Domain Event Handling¶

A domain event handler is a processing logic that handles domain events for a specific purpose. A domain event handler should handle domain events for only one purpose. Different purposes for the same domain event should have different domain event handlers.

Define Domain Event Handler¶

Install the nuget package NetCorePal.Extensions.Domain.Abstractions
```
dotnet add package NetCorePal.Extensions.Domain.Abstractions
```
A domain event handler is a class that implements the IDomainEventHandler<TDomainEvent> interface, where TDomainEvent is the type of the domain event.

Here is an example of a domain event handler:

```csharp
public class OrderCreatedDomainEventHandler(IMediator mediator) : IDomainEventHandler<OrderCreatedDomainEvent>
{
    public Task Handle(OrderCreatedDomainEvent notification, CancellationToken cancellationToken)
    {
        return mediator.Send(new DeliverGoodsCommand(notification.Order.Id), cancellationToken);
    }
}
```

Domain Event Handlers Must¶

Domain event handlers must be idempotent, meaning that processing the same domain event multiple times should result in the same outcome;
A single domain event handler can only issue one command and is not allowed to issue multiple commands within the same handler;

Note: In our framework, domain event handlers are executed synchronously, and the commands they call are in the same transaction as the CommandHandler that triggers the domain event.

Domain Event Handlers Can¶

A single domain event can correspond to multiple domain event handlers;
Domain event handlers can use the MediatR framework to send commands;
Domain event handlers can retrieve data from multiple queries;
Domain event handlers can call external services to complete some information organization and validation;
Domain event handlers can publish integration events to transmit events to other systems;

Domain Event Handlers Should Not¶

Domain event handlers should not contain operations on domain models. Domain models should be operated and persisted by CommandHandlers.

Note: Since our framework only manages transactions for CommandHandlers, operations in domain event handlers will not be saved to the database because the framework does not call SaveChangesAsync for them.