updated information on default frontends

Author: Erwinvandervalk

src/content/docs/bff/fundamentals/multi-frontend/index.mdx

@@ -112,6 +112,14 @@ and removed from the [`Path`](https://learn.microsoft.com/en-us/dotnet/api/micro
 that happens in the application is relative to the path of the frontend. This also includes the OpenID callback paths. 
 :::
 
+### Implicit frontend disabled
+
+When you don't add any frontends, then the BFF creates an 'implicit' default frontend. This allows the BFF to function correctly in 'single frontend' mode.
+As soon as you add a frontend, then this 'implicit' frontend is disabled. 
+
+This means that, if you want to use both explicitly matching frontends (on host headers or paths) and a default (fallback) frontend, that you should explicitly add
+this default frontend. 
+
 ## Adding A Frontend During Startup
 
 The simplest way to add frontends is during startup. 

src/content/docs/bff/getting-started/single-frontend.mdx

@@ -12,6 +12,10 @@ Duende.BFF (Backend for Frontend) is a library that helps you build secure, mode
 
 :::note
 Duende.BFF V4 introduced a new way of configuring the BFF, which automatically configures the BFF using recommended practices. If you're upgrading from V3, please refer to the [upgrade guide](/bff/upgrading/bff-v3-to-v4.md).
+
+When in single frontend mode, an 'implicit' default frontend is automatically registered. This ensures all the management routes and openid connect handling routes are available for your frontend. 
+When you call .AddFrontend() to add a new frontend, the system switches to Multi-frontend mode. If you wish to have a default frontend in multi-frontend mode, you'll need to explicitly add it. See [Multi-frontend support](bff/fundamentals/multi-frontend/) for more information on this topic.
+
 :::
 
 ## Prerequisites

src/content/docs/bff/upgrading/bff-v3-to-v4.md

@@ -118,18 +118,65 @@ This enables you to configure your OpenID Connect options, including secrets, an
 
 See the type `BffConfiguration` to see what settings can be configured.
 
-#### Index HTML Retrieval
+## Handling SPA static assets
 
-It's fairly common to deploy your application in such a way to have the BFF be the first entrypoint for your application. It should serve an index.html that will bootstrap your frontend. However, your static content should be loaded from a CDN.
+The BFF can be configured to handle the static file assets that are typically used when developing SPA based apps. 
 
-If you publish your frontend code to a cdn with absolute paths (for example by specifying a base path in your vite config), then all static content is loaded directly from the CDN.
+### Proxying the Index.HTML only
+
+When deploying a multi-frontend BFF, it makes most sense to have the frontend's configured with an `index.html` file that is retrieved from a Content Delivery Network (CDN). 
+
+This can be done in various ways. For example, if you use Vite, you can publish static assets with a 'base' configured. This will make sure that any static asset,
+(such as images, scripts, etc) are retrieved directly from the CDN. This gives the best performance. 
+
+```csharp
+var frontend = new BffFrontend(BffFrontendName.Parse("frontend1"))
+   .WithCdnIndexHtml(new Uri("https://my_cdn/some_app/index.html"))
+```
+
+When you do this, the BFF automatically wires up a catch-all route that serves the `index.html` for that specific frontend. 
+See [Serve the index page from the BFF host](/bff/architecture/ui-hosting.md#serve-the-index-page-from-the-bff-host) for more information. 
+
+### Proxying all static assets 
+
+When developing a SPA based application, it's very common to use a development webserver such as Vite. While vite can publish static assets with a 'base',
+this doesn't work well during development.
+
+The best development experience can be achieved by configuring the BFF to proxy all static assets from the development server:
 
-You can configure the location of your `index.html` by specifying:
 
 ```csharp
-.WithIndexHtmlUrl(new Uri("https://localhost:5005/static/index.html"))
+var frontend = new BffFrontend(BffFrontendName.Parse("frontend1"))
+   .WithProxiedStaticAssets(new Uri("https://localhost:3000")); // https://localhost:3000 would be the URL of your development web server.
 ```
 
+While this can also be done in production, it will proxy all static assets through the BFF. This will increase the bandwidth consumed by the BFF 
+and reduce the overall performance of your application. 
+
+
+### Proxying assets based on environment
+
+If you're using a local development server during development, but a CDN in production, you can also configure this as follows:
+
+``` csharp
+
+// In this example, the environment name from the application builder is used to determine
+// if we're running in production or not. 
+var runningInProduction = () => builder.Environment.EnvironmentName == Environments.Production;
+
+// Then, when configuring the frontend, you can switch when the static assets will be proxied. 
+new BffFrontend(BffFrontendName.Parse("default-frontend"))
+    .WithBffStaticAssets(new Uri("https://localhost:5010/static"), useCdnWhen: runningInProduction);
+
+```
+
+:::note
+This function is evaluated immediately when calling the method `.WithBffStaticAssets` is called. If you call this method during startup,
+then the condition is only evaluated at startup time. It's not evaluated at runtime for every request. 
+:::
+
+
+
 ### Server Side Sessions Database Migrations
 
 When using the server side sessions feature backed by the `Duende.BFF.EntityFramework` package, you will need to script [Entity Framework database migrations](/bff/fundamentals/session/server-side-sessions.mdx#entity-framework-migrations) and apply these changes to your database.