationships between them. The container resource is named "cache" and the project resources are named "apiservice" and "webfrontend". The web frontend project references the cache and API service projects. By expressing a reference in this way, the web frontend project is saying that it depends on these two resources.
+Each resource must be uniquely named. This diagram shows each resource and the relationships between them. The container resource is named "cache" and the project resources are named "apiservice" and "webfrontend". The web frontend project references the cache and API service projects. When you're expressing references in this way, the web frontend project is saying that it depends on these two resources, the "cache" and "apiservice" respectively.
 
 ## Built-in resource types
 
-.NET Aspire projects are made up of a set of resources. The primary base resource types in the [Aspire.Hosting.AppHost](https://www.nuget.org/packages/Aspire.Hosting.AppHost) Nuget package are described in the following table:
+.NET Aspire projects are made up of a set of resources. The primary base resource types in the [Aspire.Hosting.AppHost](https://www.nuget.org/packages/Aspire.Hosting.AppHost) NuGet package are described in the following table:
 
 | Method | Resource type | Description |
 |--|--|--|
@@ -75,7 +86,7 @@ var apiservice = builder.AddProject<Projects.AspireApp_ApiService>("apiservice")
 
 ## Reference resources
 
-A reference represents a dependency between resources. Consider the following:
+A reference represents a dependency between resources. Consider the following example app host `Program` C# code:
 
 ```csharp
 var builder = DistributedApplication.CreateBuilder(args);
@@ -96,7 +107,7 @@ The "webfrontend" project resource uses <xref:Aspire.Hosting.ResourceBuilderExte
 
 To express a container resource, use the <xref:Aspire.Hosting.ContainerResourceBuilderExtensions.AddContainer%2A> method:
 
-## [Docker](#tab/docker)
+#### [Docker](#tab/docker)
 
 ```csharp
 var builder = DistributedApplication.CreateBuilder(args);
@@ -111,7 +122,7 @@ var ollama = builder.AddContainer("ollama", "ollama/ollama")
 
 For more information, see [GPU support in Docker Desktop](https://docs.docker.com/desktop/gpu/).
 
-## [Podman](#tab/podman)
+#### [Podman](#tab/podman)
 
 ```csharp
 var builder = DistributedApplication.CreateBuilder(args);
@@ -148,14 +159,14 @@ builder.AddProject<Projects.AspireApp_Web>("webfrontend")
        .WithReference(apiservice);
 ```
 
-Project-to-project references are handled differently than resources that have well defined connection strings. Instead of connection string being injected into the "webfrontend" resource, environment variables to support service discovery are injected.
+Project-to-project references are handled differently than resources that have well-defined connection strings. Instead of connection string being injected into the "webfrontend" resource, environment variables to support service discovery are injected.
 
 | Method | Environment variable |
 |--|--|
 | `WithReference(cache)` | `ConnectionStrings__cache="localhost:62354"` |
 | `WithReference(apiservice)` | `services__apiservice__http__0="http://localhost:5455"` <br /> `services__apiservice__https__0="https://localhost:7356"` |
 
-Adding a reference to the "apiservice" project results in service discovery environment variables being added to the front-end. This is because typically, project to project communication occurs over HTTP/gRPC. For more information, see [.NET Aspire service discovery](../service-discovery/overview.md).
+Adding a reference to the "apiservice" project results in service discovery environment variables being added to the frontend. This is because typically, project-to-project communication occurs over HTTP/gRPC. For more information, see [.NET Aspire service discovery](../service-discovery/overview.md).
 
 It's possible to get specific endpoints from a container or executable using the <xref:Aspire.Hosting.ResourceBuilderExtensions.WithEndpoint%2A> and calling the <xref:Aspire.Hosting.ResourceBuilderExtensions.GetEndpoint%2A>:
 
@@ -179,11 +190,11 @@ The `port` parameter is the port that the container is listening on. For more in
 
 ### Service endpoint environment variable format
 
-In the preceding section, the `WithReference` method is used to express dependencies between resources. When service endpoints result in environment variables being injected into the dependent resource, the format may not be obvious. This section provides details on this format.
+In the preceding section, the `WithReference` method is used to express dependencies between resources. When service endpoints result in environment variables being injected into the dependent resource, the format might not be obvious. This section provides details on this format.
 
 When one resource depends on another resource, the app host injects environment variables into the dependent resource. These environment variables configure the dependent resource to connect to the resource it depends on. The format of the environment variables is specific to .NET Aspire and expresses service endpoints in a way that is compatible with [Service Discovery](../service-discovery/overview.md).
 
-Service endpoint environment variable names are prefixed with `services__` (double underscore), then the service name, the endpoint name, and finally the index. The index supports multiple endpoints for a single service, starting with `0` for the first endpoint and incrementing for each additional endpoint.
+Service endpoint environment variable names are prefixed with `services__` (double underscore), then the service name, the endpoint name, and finally the index. The index supports multiple endpoints for a single service, starting with `0` for the first endpoint and incrementing for each endpoint.
 
 Consider the following environment variable examples:
 
@@ -199,9 +210,15 @@ services__apiservice__myendpoint__0
 
 In the preceding example, the `apiservice` service has a named endpoint called `myendpoint`. The value of the environment variable is the URL of the service endpoint.
 
+## Reference existing resources
+
+Some situations warrant that you reference an existing resource, perhaps one that is deployed to a cloud provider. For example, you might want to reference an Azure database. In this case, you'd rely on the [Execution context](#execution-context) to dynamically determine whether the app host is running in "run" mode or "publish" mode. If you're running locally and want to rely on a cloud resource, you can use the `IsRunMode` property to conditionally add the reference. You might choose to instead create the resource in publish mode. Some [hosting integrations](integrations-overview.md#hosting-integrations) support providing a connection string directly, which can be used to reference an existing resource.
+
+Likewise, there might be use cases where you want to integrate .NET Aspire into an existing solution. One common approach is to add the .NET Aspire app host project to an existing solution. Within your app host, you express dependencies by adding project references to the app host and [building out the app model](#define-the-app-model). For example, one project might depend on another. These dependencies are expressed using the <xref:Aspire.Hosting.ResourceBuilderExtensions.WithReference%2A> method. For more information, see [Add .NET Aspire to an existing .NET app](../get-started/add-aspire-existing-app.md).
+
 ## Execution context
 
-The <xref:Aspire.Hosting.IDistributedApplicationBuilder> exposes an execution context (<xref:Aspire.Hosting.DistributedApplicationExecutionContext>), which provides information about the current execution of the app host. This context can be used to evaluate whether or not the app host is executing as "run" mode, or as part of a publish operation. Consider the following:
+The <xref:Aspire.Hosting.IDistributedApplicationBuilder> exposes an execution context (<xref:Aspire.Hosting.DistributedApplicationExecutionContext>), which provides information about the current execution of the app host. This context can be used to evaluate whether or not the app host is executing as "run" mode, or as part of a publish operation. Consider the following properties:
 
 - <xref:Aspire.Hosting.DistributedApplicationExecutionContext.IsRunMode%2A>: Returns `true` if the current operation is running.
 - <xref:Aspire.Hosting.DistributedApplicationExecutionContext.IsPublishMode%2A>: Returns `true` if the current operation is publishing.