This article explains how to implement reliable event handling using the Outbox/Inbox pattern in a modular monolithic application with multiple databases. We'll use the ModularCRM project as an example (how that project was created is explained in this document).

Project Background

ModularCRM is a monolithic application that integrates multiple ABP framework open-source modules, including:

• Account
• Identity
• Tenant Management
• Permission Management
• Setting Management
• And other open-source modules

Besides the ABP framework modules, the project contains three business modules:

• Order module (Ordering), using MongoDB database
• Product module (Products), using SQL Server database
• Payment module (Payment), using MongoDB database

The project configures separate database connection strings for ModularCRM and the three business modules in appsettings.json:

{
  "ConnectionStrings": {
    "Default": "Server=localhost,1434;Database=ModularCrm;User Id=sa;Password=1q2w3E***;TrustServerCertificate=true",
    "Products": "Server=localhost,1434;Database=ModularCrm_Products;User Id=sa;Password=1q2w3E***;TrustServerCertificate=true",
    "Ordering": "mongodb://localhost:27017/ModularCrm_Ordering?replicaSet=rs0",
    "Payment": "mongodb://localhost:27017/ModularCrm_Payment?replicaSet=rs0"
  }
}

Business Scenario

These modules communicate through the ABP framework's DistributedEventBus to implement the following business flow:

This is a simple example flow. Real business flows are more complex. The sample code is for demonstration purposes.

1. Order module: Publishes OrderPlacedEto event when an order is placed
2. Product module: Subscribes to OrderPlacedEto event and reduce product stock
3. Payment module: Subscribes to OrderPlacedEto event, processes payment, then publishes PaymentCompletedEto event
4. Order module: Subscribes to PaymentCompletedEto event and updates order status to Delivered

When implementing this flow, we need to ensure:

• Transaction consistency between order creation and event publishing
• Transaction consistency when modules process messages
• Reliable message delivery (including persistence, confirmation, and retry mechanisms)

Using the default implementation of the ABP framework's distributed event bus cannot meet these requirements, so we need to add a new mechanism that is also provided by the ABP Framework.

Outbox/Inbox Pattern Solution

To meet these requirements, we use the Outbox/Inbox pattern:

Outbox Pattern

• Saves distributed events with database operations in the same transaction
• Sends events to distributed message service through background jobs
• Ensures consistency between data updates and event publishing
• Prevents message loss during system failures

Inbox Pattern

• First saves received distributed events to the database
• Processes events in a transactional way
• Ensures messages are processed only once by saving processed message records
• Maintains processing state for reliable handling

For how to enable and configure Outbox/Inbox in projects and modules, see: https://abp.io/docs/latest/framework/infrastructure/event-bus/distributed#outbox-inbox-for-transactional-events

Module Configuration

Each module needs to configure separate Outbox/Inbox. Since it's a monolithic application, all message processing classes are in the same project, so we need to configure Outbox/Inbox for each module with Selector/EventSelector to ensure that the module only sends and receives the messages it cares about, avoiding message duplication processing.

ModularCRM Main Application Configuration

It will send and receive messages from all ABP framework open-source modules.

// This selector will match all abp built-in modules and the current module.
Func<Type, bool> abpModuleSelector = type => type.Namespace != null && (type.Namespace.StartsWith("Volo.") || type.Assembly == typeof(ModularCrmModule).Assembly);

Configure<AbpDistributedEventBusOptions>(options =>
{
    options.Inboxes.Configure("ModularCrm", config =>
    {
        config.UseDbContext<ModularCrmDbContext>();
        config.EventSelector = abpModuleSelector;
        config.HandlerSelector = abpModuleSelector;
    });

    options.Outboxes.Configure("ModularCrm", config =>
    {
        config.UseDbContext<ModularCrmDbContext>();
        config.Selector = abpModuleSelector;
    });
});

Order Module Configuration

It only sends OrderPlacedEto events and receives PaymentCompletedEto events and executes OrderPaymentCompletedEventHandler.

Configure<AbpDistributedEventBusOptions>(options =>
{
    options.Inboxes.Configure(OrderingDbProperties.ConnectionStringName, config =>
    {
        config.UseMongoDbContext<IOrderingDbContext>();
        config.EventSelector = type => type == typeof(PaymentCompletedEto);
        config.HandlerSelector = type => type == typeof(OrderPaymentCompletedEventHandler);
    });

    options.Outboxes.Configure(OrderingDbProperties.ConnectionStringName, config =>
    {
        config.UseMongoDbContext<IOrderingDbContext>();
        config.Selector = type => type == typeof(OrderPlacedEto);
    });
});

Here, the EventSelector and HandlerSelector checks only a single type. If you have multiple events and event handlers, you can check the given type if it is included in an array of types.

Product Module Configuration

It only receives EntityCreatedEto<UserEto> and OrderPlacedEto events and executes ProductsOrderPlacedEventHandler and ProductsUserCreatedEventHandler. It does not send any events now.

Configure<AbpDistributedEventBusOptions>(options =>
{
    options.Inboxes.Configure(ProductsDbProperties.ConnectionStringName, config =>
    {
        config.UseDbContext<IProductsDbContext>();
		config.EventSelector = type => type == typeof(EntityCreatedEto<UserEto>) || type == typeof(OrderPlacedEto);
        config.HandlerSelector = type => type == typeof(ProductsOrderPlacedEventHandler) || type == typeof(ProductsUserCreatedEventHandler);
    });

    // Outboxes are not used in this module
	options.Outboxes.Configure(ProductsDbProperties.ConnectionStringName, config =>
	{
		config.UseDbContext<IProductsDbContext>();
		config.Selector = type => false;
	});
});

Payment Module Configuration

It only sends PaymentCompletedEto events and receives OrderPlacedEto events and executes PaymentOrderPlacedEventHandler.

Configure<AbpDistributedEventBusOptions>(options =>
{
    options.Inboxes.Configure(PaymentDbProperties.ConnectionStringName, config =>
    {
        config.UseMongoDbContext<IPaymentMongoDbContext>();
        config.EventSelector = type => type == typeof(OrderPlacedEto);
        config.HandlerSelector = type => type == typeof(PaymentOrderPlacedEventHandler);
    });

    options.Outboxes.Configure(PaymentDbProperties.ConnectionStringName, config =>
    {
        config.UseMongoDbContext<IPaymentMongoDbContext>();
        config.Selector = type => type == typeof(PaymentCompletedEto);
    });
});

Running ModularCRM Simulation Business Flow

1. Run the following command in the ModularCrm directory:

docker-compose up -d

abp install-lib              

dotnet run --project ModularCrm --migrate-database 

dotnet run --project ModularCrm

2. Navigate to https://localhost:44303/ to view the application homepage

3. Enter a customer name and select a product, then submit an order. After a moment, refresh the page to see the order, product, and payment information.

Application logs display the complete processing flow:

[Ordering Module] Order created:  OrderId: b7ad3f47-0e77-bb81-082f-3a1834503e88, ProductId: 0f95689f-4cb6-36f5-68bd-3a18344d32c9, CustomerName: john

[Products Module] OrderPlacedEto event received: OrderId: b7ad3f47-0e77-bb81-082f-3a1834503e88, CustomerName: john, ProductId: 0f95689f-4cb6-36f5-68bd-3a18344d32c9
[Products Module] Stock count decreased for ProductId: 0f95689f-4cb6-36f5-68bd-3a18344d32c9

[Payment Module] OrderPlacedEto event received: OrderId: b7ad3f47-0e77-bb81-082f-3a1834503e88, CustomerName: john, ProductId: 0f95689f-4cb6-36f5-68bd-3a18344d32c9
[Payment Module] Payment processing completed for OrderId: b7ad3f47-0e77-bb81-082f-3a1834503e88

[Ordering Module] PaymentCompletedEto event received: OrderId: b7ad3f47-0e77-bb81-082f-3a1834503e88, PaymentId: d0a41ead-ee0f-714c-e254-3a1834504d65, PaymentMethod: CreditCard, PaymentAmount: ModularCrm.Payment.Payment.PaymentCompletedEto
[Ordering Module] Order state updated to Delivered for OrderId: b7ad3f47-0e77-bb81-082f-3a1834503e88

In addition, when a new user registers, the product module will also receive the EntityCreatedEto<UserEto> event, and we will send an email to the new user, just to demonstrate the Outbox/Inbox Selector mechanism.

[Products Module] UserCreated event received: UserId: "9a1f2bd0-5b28-210a-9e56-3a18344d310a", UserName: admin
[Products Module] Sending a popular products email to admin@abp.io...

Summary

By introducing the Outbox/Inbox pattern, we have achieved:

1. Transactional message sending and receiving
2. Reliable message processing mechanism
3. Modular event processing in a multi-database environment

ModularCRM project not only implements reliable message processing but also demonstrates how to handle multi-database scenarios gracefully in a monolithic application. Project source code: https://github.com/abpframework/abp-samples/tree/master/ModularCrm-OutboxInbox-Pattern

Reference

• Outbox/Inbox for transactional events
• ConnectionStrings
• ABP Studio: Single Layer Solution Template