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Your First Subscribers

If we look back at our task requirements, the last one states that; The bank would like to keep track of how much money it has gained as revenue as a result of the transaction fees.

One way to satisfy this requirement is to use a DomainEventSubscriber that Subscribes to the FeesDeductedEvent domain event, which then tells the persistence mechanism to store the result so that it can later be read by a Projection.

//Walkthrough.Domain/Subscribers/Revenue/RevenueSubscriber.cs
public class RevenueSubscriber : DomainEventSubscriber,
    ISubscribeTo<Account,AccountId,FeesDeductedEvent>
{
    public IActorRef RevenueRepository { get; }
    public RevenueSubscriber(IActorRef revenueRepository)
    {
        RevenueRepository = revenueRepository;
    }

    public Task Handle(IDomainEvent<Account, AccountId, FeesDeductedEvent> domainEvent)
    {
        var command = new AddRevenueCommand(domainEvent.AggregateEvent.Amount);
        RevenueRepository.Tell(command);

        return Task.CompletedTask;
    }
}

Our mock revenue repository is going to be modelled as an actor, but typically this would be a type of persistent storage that applies to your use case.

//Walkthrough.Domain/Repositories/RevenueRepository.cs
public class RevenueRepository : ReceiveActor
{
    public Money Revenue { get; private set; } = new Money(0.00m);
    public int Transactions { get; private set; } = 0;

    public RevenueRepository()
    {
        Receive<AddRevenueCommand>(Handle);
        Receive<GetRevenueQuery>(Handle);
    }

    private bool Handle(AddRevenueCommand command)
    {
        Revenue = Revenue + command.AmountToAdd;
        Transactions++;
        return true;
    }

    private bool Handle(GetRevenueQuery query)
    {
        var projection = new RevenueProjection(Revenue, Transactions);
        Sender.Tell(projection);
        return true;
    }
}

This repository handles a AddRevenueCommand which is just a data transfer object (dto) that holds the amount of money to add to the current revenue. The repository also handles a query GetRevenueQuery. You can now see a hint of how the read/query side of an Akkatecture application might work. On to the next section on projection.

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