EventDriven.EventBus.Dapr

An event bus abstraction over Dapr pub/sub.

Prerequisites

• .NET Core SDK (6.0 or greater)
• Docker Desktop
• MongoDB Docker: docker run --name mongo -d -p 27017:27017 -v /tmp/mongo/data:/data/db mongo
• MongoDB Client:
  • Download Robo 3T only.
  • Add connection to localhost on port 27017.

Introduction

Dapr, which stands for Distributed Application Runtime, uses a sidecar pattern to provide a pub/sub abstraction over message brokers and queuing systems, including AWS SNS+SQS, GCP Pub/Sub, Azure Events Hub and several others.

The Dapr .NET SDK provides an API to perform pub/sub from an ASP.NET service, but it requires the application to be directly aware of Dapr. Publishers need to use DaprClient to publish an event, and subscribers need to decorate controller actions with the Topic attribute.

The purpose of the Dapr Event Bus project is to provide a thin abstraction layer over Dapr pub/sub so that applications may publish events and subscribe to topics without any knowledge of Dapr. This allows for better testability and flexibility, especially for worker services that do not natively include an HTTP stack.

Packages

• EventDriven.EventBus.Abstractions
• EventDriven.EventBus.Dapr
• EventDriven.EventBus.Dapr.EventCache.Mongo
• EventDriven.SchemaRegistry.Mongo

Usage

1. In both the publisher and subscriber, you need to register the Dapr Event Bus with DI.

   • First add the following to appsettings.json.

   "DaprEventBusOptions": {
     "PubSubName": "pubsub"
   },
   "DaprEventCacheOptions": {
     "DaprStateStoreOptions": {
       "StateStoreName": "statestore-mongodb"
     }
   },
   "DaprStoreDatabaseSettings": {
     "ConnectionString": "mongodb://localhost:27017",
     "DatabaseName": "daprStore",
     "CollectionName": "daprCollection"
   },
   "DaprEventBusSchemaOptions": {
     "UseSchemaRegistry": true,
     "SchemaValidatorType": "Json",
     "SchemaRegistryType": "Mongo",
     "AddSchemaOnPublish": true,
     "MongoStateStoreOptions": {
       "ConnectionString": "mongodb://localhost:27017",
       "DatabaseName": "schema-registry",
       "SchemasCollectionName": "schemas"
     }
   }

   • Call services.AddDaprEventBus in Startup.ConfigureServices.
   • Then call services.AddDaprMongoEventCache.

   public void ConfigureServices(IServiceCollection services)
   {
       // Add Dapr Event Bus
       services.AddDaprEventBus(Configuration);
   
       // Add Dapr Mongo event cache
       services.AddDaprMongoEventCache(Configuration);
   }

2. Define a C# record that extends IntegrationEvent. For example, the following WeatherForecastEvent record does so by adding a WeatherForecasts property.

   public record WeatherForecastEvent(IEnumerable<WeatherForecast> WeatherForecasts) : IntegrationEvent;

3. In the publisher inject IEventBus into the constructor of a controller (Web API projects) or worker class (Worker Service projects). Then call EventBus.PublishAsync, passing the event you defined in step 2.

   public class Worker : BackgroundService
   {
       public Worker(IEventBus eventBus)
       {
           _eventBus = eventBus;
       }
   
       protected override async Task ExecuteAsync(CancellationToken stoppingToken)
       {
           while (!stoppingToken.IsCancellationRequested)
           {
               // Publish event
               await _eventBus.PublishAsync(new WeatherForecastEvent(weathers));
   
               // Pause
               await Task.Delay(5000, stoppingToken);
           }
       }
   }

4. In the subscriber create the same IntegrationEvent derived class as in the publisher. Then create an event handler that extends IntegrationEventHandler<TIntegrationEvent> where TIntegrationEvent is the event type you defined earlier.

   • Override HandleAsync to perform a task when an event is received.
   • For example, WeatherForecastEventHandler sets WeatherForecasts on WeatherForecastRepository to the WeatherForecasts property of WeatherForecastEvent.

   public class WeatherForecastEventHandler : IntegrationEventHandler<WeatherForecastEvent>
   {
       private readonly WeatherForecastRepository _weatherRepo;
   
       public WeatherForecastEventHandler(WeatherForecastRepository weatherRepo)
       {
           _weatherRepo = weatherRepo;
       }
   
       public override Task HandleAsync(WeatherForecastEvent @event)
       {
           _weatherRepo.WeatherForecasts = @event.WeatherForecasts;
           return Task.CompletedTask;
       }
   }

5. Lastly, in Startup.Configure in app.UseEndpoints call endpoints.MapDaprEventBus, passing an action that subscribes to DaprEventBus events with one or more event handlers.

   • Also call app.UseRouting, app.UseCloudEvents, endpoints.MapSubscribeHandler.
   • Make sure to add parameters to Startup.Configure to inject each handler you wish to use.
   • For example, to add the weather forecast handler, you must add a WeatherForecastEventHandler parameter to the Configure method.

   public void Configure(IApplicationBuilder app, IWebHostEnvironment env,
       WeatherForecastEventHandler forecastEventHandler)
   {
       app.UseRouting();
       app.UseCloudEvents();
       app.UseEndpoints(endpoints =>
       {
           // Map SubscribeHandler and DapEventBus
           endpoints.MapSubscribeHandler();
           endpoints.MapDaprEventBus(eventBus =>
           {
               // Subscribe with a handler
               eventBus.Subscribe(forecastGeneratedEventHandler);
           });
       });
   }

Schema Registry

When you enable Schema Registry for the Dapr Event Bus, messages sent to the Event Bus will be validated using schemas registered for a given topic. By default Json Schema will be used to validate messages (other schema types may be supported in the future). This helps ensure that message schemas will not change in a way that will cause deserialization errors when consumers receive messages for a specific topic.

Note: Schema evolution rules for Json allow the addition of fields, which are then ignored by consumers. If fields are not required, they can be omitted and consumers will get default values when they deserialize messages.

The Schema Registry only validates messages when they are published to the Event Bus. Therefore, it is only necessary to enable Schema Registry for publishers, not subscribers.

UseSchemaRegistry enables use of the Schema Registry. SchemaValidatorType specifies the type of schema validator to use (the default is Json). AddSchemaOnPublish will add a generated schema to the Schema Registry if no schema has been previously registered for a given topic.

Note: EventDriven.SchemaValidator.Json uses JSchemaGenerator from Newtonsoft.Json.Schema.Generation, which makes all fields required by default. To make fields optional, you need to use EventDriven.SchemaRegistry.Api to update the schema by removing required fields.

To view all the registered schemas you can connect to the schema datastore directly, for example, using a MongoDB client such as Robot 3T.

Disabling Schema Registry

It is recommended you enable schema registry to so that subscribers may be protected from breakage should a publisher change an event schema in a way that is not backwards compatible.

However, if you wish to disable schema registry in a publisher, you may do so as follows.

• Update the DaprEventBusSchemaOptions section in your appsettings.json file by setting UseSchemaRegistry to false.
• Or simply remove the DaprEventBusSchemaOptions section altogether.

Disabling Event Cache

It is recommended you keep event cache enabled to make your subscriber idempotent and filter out duplicate events. This is the default behavior.

However, if you wish to disable event cache in a subscriber, you may do so as follows.

• Add a MongoEventCacheOptions section in your appsettings.json file and set EnableEventCache to false.

Samples

The samples folder contains two sample applications which use the Dapr Event Bus: SimplePubSub and DuplexPubSub.

1. The SimplePubSub sample contains two projects: Publisher and Subscriber. Every 5 seconds the Publisher creates a new set of weather forecasts and publishes them to the event bus. The Subscriber subscribes to the event by setting the WeatherForecasts property of WeatherForecastRepository, which is returned by the Get method of WeatherForecastController.
2. The DuplexPubSub sample contains four projects: Frontend, Backend, WeatherGenerator and Common. The Backend publishes a WeatherForecastRequestedEvent to the event bus in the Get method of the WeatherForecastController. The WebGenerator handles the event by creating a set of weather forecasts and publishing them to the event bus with a WeatherForecastGeneratedEvent, which is handled by the Backend by setting the WeatherForecasts property of the WeatherForecastRepository, so that new weather forecasts are returned by the WeatherForecastController. The Frontend initiates the pub/sub process by using an HttpClient to call the Backend when the user clicks the "Get Weather Forecasts" button.

Packages

EventDriven.EventBus.Abstractions

The EventDriven.EventBus.Abstractions package includes interfaces and abstract classes which provide an abstraction layer for interacting with any messsaging subsystem. This allows you to potentially exchange the Dapr implementation with another one, such as NServiceBus or MassTransit, without altering application code.

This package contains an IEventBus interface implemented by an EventBus abstract class.

public interface IEventBus
{
    Dictionary<string, List<IIntegrationEventHandler>> Topics { get; }

    void Subscribe(
        IIntegrationEventHandler handler,
        string topic = null,
        string prefix = null);

    void UnSubscribe(
        IIntegrationEventHandler handler,
        string topic = null,
        string prefix = null);

    Task PublishAsync<TIntegrationEvent>(
        TIntegrationEvent @event,
        string topic = null,
        string prefix = null)
        where TIntegrationEvent : IIntegrationEvent;
}

When a subscriber calls Subscribe, it passes a class that extends IntegrationEventHandler, which implements IIntegrationEventHandler. The event handler is added to a topic which can have one more handlers. A topic name may be specified explicitly, as well as a prefix which may contain a version number. There are non-generic and generic versions of IIntegrationEventHandler.

public interface IIntegrationEventHandler
{
    string Topic { get; set; }

    Task HandleAsync(IIntegrationEvent @event);
}

public interface IIntegrationEventHandler<in TIntegrationEvent> : IIntegrationEventHandler
    where TIntegrationEvent : IIntegrationEvent
{
    Task HandleAsync(TIntegrationEvent @event);
}

The generic version of IntegrationEventHandler includes a TIntegrationEvent type argument that must implement IIntegrationEvent. The IntegrationEvent abstract record provides defaults for both Id and CreationDate properties.

public interface IIntegrationEvent
{
    string Id { get; }

    DateTime CreationDate { get; }
}

public abstract record IntegrationEvent : IIntegrationEvent
{
    public string Id { get; init; } = Guid.NewGuid().ToString();

    public DateTime CreationDate { get; init; } = DateTime.UtcNow;
}

EventDriven.EventBus.Dapr

The EventDriven.EventBus.Dapr package has a DaprEventBus class that extends EventBus by injecting DaprClient. It also injects DaprEventBusOptions for the pubsub component name needed by DaprClient.PublishAsync. The event topic defaults to the type name of the the event, but it can also be supplied explicitly.

public class DaprEventBus : EventBus
{
    private readonly IOptions<DaprEventBusOptions> _options;
    private readonly DaprClient _dapr;

    public DaprEventBus(IOptions<DaprEventBusOptions> options, DaprClient dapr)
    {
        _options = options ?? throw new ArgumentNullException(nameof(options));
        _dapr = dapr ?? throw new ArgumentNullException(nameof(dapr));
    }

    public override async Task PublishAsync<TIntegrationEvent>(
        TIntegrationEvent @event,
        string topic = null,
        string prefix = null)
    {
        if (@event is null) throw new ArgumentNullException(nameof(@event));
        var topicName = GetTopicName(@event.GetType(), topic, prefix);
        await _dapr.PublishEventAsync(_options.Value.PubSubName, topicName, @event);
    }
}

The ServiceCollectionExtensions class has a AddDaprEventBus method that registers DaprClient and DaprEventBus, and it configures DaprEventBusOptions for specifying the PubSubName option.

The DaprEventBusEndpointRouteBuilderExtensions class has a MapDaprEventBus method that allows the caller to subscribe to DaprEventBus by adding handlers. It maps an HTTP Post endpoint for each event bus topic that is called by Dapr when a message is sent to the registered pub/sub component. The default component Redis, but Dapr can be configured to use another message broker, such as AWS SNS+SQS.