Primitives

The basic core primitives of Akkatecture are:

A ValueObject is an immutable type that is distinguishable only by the state of its properties. That is, unlike an Entity, which has a unique identifier and remains distinct even if its properties are otherwise identical, two ValueObjects with the exact same properties can be considered equal. An Entity always has a globally unique identifier, that is, if two entities have the same identity, they are the same entity, regardless if their member values are different. Akkatecture uses these primitives all over the project, and you are highly encouraged to use them as well, so that your domain design is highly expressive, and readable.

Value Objects

In Akkatecture, the ValueObject abstract class type is used to represent value object. These should be immutable by implementation. There also is the SingleValueObject<> generic primitive, that allows for the modelling of domain value objects that are only valued by a single c# dotnet primitive.

public class AccountNumber : SingleValueObject<string>
{
  public AccountNumber(string value)
    : base(value)
  {
      //do some model validation, null checks etc.
  }
}

It is recommended to use value as the parameter name, if you intend to serialize value objects. With Akkatecture's built in serialization converters.

Identities

The Identity<> value object provides generic functionality to model and validate the identities of e.g. aggregate roots. Its esentially a wrapper around a Guid. It is possible to model your own Identity<> by implementing the IIdentity<> interface.

public class AccountId : Identity<AccountId>
{
  public AccountId(string value)
    : base(value)
  {
  }
}
  1. The identity follow the form {class without "Id"}-{guid} e.g. account-c93fdb8c-5c9a-4134-bbcd-87c0644ca34f for the above AccountId example.

  2. The internal Guid can be generated using one of the following methods/properties as described by points 3-5 on this list. You can access the Guid factories directly by accessing the static methods on the GuidFactories class.

  3. New: Uses the standard Guid.NewGuid().

  4. NewDeterministic(...): Creates a name-based Guid using the algorithm from RFC 4122 §4.3, which allows identities to be generated based on known data, e.g. an e-mail, i.e., it always returns the same identity for the same arguments.

  5. NewComb(): Creates a sequential Guid that can be used to e.g. avoid database fragmentation.

  6. A string can be tested to see if its a valid identity using the static bool IsValid(string) method.

  7. Any validation errors can be gathered using the static IEnumerable<string> Validate(string) method.

Its very important to name the constructor argument value as it is significant when the identity type is deserialized.

Here's some examples on we can use our newly created AccountId

// Uses the default Guid.NewGuid()
// as described in point 3 above
var accountId = AccountId.New
// Create a namespace, put this in a constant somewhere
var emailNamespace = Guid.Parse("769077C6-F84D-46E3-AD2E-828A576AAAF3");

// Creates an identity with the value "account-9181a444-af25-567e-a866-c263b6f6119a",
// useful to use when you want to create Id's
// deterministically from other real world "identifiers",
// especially in distributed situations
// as described in point 4 above
var accountId = AccountId.NewDeterministic(emailNamespace, "[email protected]");
// Creates a new identity every time, but an identity when used in e.g.
// database indexes, minimizes fragmentation
// as described in point 5 above
var accountId = AccountId.NewComb()

You are not forced to use the Identity<> implementation from Akkatecture. If you make your own Identity that implements IIdentity then it can still plug into all of the Akkatecture constructs as variants.

Entities

An Entity is an object that has some intrinsic identity, apart from the rest of its state. Even if its properties are the same as another instance of the same type, it remains distinct because of its unique identity. The Entity<> In Akkatecture is itself a ValueObject however it implements the IEntity<Identity> interface, which requires it to have a member called Identity Id. Now you can see the relationship between Entity<>, SingleValueObject<>, and Identity<>. A sample Entity running onwards from the Account example above, could be:

public class Account : Entity<AccountId>
{
  public AccountNumber AccountNumber {get;}

  public Account(
    AccountId entityId,
    AccountNumber accountNumber)
    : base(entityId)
  {
      //do some model validation, null checks etc.
      AccountNumber = accountNumber;
  }
}

When you look at the Account class/model definition above, you could describe it using object oriented programming language. That is, The Account class/model is-an Entity<> that has-an AccountId, and has-an AccountNumber.

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