Unique event type constraint

Use builder.Unique<TEventType>() inside an IConstraint implementation to enforce that only one event of a specific type can be appended per event source identifier. Use this when the event itself is a unique fact — for example, initializing a project can only happen once per project.

Chronicle discovers all IConstraint implementations automatically — no registration is needed.

Defining a unique event type constraint

using Cratis.Chronicle.Events.Constraints;

public class UniqueProjectInitialization : IConstraint
{
    public void Define(IConstraintBuilder builder) =>
        builder.Unique<ProjectInitialized>();
}

Violation message

Provide a static message string:

using Cratis.Chronicle.Events.Constraints;

public class UniqueProjectInitialization : IConstraint
{
    public void Define(IConstraintBuilder builder) =>
        builder.Unique<ProjectInitialized>(
            message: "A project can only be initialized once.");
}

Or use a callback to compose the message dynamically from violation context:

using Cratis.Chronicle.Events.Constraints;

public class UniqueProjectInitialization : IConstraint
{
    public void Define(IConstraintBuilder builder) =>
        builder.Unique<ProjectInitialized>(
            messageCallback: violation => $"Project '{violation.EventSourceId}' has already been initialized.");
}

Constraint name

An optional name can be provided to identify the constraint. When not provided, Chronicle uses the event type name as the default constraint name:

using Cratis.Chronicle.Events.Constraints;

public class UniqueProjectInitialization : IConstraint
{
    public void Define(IConstraintBuilder builder) =>
        builder.Unique<ProjectInitialized>(
            name: "UniqueProjectInitialization",
            message: "A project can only be initialized once.");
}

How constraints are enforced

When a constraint is registered, the Chronicle Kernel creates the indexes required to enforce it. Constraints are evaluated server-side during append, ensuring data integrity regardless of the client.