Your First Aggregate

On analysis of the business requirements, it is apparent that the main aggregate entity that exists under the Bank domain context is an Account. The account aggregate needs the ability to be Opened and to Transfer Money to other Accounts. Inversely, Accounts need to also Receive Money. Lets call our aggregate responsible for representing an account the AccountAggregate.

The Account Aggregate

First we need to make an Identity<> for our aggregate.

//Walkthrough.Domain/Model/Account/AccountId.cs
public class AccountId : Identity<AccountId>
{
    public AccountId(string value)
        : base(value)
    {
    }
}

Then we need to make the Account aggregate state model that will hold our balance:

//Walkthrough.Domain/Model/Account/AccountState.cs
public class AccountState : AggregateState<Account, AccountId>
{
    public Money Balance { get; set; }
}

Lets make a simple Money ValueObject<decimal> that will represent money in our application, lets put some simple domain logic into the value object that states that its value must be non-negative.

//Walkthrough.Domain/Model/Account/ValueObjects/Money.cs
public class Money : SingleValueObject<decimal\>
{
    public Money(decimal value)
        : base(value)
    {
        if(value < 0) throw new ArgumentException(nameof(value));
    }

    //overload the + and - operators to support the
    //addition and subtraction of money
}

Now we have a value object that represents money in our domain. In the fully worked walkthrough in the Akkatecture repository we have overloaded the + and - operators accordingly.

Now we can make our Account aggregate root.

//Walkthrough.Domain/Model/Account/Account.cs
public class Account : AggregateRoot<Account, AccountId, AccountState>
{
    public Account(AccountId aggregateId)
        : base(aggregateId)
    {
    }
}

And finally we need to make our aggregate root manager, which will be responsible for supervising, and creating the aggregate roots.

//Walkthrough.Domain/Model/Account/AccountManager.cs
public class AccountManager : AggregateManager<Account, AccountId, Command<Account, AccountId>>
{
}

We have made the basic skeleton of our aggregate domain. To interact with our aggregate we need to use commands. Lets proceed to the next part of the walkthrough.

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