The Microsoft.FeatureManagement library enables developers to use feature flags and dynamic features inside of their applications. Feature flags can be used to turn features on or off dynamically. Developers can use feature flags in simple use cases like conditional statements to more advanced scenarios like conditionally adding routes or MVC filters. Dynamic features can be used to select different variants of a feature's configuration. This enables the possibility of using one version of a feature for one set of users, and another version of the feature for the remaining users.
Here are some of the benefits of using this library:
- A common convention for feature management
- Low barrier-to-entry
- Built on
IConfiguration - Supports JSON file feature flag setup
- Built on
- Feature flag lifetime management
- Configuration values can change in real-time, feature flags can be consistent across the entire request
- Simple to complex scenarios covered
- Toggle on/off features through declarative configuration file
- Use different variants of a feature in different circumstances
- API extensions for ASP.NET Core and MVC framework
- Routing
- Filters
- Action Attributes
API Reference: https://go.microsoft.com/fwlink/?linkid=2091700
The content of this README relates to version 3 of the Microsoft.FeatureManagement library.
The README for version 2 can be found here.
Feature flags can either be on or off. They are composed of two parts, a name and a list of feature-filters that are used to turn the feature on.
The .NET Core configuration system is used to determine the state of features. The foundation of this system is IConfiguration. Any provider for IConfiguration can be used as the feature state provider for the feature management library. This enables scenarios ranging from appsettings.json to Azure App Configuration and more.
The feature management library supports appsettings.json as a feature flag source since it is a provider for .NET Core's IConfiguration system. Below we have an example of the format used to set up feature flags in a json file. The example below uses the 2.0.0 configuration schema which is supported in Microsoft.FeatureManagement version 3. For previous schema versions see the configuration schema details.
{
// Define feature flags in a json configuration file
"FeatureManagement": {
"FeatureFlags": {
"FeatureT": {
"EnabledFor": [
{
"Name": "AlwaysOn"
}
]
},
"FeatureU": {
"EnabledFor": [
{
"Name": "TimeWindow",
"Parameters": {
"Start": "Wed, 01 May 2019 13:59:59 GMT",
"End": "Mon, 01 July 2019 00:00:00 GMT"
}
}
]
}
}
}
}The FeatureManagement section of the json document is used by convention to load feature flag settings. In the section above, we see that we have provided two different features. Features define their feature filters using the EnabledFor property. In the feature filters for FeatureT we see AlwaysOn. This feature filter is built-in and if specified will always enable the feature. The AlwaysOn feature filter does not require any configuration so it only has the Name property. FeatureU specifies a feature filter named TimeWindow. This is an example of a configurable feature filter. We can see in the example that the filter has a parameter's property. This is used to configure the filter. In this case, the start and end times for the feature to be active are configured.
The following snippet demonstrates an alternative way to define a feature that can be used for on/off features.
{
// Define feature flags in a json configuration file
"FeatureManagement": {
"FeatureFlags": {
"FeatureT": true, // On feature
"FeatureX": false // Off feature
}
}
}Feature flags rely on .NET Core dependency injection. We can register the feature management services using standard conventions.
using Microsoft.FeatureManagement;
using Microsoft.FeatureManagement.FeatureFilters;
public class Startup
{
public void ConfigureServices(IServiceCollection services)
{
services.AddFeatureManagement()
.AddFeatureFilter<PercentageFilter>()
.AddFeatureFilter<TimeWindowFilter>();
}
}This tells the feature manager to use the "FeatureManagement" section from the configuration for feature flag settings. It also registers two built-in feature filters named PercentageFilter and TimeWindowFilter. When filters are referenced in feature flag settings (appsettings.json) the Filter part of the type name can be omitted.
Advanced: The feature manager looks for feature definitions in a configuration section named "FeatureManagement". If the "FeatureManagement" section does not exist, it falls back to the root of the provided configuration.
The simplest use case for feature flags is to do a conditional check for whether a feature is enabled to take different paths in code. The uses cases grow from there as the feature flag API begins to offer extensions into ASP.NET Core.
The basic form of feature management is checking if a feature is enabled and then performing actions based on the result. This is done through the IFeatureManager's IsEnabledAsync method.
…
IFeatureManager featureManager;
…
if (await featureManager.IsEnabledAsync("FeatureU"))
{
// Do something
}When using the feature management library with MVC, the IFeatureManager can be obtained through dependency injection.
public class HomeController : Controller
{
private readonly IFeatureManager _featureManager;
public HomeController(IFeatureManager featureManager)
{
_featureManager = featureManager;
}
}The feature management library provides functionality in ASP.NET Core and MVC to enable common feature flag scenarios in web applications. These capabilities are available by referencing the Microsoft.FeatureManagement.AspNetCore NuGet package.
MVC controller and actions can require that a given feature flag, or one of any list of feature flags, be enabled in order to execute. This can be done by using a FeatureGateAttribute, which can be found in the Microsoft.FeatureManagement.Mvc namespace.
[FeatureGate("FeatureX")]
public class HomeController : Controller
{
…
}The HomeController above is gated by "FeatureX". "FeatureX" must be enabled before any action the HomeController contains can be executed.
[FeatureGate("FeatureY")]
public IActionResult Index()
{
return View();
}The Index MVC action above requires "FeatureY" to be enabled before it can execute.
When an MVC controller or action is blocked because none of the feature flags it specifies are enabled, a registered IDisabledFeaturesHandler will be invoked. By default, a minimalistic handler is registered which returns HTTP 404. This can be overridden using the IFeatureManagementBuilder when registering feature flags.
public interface IDisabledFeaturesHandler
{
Task HandleDisabledFeature(IEnumerable<string> features, ActionExecutingContext context);
}In MVC views <feature> tags can be used to conditionally render content based on whether a feature is enabled or not.
<feature name="FeatureX">
<p>This can only be seen if 'FeatureX' is enabled.</p>
</feature>The <feature> tag requires a tag helper to work. This can be done by adding the feature management tag helper to the ViewImports.cshtml file.
@addTagHelper *, Microsoft.FeatureManagement.AspNetCoreThe feature tag can also be used to show content if a feature is disabled. This is done by using the negate attribute.
<feature name="FeatureX" negate="true">
<p>This can only be seen if 'FeatureX' is disabled.</p>
</feature>MVC action filters can be set up to conditionally execute based on the state of a feature flag. This is done by registering MVC filters in a feature flag aware manner.
The feature management pipeline supports async MVC Action filters, which implement IAsyncActionFilter.
services.AddMvc(o =>
{
o.Filters.AddForFeature<SomeMvcFilter>("FeatureV");
});The code above adds an MVC filter named SomeMvcFilter. This filter is only triggered within the MVC pipeline if the feature flag it specifies, "FeatureV", is enabled.
MVC Razor pages can require that a given feature, or one of any list of features, be enabled in order to execute. This can be done by using a FeatureGateAttribute, which can be found in the Microsoft.FeatureManagement.Mvc namespace.
[FeatureGate(MyFeatureFlags.FeatureU)]
public class IndexModel : PageModel
{
public void OnGet()
{
}
}The code above sets up a Razor page to require the "FeatureU" to be enabled. If the feature is not enabled, the page will generate an HTTP 404 (NotFound) result.
When used on Razor pages, the FeatureGateAttribute must be placed on the page handler type. It cannot be placed on individual handler methods.
The feature management library can be used to add application branches and middleware that execute conditionally based on feature flag state.
app.UseMiddlewareForFeature<ThirdPartyMiddleware>("FeatureU");With the above call, the application adds a middleware component that only appears in the request pipeline if the feature flag "FeatureU" is enabled. If the feature flag is enabled/disabled during runtime, the middleware pipeline can be changed dynamically.
This builds off the more generic capability to branch the entire application based on a feature flag.
app.UseForFeature(featureName, appBuilder =>
{
appBuilder.UseMiddleware<T>();
});Feature filters define a scenario for when a feature flag should be enabled. When a feature flag is evaluated for whether it is on or off, its list of feature-filters are traversed until one of the filters decides the feature flag should be enabled. At this point the feature flag is considered enabled and traversal through the feature filters stops. If no feature filter indicates that the feature flag should be enabled, then it will be considered disabled.
As an example, a Microsoft Edge browser feature filter could be designed. This feature filter would activate any features it is attached to as long as an HTTP request is coming from Microsoft Edge.
Creating a feature filter provides a way to enable feature flags based on criteria that you define. To implement a feature filter, the IFeatureFilter interface must be implemented. IFeatureFilter has a single method named EvaluateAsync. When a feature flag specifies that it can be enabled for a feature filter, the EvaluateAsync method is called. If EvaluateAsync returns true it means the feature flag should be enabled.
Feature filters are registered by the IFeatureManagementBuilder when AddFeatureManagement is called. These feature filters have access to the services that exist within the service collection that was used to add feature flags. Dependency injection can be used to retrieve these services.
Some feature filters require parameters to decide whether a feature flag should be turned on or not. For example a browser feature filter may turn on a feature flag for a certain set of browsers. It may be desired that Edge and Chrome browsers enable a feature flag, while Firefox does not. To do this a feature filter can be designed to expect parameters. These parameters would be specified in the feature configuration, and in code would be accessible via the FeatureFilterEvaluationContext parameter of IFeatureFilter.EvaluateAsync.
public class FeatureFilterEvaluationContext
{
/// <summary>
/// The name of the feature flag being evaluated.
/// </summary>
public string FeaturFlagName { get; set; }
/// <summary>
/// The settings provided for the feature filter to use when evaluating whether the feature flag should be enabled.
/// </summary>
public IConfiguration Parameters { get; set; }
}FeatureFilterEvaluationContext has a property named Parameters. These parameters represent a raw configuration that the feature filter can use to decide how to evaluate whether the feature flag should be enabled or not. To use the browser feature filter as an example once again, the filter could use Parameters to extract a set of allowed browsers that would have been specified for the feature flag and then check if the request is being sent from one of those browsers.
[FilterAlias("Browser")]
public class BrowserFilter : IFeatureFilter
{
… Removed for example
public Task<bool> EvaluateAsync(FeatureFilterEvaluationContext context)
{
BrowserFilterSettings settings = context.Parameters.Get<BrowserFilterSettings>() ?? new BrowserFilterSettings();
//
// Here we would use the settings and see if the request was sent from any of BrowserFilterSettings.AllowedBrowsers
}
}When a feature filter is registered to be used for a feature flag, the alias used in configuration is the name of the feature filter type with the filter suffix, if any, removed. For example MyCriteriaFilter would be referred to as MyCriteria in configuration.
"MyFeature": {
"EnabledFor": [
{
"Name": "MyCriteria"
}
]
}This can be overridden through the use of the FilterAliasAttribute. A feature filter can be decorated with this attribute to declare the name that should be used in configuration to reference this feature filter within a feature flag.
If a feature flag is configured to be enabled for a specific feature filter and that feature filter hasn't been registered, then an exception will be thrown when the feature flag is evaluated. The exception can be disabled by using the feature management options.
services.Configure<FeatureManagementOptions>(options =>
{
options.IgnoreMissingFeatureFilters = true;
});Feature filters can evaluate whether a feature flag should be enabled based off the properties of an HTTP Request. This is performed by inspecting the HTTP Context. A feature filter can get a reference to the HTTP Context by obtaining an IHttpContextAccessor through dependency injection.
public class BrowserFilter : IFeatureFilter
{
private readonly IHttpContextAccessor _httpContextAccessor;
public BrowserFilter(IHttpContextAccessor httpContextAccessor)
{
_httpContextAccessor = httpContextAccessor ?? throw new ArgumentNullException(nameof(httpContextAccessor));
}
}The IHttpContextAccessor must be added to the dependency injection container on startup for it to be available. It can be registered in the IServiceCollection using the following method.
public void ConfigureServices(IServiceCollection services)
{
…
services.TryAddSingleton<IHttpContextAccessor, HttpContextAccessor>();
…
}In console applications there is no ambient context such as HttpContext that feature filters can acquire and utilize to check if a feature should be on or off. In this case, applications need to provide an object representing a context into the feature management system for use by feature filters. This is done by using IFeatureManager.IsEnabledAsync<TContext>(string featureFlagName, TContext appContext). The appContext object that is provided to the feature manager can be used by feature filters to evaluate the state of a feature flag.
MyAppContext context = new MyAppContext
{
AccountId = current.Id;
}
if (await featureManager.IsEnabledAsync(feature, context))
{
…
}Contextual feature filters implement the IContextualFeatureFilter<TContext> interface. These special feature filters can take advantage of the context that is passed in when IFeatureManager.IsEnabledAsync<TContext> is called. The TContext type parameter in IContextualFeatureFilter<TContext> describes what context type the filter is capable of handling. This allows the developer of a contextual feature filter to describe what is required of those who wish to utilize it. Since every type is a descendant of object, a filter that implements IContextualFeatureFilter<object> can be called for any provided context. To illustrate an example of a more specific contextual feature filter, consider a feature flag that is enabled if an account is in a configured list of enabled accounts.
public interface IAccountContext
{
string AccountId { get; set; }
}
[FilterAlias("AccountId")]
class AccountIdFilter : IContextualFeatureFilter<IAccountContext>
{
public Task<bool> EvaluateAsync(FeatureFilterEvaluationContext featureEvaluationContext, IAccountContext accountId)
{
//
// Evaluate if the feature should be on with the help of the provided IAccountContext
}
}We can see that the AccountIdFilter requires an object that implements IAccountContext to be provided to be able to evalute the state of a feature flag. When using this feature filter, the caller needs to make sure that the passed in object implements IAccountContext.
Note: Only a single feature filter interface can be implemented by a single type. Trying to add a feature filter that implements more than a single feature filter interface will result in an ArgumentException.
There a few feature filters that come with the Microsoft.FeatureManagement package. These feature filters are not added automatically, but can be referenced and registered as soon as the package is registered.
Each of the built-in feature filters have their own parameters. Here is the list of feature filters along with examples.
This filter provides the capability to enable a feature flag based on a set percentage.
"EnhancedPipeline": {
"EnabledFor": [
{
"Name": "Microsoft.Percentage",
"Parameters": {
"Value": 50
}
}
]
}This filter provides the capability to enable a feature flag based on a time window. If only End is specified, the feature flag will be considered on until that time. If only start is specified, the feature flag will be considered on at all points after that time.
"EnhancedPipeline": {
"EnabledFor": [
{
"Name": "Microsoft.TimeWindow",
"Parameters": {
"Start": "Wed, 01 May 2019 13:59:59 GMT",
"End": "Mon, 01 July 2019 00:00:00 GMT"
}
}
]
}This filter provides the capability to enable a feature flag for a target audience. An in-depth explanation of targeting is explained in the targeting section below. The filter parameters include an audience object which describes users, groups, and a default percentage of the user base that should have access to the feature flag. Each group object that is listed in the target audience must also specify what percentage of the group's members should have access. If a user is specified in the users section directly, or if the user is in the included percentage of any of the group rollouts, or if the user falls into the default rollout percentage then that user will have the feature flag enabled.
"EnhancedPipeline": {
"EnabledFor": [
{
"Name": "Microsoft.Targeting",
"Parameters": {
"Audience": {
"Users": [
"Jeff",
"Alicia"
],
"Groups": [
{
"Name": "Ring0",
"RolloutPercentage": 100
},
{
"Name": "Ring1",
"RolloutPercentage": 50
}
],
"DefaultRolloutPercentage": 20
}
}
}
]
}All of the built-in feature filter alias' are in the 'Microsoft' feature filter namespace. This is to prevent conflicts with other feature filters that may share the same simple alias. The segments of a feature filter namespace are split by the '.' character. A feature filter can be referenced by its fully qualified alias such as 'Microsoft.Percentage' or by the last segment which in the case of 'Microsoft.Percentage' is 'Percentage'.
When new features are being added to an application there may come a time when a feature has multiple different proposed design options. A common pattern when this happens is to do some form of A/B testing. That is, provide a different version of the feature to different segments of the user base, and judge off user interaction which is better. The dynamic feature functionality contained in this library aims to proivde a simplistic, standardized method for developers to perform this form of A/B testing.
In the scenario above, the different proposals for the design of a feature are referred to as variants of the feature. The feature itself is referred to as a dynamic feature. The variants of a dynamic feature can have types ranging from object, to string, to integer and so on. There is no limit to the amount of variants a dynamic feature may have. A developer is free to choose what type should be returned when a variant of a dynamic feature is requested. They are also free to choose how many variants are available to select from.
Each variant of a dynamic feature is associated with a different configuration of the feature. Additionally, each variant of a dynamic feature contains information describing under what circumstances the variant should be used.
Dynamic features are accessible through the IDynamicFeatureManager interface.
public interface IDynamicFeatureManager
{
IAsyncEnumerable<string> GetDynamicFeatureNamesAsync(CancellationToken cancellationToken = default);
ValueTask<T> GetVariantAsync<T>(string dynamicFeature, CancellationToken cancellationToken = default);
ValueTask<T> GetVariantAsync<T, TContext>(string dynamicFeature, TContext context, CancellationToken cancellationToken = default);
}The dynamic feature manager performs a resolution process that takes the name of a feature and returns a strongly typed value to represent the variant's value.
The following steps are performed during the retrieval of a dynamic feature's variant
- Lookup the configuration of the specified dynamic feature to find the registered variants
- Assign one of the registered variants to be used.
- Resolve typed value based off of the assigned variant.
The dynamic feature manager is made available by using the AddFeatureManagement call. Make sure to add any required feature variant assigners referenced by dynamic features within the application by using AddFeatureVariantAssigner.
using Microsoft.FeatureManagement;
using Microsoft.FeatureManagement.Assigners;
public class Startup
{
public void ConfigureServices(IServiceCollection services)
{
services.AddFeatureManagement()
.AddFeatureVariantAssigner<TargetingFeatureVariantAssigner>();
}
}One possible example of when variants may be used is in a web application when there is a desire to test different visuals. In the following examples a mock of how one might assign different variants of a web page background to their users is shown.
//
// Modify view based off multiple possible variants
model.BackgroundUrl = dynamicFeatureManager.GetVariantAsync<string>("HomeBackground", cancellationToken);
return View(model);Dynamic features can be configured in a configuration file similarly to feature flags. Instead of being defined in the FeatureManagement:FeatureFlags section, they are defined in the FeatureManagement:DynamicFeatures section. Additionally, dynamic features have the following properties.
- Assigner: The assigner that should be used to select which variant should be used any time this feature is accessed.
- Variants: The different variants of the dynamic feature.
- Name: The name of the variant.
- Default: Whether the variant should be used if no variant could be explicitly assigned. One and only one default variant is required.
- ConfigurationReference: A reference to the configuration of the variant to be used as typed options in the application.
- AssignmentParameters: The parameters used in the assignment process to determine if this variant should be used.
An example of a dynamic feature named "ShoppingCart" is shown below.
{
"FeatureManagement":
{
"DynamicFeatures": {
"ShoppingCart": {
"Assigner": "Targeting",
"Variants": [
{
"Default": true,
"Name": "Big",
"ConfigurationReference": "ShoppingCart:Big",
"AssignmentParameters": {
"Audience": {
"Users": [
"Alec"
],
"Groups": [
]
}
}
},
{
"Name": "Small",
"ConfigurationReference": "ShoppingCart:Small",
"AssignmentParameters": {
"Audience": {
"Users": [
],
"Groups": [
{
"Name": "Ring1",
"RolloutPercentage": 50
}
],
"DefaultRolloutPercentage": 30
}
}
}
]
}
}
},
"ShoppingCart": {
"Big": {
"Size": 400,
"Color": "green"
},
"Small": {
"Size": 150,
"Color": "gray"
}
}
}In the example above we see the declaration of a dynamic feature in a json configuration file. The dynamic feature is defined in the FeatureManagement:DynamicFeatures section of configuration. The name of this dynamic feature is ShoppingCart. A dynamic feature must declare a feature variant assigner that should be used to select a variant when requested. In this case the built-in Targeting feature variant assigner is used. The dynamic feature has two different variants that are available to the application. One variant is named Big and the other is named Small. Each variant contains a configuration reference denoted by the ConfigurationReference property. The configuration reference is a pointer to a section of application configuration that contains the options that should be used for that variant. The variant also contains assignment parameters denoted by the AssignmentParameters property. The assignment parameters are used by the assigner associated with the dynamic feature. The assigner reads the assignment parameters at run time when a variant of the dynamic feature is requested to choose which variant should be returned.
An application that is configured with this ShoppingCart dynamic feature may request the value of a variant of the feature at runtime through the use of IDynamicFeatureManager.GetVariantAsync. The dynamic feature uses targeting for variant assignment so each of the variants' assignment parameters specify a target audience that should receive the variant. For a walkthrough of how the targeting assigner would choose a variant in this scenario reference the Microsoft.Targeting Assigner section. When the feature manager chooses one of the variants it resolves the value of the variant by resolving the configuration reference declared in the variant. The example above includes the configuration that is referenced by the ConfigurationReference of each variant.
A feature variant assigner is a component that uses contextual information within an application to decide which feature variant should be chosen when a variant of a dynamic feature is requested.
When requesting the value of a dynamic feature, the feature manager needs to determine which variant to use. The act of choosing which of the variants to be used is called "assignment." A built-in method of assignment allows the variants of a dynamic feature to be assigned to segments of an application's audience. This is the same targeting strategy used by the targeting feature filter.
To perform assignments, the feature manager uses components known as feature variant assigners. Feature variant assigners choose which of the variants of a dynamic feature should be assigned when a dynamic feature is requested. Each variant of a dynamic feature defines assignment parameters so that when an assigner is invoked, the assigner can tell under which conditions each variant should be selected. It is possible that an assigner is unable to choose between the list of available variants based on the configured assignment parameters. In this case, the feature manager chooses the default variant. The default variant is a variant that is marked explicitly as the default. It is required to have a default variant when configuring a dynamic feature in order to handle the possibility that an assigner is not able to select a variant of a dynamic feature.
There may come a time when custom criteria is needed to decide which variant of a feature should be assigned when a feature is referenced. This is made possible by an extensibility model that allows the act of assignment to be overridden. Every feature registered in the feature management system that uses feature variants specifies what assigner should be used to choose a variant.
public interface IFeatureVariantAssigner : IFeatureVariantAssignerMetadata
{
/// <summary>
/// Assign a variant of a feature to be used based off of customized criteria.
/// </summary>
/// <param name="variantAssignmentContext">Information provided by the system to be used during the assignment process.</param>
/// <param name="cancellationToken">The cancellation token to cancel the operation.</param>
/// <returns>The variant that should be assigned for a given feature.</returns>
ValueTask<FeatureVariant> AssignVariantAsync(FeatureVariantAssignmentContext variantAssignmentContext, CancellationToken cancellationToken);
}
}An example implementation can be found in this example.
There is a built-in feature variant assigner that uses targeting. It comes with the Microsoft.FeatureManagement package. This assigner is not added automatically, but it can be referenced and registered as soon as the package is registered.
This feature variant assigner provides the capability to assign the variants of a dynamic feature to targeted audiences. An in-depth explanation of targeting is explained in the targeting section.
The assignment parameters used by the targeting feature variant assigner include an audience object which describes the user base that should receive the associated variant. The audience is made of users, groups, and a percentage of the entire user base. Each group object that is listed in the target audience is required to specify what percentage of the group's members should have receive the variant. If a user is specified in the users section directly, or if the user is in the included percentage of any of the group rollouts, or if the user falls into the default rollout percentage then that user will receive the associated variant.
"ShoppingCart": {
"Assigner": "Targeting",
"Variants": [
{
"Default": true,
"Name": "Big",
"ConfigurationReference": "ShoppingCart:Big",
"AssignmentParameters": {
"Audience": {
"Users": [
"Alec"
],
"Groups": [
{
"Name": "Ring0",
"RolloutPercentage": 100
},
{
"Name": "Ring1",
"RolloutPercentage": 50
}
]
}
}
},
{
"Name": "Small",
"ConfigurationReference": "ShoppingCart:Small",
"AssignmentParameters": {
"Audience": {
"Users": [
"Susan",
],
"Groups": [
{
"Name": "Ring1",
"RolloutPercentage": 50
}
],
"DefaultRolloutPercentage": 80
}
}
}
]
}Based on the configured audiences for the variants included in this feature, if the application is executed under the context of a user named Alec then the value of the Big variant will be returned. If the application is executing under the context of a user named Susan then the value of the Small variant will be returned. If a user match does not occur, then group matches are evaluated. If the application is executed under the context of a user in the group Ring0 then the Big variant will be returned. If the user's group is Ring1 instead, then the user has a 50% chance of being assigned to Small. If there is no user match nor group match, then the default rollout percentage is used. In this case, 80% of unmatched users will get the Small variant, leaving the other 20% to get the Big variant since it is marked as the Default.
Example usage of this assigner can be found in the FeatureFlagDemo example.
When using the targeting feature variant assigner, make sure to register it as well as an implementation of ITargetingContextAccessor.
services.AddSingleton<ITargetingContextAccessor, HttpContextTargetingContextAccessor>();
services.AddFeatureManagement();
.AddFeatureVariantAssigner<TargetingFeatureVariantAssigner>();When a variant of a dynamic feature has been chosen, the feature management system resolves the configuration reference associated with that variant. The resolution is done through the ConfigurationReference property. In the "Configuring a Dynamic Feature" section we see a dynamic feature named ShoppingCart. The first variant of the feature, is named "Big", and is being referenced in the feature variant as ShoppingCart:Big in the configuration reference. The referenced section is shown below.
"ShoppingCart": {
"Big": {
"Size": 400,
"Color": "green"
}
}The feature management system resolves the configuration reference and binds the resolved configuration section to the type specified when a variant of a dynamic feature is requested. This is performed by an implementation of the IFeatureVariantOptionsResolver. By providing a custom implementation of IFeatureVariantOptionsResolver, a developer can resolve configuration references from sources other than configuration.
Targeting is a feature management strategy that enables developers to progressively roll out new features to their user base. The strategy is built on the concept of targeting a set of users known as the target audience. An audience is made up of specific users, groups, and a designated percentage of the entire user base. The groups that are included in the audience can be broken down further into percentages of their total members.
The following steps demonstrate an example of a progressive rollout for a new 'Beta' feature:
- Individual users Jeff and Alicia are granted access to the Beta
- Another user, Mark, asks to opt-in and is included.
- Twenty percent of a group known as "Ring1" users are included in the Beta.
- The number of "Ring1" users included in the beta is bumped up to 100 percent.
- Five percent of the user base is included in the beta.
- The rollout percentage is bumped up to 100 percent and the feature is completely rolled out.
This strategy for rolling out a feature is built in to the library through the included Microsoft.Targeting feature filter.
An example web application that uses the targeting feature filter is available in the FeatureFlagDemo example project.
To begin using the TargetingFilter in an application it must be added to the application's service collection just as any other feature filter. Unlike other built in filters, the TargetingFilter relies on another service to be added to the application's service collection. That service is an ITargetingContextAccessor.
The implementation type used for the ITargetingContextAccessor service must be implemented by the application that is using the targeting filter. Here is an example setting up feature management in a web application to use the TargetingFilter with an implementation of ITargetingContextAccessor called HttpContextTargetingContextAccessor.
services.AddSingleton<ITargetingContextAccessor, HttpContextTargetingContextAccessor>();
services.AddFeatureManagement();
.AddFeatureFilter<TargetingFilter>();To use the TargetingFilter in a web application an implementation of ITargetingContextAccessor is required. This is because when a targeting evaluation is being performed information such as what user is currently being evaluated is needed. This information is known as the targeting context. Different web applications may extract this information from different places. Some common examples of where an application may pull the targeting context are the request's HTTP context or a database.
An example that extracts targeting context information from the application's HTTP context is included in the FeatureFlagDemo example project. This method relies on the use of IHttpContextAccessor which is discussed here.
The targeting filter relies on a targeting context to evaluate whether a feature should be turned on. This targeting context contains information such as what user is currently being evaluated, and what groups the user in. In console applications there is typically no ambient context available to flow this information in to the targeting filter, thus it must be passed directly when FeatureManager.IsEnabledAsync is called. This is supported through the use of the ContextualTargetingFilter. Applications that need to float the targeting context into the feature manager should use this instead of the TargetingFilter.
services.AddFeatureManagement()
.AddFeatureFilter<ContextualTargetingFilter>();Since ContextualTargetingFilter is an IContextualTargetingFilter<ITargetingContext>, an implementation of ITargetingContext must be passed in to IFeatureManager.IsEnabledAsync for it to be able to evaluate and turn a feature on.
IFeatureManager fm;
…
// userId and groups defined somewhere earlier in application
TargetingContext targetingContext = new TargetingContext
{
UserId = userId,
Groups = groups;
}
await fm.IsEnabledAsync(featureName, targetingContext);The ContextualTargetingFilter still uses the feature filter alias Microsoft.Targeting, so the configuration for this filter is consistent with what is mentioned in that section.
An example that uses the ContextualTargetingFilter in a console application is available in the TargetingConsoleApp example project.
Options are available to customize how targeting evaluation is performed across all features. These options can be configured when setting up feature management.
services.Configure<TargetingEvaluationOptions>(options =>
{
options.IgnoreCase = true;
});The concept of targeting can be extended to dynamic features. Instead of targeting an audience to see a feature as enabled, the variants of a dynamic feature can be configured to target different audiences. For an in depth view of how this can be done see the targeting feature variant assigner section.
Feature state is provided by the IConfiguration system. Any caching and dynamic updating is expected to be handled by configuration providers. The feature manager asks IConfiguration for the latest value of a feature's state whenever a feature is checked to be enabled.
There are scenarios which require the state of a feature to remain consistent during the lifetime of a request. The values returned from the standard IFeatureManager may change if the IConfiguration source which it is pulling from is updated during the request. This can be prevented by using IFeatureManagerSnapshot. IFeatureManagerSnapshot can be retrieved in the same manner as IFeatureManager. IFeatureManagerSnapshot implements the interface of IFeatureManager, but it caches the first evaluated state of a feature during a request and will return the same state of a feature during its lifetime. Symmetric functionality is available for dynamic features through the use of IDynamicFeatureManagerSnapshot.
Implementing a custom feature provider enable developers to pull feature flags from sources such as a database or a feature management service. The included feature provider that is used by default pulls feature flags from .NET Core's configuration system. This allows for features to be defined in an appsettings.json file or in configuration providers like Azure App Configuration. This behavior can be substituted to provide complete control of where feature definitions are read from.
To customize the loading of feature flag definitions, one must implement the IFeatureFlagDefinitionProvider interface.
public interface IFeatureFlagDefinitionProvider
{
Task<FeatureFlagDefinition> GetFeatureFlagDefinitionAsync(string featureflagName, CancellationToken cancellationToken = default);
IAsyncEnumerable<FeatureFlagDefinition> GetAllFeatureFlagDefinitionsAsync(CancellationToken cancellationToken = default);
}To use an implementation of IFeatureFlagDefinitionProvider it must be added into the service collection before adding feature management. The following example adds an implementation of IFeatureFlagDefinitionProvider named InMemoryFeatureDefinitionProvider.
services.AddSingleton<IFeatureFlagDefinitionProvider, InMemoryFeatureDefinitionProvider>()
.AddFeatureManagement()It is also possible to provide custom dynamic feature definitions. This is done by implementing the IDynamicFeatureDefinitionProvider interface and registering it as mentioned above.
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