README.md

Blazor Server App

This sample shows the recommended way to use the Feature Management library in a Blazor Server App.

Quickstart

To run this sample, follow these steps:

1. Set the project as the startup project.
2. Run the project.
3. Refresh the page until the BETA content occurs.

About the App

This app demonstrates how to use the Feature Management library in Blazor apps.

This app uses two feature flags: "BrowserEnhancement" and "Beta".

"feature_management": {
  "feature_flags": [
    {
      "id": "BrowserEnhancement",
      "enabled": true,
      "conditions": {
        "client_filters": [
          {
            "name": "Browser",
            "parameters": {
              "AllowedBrowsers": [ "Edge" ]
            }
          }
        ]
      }
    },
    {
      "id": "Beta",
      "enabled": true,
      "conditions": {
        "client_filters": [
          {
            "name": "Targeting",
            "parameters": {
              "Audience": {
                "DefaultRolloutPercentage": 50,
                "Exclusion": {
                  "Groups": [
                    "Guests"
                  ]
                }
              }
            }
          }
        ]
      }
    }
  ]
}

The "BrowserEnhancement" feature is enabled when the user is using the allowed browsers, in this example, the Edge browser. If the "BrowserEnhancement" feature is on, the color of the top bar will be dark blue. Otherwise, it will be white.

The "Beta" feature uses the Targeting filter to evaluate when to activate. The "Guests" group, consisting of the unauthenticated users, will be excluded from the audience. Other users will have 50% chance to fall into the default rollout. When the "Beta" feature is enabled, the BETA content will be displayed.

HttpContext

In regular ASP.NET Core web app, the most common way to get targeting context is through the HttpContextAccessor. However, IHttpContextAccessor must be avoided with interactive rendering of Razor component in the Blazor server app because there isn't a valid HttpContext available. More details can be found here.

The recommended approach to pass the HttpContext in Blazor apps is to copy the data into a scoped service. This app obtains the "User-Agent" information from the HttpContext and passes it to a scoped service called UserAgentContext. UserAgentContext will be consumed by the BrowserFilter through dependency injection.

User Authentication State

This app uses cookie authentication. When the app is loaded, it has 2/3 chance to be an authenticated user. The details can be found in the _Host.cshtml.

Rather than HttpContext, the AuthenticationStateProvider service is used to obtain the user authentication state information for setting targeting context. The details can be found in the MyTargetingContextAccessor.

Service Registration

Blazor applications like this one typically pull ambient contextual data from scoped services. For example, the UserAgentContext, AuthenticationStateProvider and ITargetingContextAccessor are all scoped services. This pattern breaks if the feature management services are added as singleton, which is typical in non-blazor web apps.

In Blazor, avoid the following

services.AddFeatureManagment()

And instead use

services.AddScopedFeatureManagement()

This call can be seen in the example.