Unique event type constraint

Adorn the event type class with [Unique] to enforce that only one event of this type can be appended per event source. Use this when the event itself is a unique fact — for example, registering a user can only happen once.

Chronicle discovers [Unique]-adorned types automatically and registers the constraints with the Kernel when the client starts.

Defining a unique event type constraint

C#

using Cratis.Chronicle.Events;
using Cratis.Chronicle.Events.Constraints;

[EventType]
[Unique]
public record ConstraintsModelBoundUniqueEventTypeUserRegistered(string Email, string DisplayName);

Kotlin

import io.cratis.chronicle.constraints.Unique
import io.cratis.chronicle.events.EventType

@EventType
@Unique
data class ConstraintsModelBoundUniqueEventTypeUserRegistered(val email: String, val displayName: String)

Java

import io.cratis.chronicle.constraints.Unique;
import io.cratis.chronicle.events.EventType;

@EventType
@Unique
record ConstraintsModelBoundUniqueEventTypeUserRegistered(String email, String displayName) {}

Elixir

defmodule MyApp.Events.ConstraintsModelBoundUniqueEventTypeUserRegistered do
  use Chronicle.Events.EventType, id: "constraints-model-bound-unique-event-type-user-registered"

  defstruct [:email, :display_name]

  unique_event_type()
end

TypeScript

TypeScript does not support this workflow yet.

Constraint name and violation message

An optional constraint name and violation message can be provided:

C#

using Cratis.Chronicle.Events;
using Cratis.Chronicle.Events.Constraints;

[EventType]
[Unique(name: "UniqueUser", message: "A user with this identity has already been registered.")]
public record ConstraintsModelBoundUniqueEventTypeNamedUserRegistered(string Email, string DisplayName);

Kotlin

import io.cratis.chronicle.constraints.Unique
import io.cratis.chronicle.events.EventType

@EventType
@Unique(id = "UniqueUser", message = "A user with this identity has already been registered.")
data class ConstraintsModelBoundUniqueEventTypeNamedUserRegistered(val email: String, val displayName: String)

Java

import io.cratis.chronicle.constraints.Unique;
import io.cratis.chronicle.events.EventType;

@EventType
@Unique(id = "UniqueUser", message = "A user with this identity has already been registered.")
record ConstraintsModelBoundUniqueEventTypeNamedUserRegistered(String email, String displayName) {}

Elixir

defmodule MyApp.Events.ConstraintsModelBoundUniqueEventTypeNamedUserRegistered do
  use Chronicle.Events.EventType,
    id: "constraints-model-bound-unique-event-type-named-user-registered"

  defstruct [:email, :display_name]

  # Elixir supports a custom constraint name, but not a custom violation message.
  unique_event_type(name: "UniqueUser")
end

TypeScript

TypeScript does not support this workflow yet.

The message parameter is optional. Chronicle produces a default violation message when one is not supplied.

Releasing a constraint

Apply [RemoveConstraint] to the event type that signals a domain object has been removed. When this event is appended, Chronicle releases the constraint, and the event source can accept a new event of this type:

C#

using Cratis.Concepts;
using Cratis.Chronicle.Events;
using Cratis.Chronicle.Events.Constraints;

public record ConstraintsModelBoundUniqueEventTypeUserId(Guid Value) : ConceptAs<Guid>(Value);

[EventType]
[RemoveConstraint("UniqueUser")]
public record ConstraintsModelBoundUniqueEventTypeUserRemoved(ConstraintsModelBoundUniqueEventTypeUserId UserId);

Kotlin

import io.cratis.chronicle.constraints.RemoveConstraint
import io.cratis.chronicle.events.EventType

@EventType
@RemoveConstraint("UniqueUser")
data class ConstraintsModelBoundUniqueEventTypeUserRemoved(val userId: String)

Java

import io.cratis.chronicle.constraints.RemoveConstraint;
import io.cratis.chronicle.events.EventType;

@EventType
@RemoveConstraint("UniqueUser")
record ConstraintsModelBoundUniqueEventTypeUserRemoved(String userId) {}

Elixir

defmodule MyApp.Events.ConstraintsModelBoundUniqueEventTypeUserRemoved do
  use Chronicle.Events.EventType, id: "constraints-model-bound-unique-event-type-user-removed"

  defstruct [:user_id]

  remove_constraint("UniqueUser")
end

TypeScript

TypeScript does not support this workflow yet.

The constraint name must exactly match the name provided to the [Unique] attribute. When no name was provided to [Unique], Chronicle uses the event type name as the default constraint name.

More than one event type can name the same constraint. Each of them releases it on its own, which is how a cycle that can end in more than one way is expressed with attributes.

How constraints are enforced

The Chronicle Kernel reads the [Unique] and [RemoveConstraint] attributes when the client connects and creates the indexes it needs. Every subsequent append is checked against those indexes server-side, so no constraint logic runs in client code.