New Article: Customizing Authentication Flow with OpenIddict Events in ABP Framework

docs/en/Community-Articles/2025-02-04-OpenIddict-Custom-Logic/POST.md

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+# Customizing Authentication Flow with OpenIddict Events in ABP Framework
+
+[ABP's OpenIddict Module](https://abp.io/docs/latest/modules/openiddict) provides an integration with the [OpenIddict](https://github.com/openiddict/openiddict-core) library,  which provides advanced authentication features like **single sign-on**, **single log-out**, and **API access control**.
+
+OpenIddict provides an event-driven model ([event models](https://documentation.openiddict.com/introduction#events-model)) that allows developers to customize authentication and authorization processes. This event model enables handling actions such as user **sign-in**, **sign-out**, **token validation**, and **request handling** dynamically.
+
+In this article, we will explore OpenIddict event models, their key use cases, and how to implement them effectively.
+
+## Understanding OpenIddict Event Model
+
+OpenIddict events are primarily used within the OpenIddict server component. These events provide hooks into the OpenID Connect flow, allowing developers to modify behavior at different stages of authentication & authorization processes.
+
+They are triggered during critical moments such as:
+
+* User authentication (sign-in)
+* Session termination (sign-out)
+* Token validation and generation
+* Request processing
+* Error handling
+
+OpenIddict provides multiple server events, under the `OpenIddictServerEvents` static class to make them easier to find (also provides additonal validation events under the `OpenIddictValidationEvents` static class). 
+
+Here are some of the pre-defined `OpenIddictServerEvents`:
+
+![](openiddict-server-events.png)
+
+Each event represents a specific checkpoint in the **request processing pipeline**, such as validating an OpenID Connect request, extracting request parameters, processing the request, or generating a response. As an application developer, you simply need to create event handlers that subscribe to these predefined events to implement your custom logic at the desired pipeline stage.
+
+## Example: How to add custom logic when a user signs out?
+
+Let's walkthrough a practical example of implementing custom sign-out logic using OpenIddict events.
+
+### Step 1: Create a Custom Event Handler
+
+First, create a handler that implements `IOpenIddictServerHandler<OpenIddictServerEvents.ProcessSignOutContext>`:
+
+```csharp
+using System.Threading.Tasks;
+using OpenIddict.Server;
+
+namespace MySolution;
+
+public class SignOutEventHandler : IOpenIddictServerHandler<OpenIddictServerEvents.ProcessSignOutContext>
+{
+    public static OpenIddictServerHandlerDescriptor Descriptor { get; }
+        = OpenIddictServerHandlerDescriptor.CreateBuilder<OpenIddictServerEvents.ProcessSignOutContext>()
+            .UseSingletonHandler<SignOutEventHandler>()
+            .SetOrder(100_000)
+            .SetType(OpenIddictServerHandlerType.Custom)
+            .Build();
+
+    public ValueTask HandleAsync(OpenIddictServerEvents.ProcessSignOutContext context)
+    {
+        // Implement your custom sign-out logic here
+
+        // Examples:
+        // - Clear custom session data
+        // - Perform audit logging
+        // - Notify other services
+        // - Clean up user-specific resources
+
+        return ValueTask.CompletedTask;
+    }
+}
+```
+
+The handler configuration includes several important components:
+
+* `Descriptor` - Defines how the handler should be registered and executed
+* `SetOrder` - Determines the execution order when multiple handlers exist
+* `SetType` - Specifies this as a custom handler implementation
+* `UseSingletonHandler` - Sets lifetime of the class as _Singleton_
+
+### Step 2: Register the Event Handler
+
+Register your custom handler in your application's module configuration:
+
+```csharp
+//...
+
+public class MySolutionAuthServerModule : AbpModule
+{
+    public override void PreConfigureServices(ServiceConfigurationContext context)
+    {
+        PreConfigure<OpenIddictServerBuilder>(serverBuilder =>
+        {
+            serverBuilder.AddEventHandler(SignOutEventHandler.Descriptor);
+        });
+    }
+
+    //...
+}
+```
+
+That's it! After these steps, your `SignOutEventHandler.HandleAsync()` method should be triggered after each signout request. You can also use other pre-defined server events for other stages of the authentication & authorization processes such as;
+
+* `OpenIddictServerEvents.ProcessSignInContext` -> after each sign-in,
+* `OpenIddictServerEvents.ProcessErrorContext` -> when an error occurs in the authentication,
+* `OpenIddictServerEvents.ProcessChallengeContext` -> called when processing a challenge operation,
+* and other 40+ server events...
+
+Each event provides access to the relevant context, allowing you to access and modify the authentication flow's behavior.
+
+## Conclusion
+
+ABP Framework integrates OpenIddict as its authentication and authorization module. OpenIddict provides an event-driven model that allows developers to customize authentication and authorization processes within their ABP applications. It's pre-installed & pre-configured in the ABP's startup templates.
+
+OpenIddict provides a powerful and flexible way to customize authentication flows. By leveraging these events, developers can implement complex authentication scenarios while maintaining clean, maintainable code.
+
+## References
+
+* [OpenIddict Documentation](https://documentation.openiddict.com/introduction#events-model)
+* [ABP OpenIddict Module Documentation](https://abp.io/docs/latest/modules/openiddict)
+* [Advanced OpenIddict Scenarios](https://kevinchalet.com/2018/07/02/implementing-advanced-scenarios-using-the-new-openiddict-rc3-events-model/)