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Cratis

Unique And Once

Use this when the rule is that a value is used once, or that something happens once per entity. A constraint is the right tool. It needs no read and no read protection, and it holds for every writer.

RuleOne library card per member. A username is unique across members
Mechanism[Unique] on the event class (once per event source), or on a property (once per value)
AlternativesAn expected-empty scope for a one-off claim (Chronicle used directly). A new aggregate, which expects an empty history
NotA read that asks “does it exist?” before the append. A decision read for value uniqueness: it guards one key’s history, not a value across keys
using Cratis.Arc.Commands.ModelBound;
using Cratis.Chronicle.Events;
using Cratis.Chronicle.Events.Constraints;


[EventType]
[Unique(message: "This member already has a library card.")]
public record LibraryCardIssued();


[Command]
public record IssueLibraryCard(EventSourceId Member)
{
    public LibraryCardIssued Handle() => new();
}


[EventType]
public record UserRegistered([property: Unique(message: "That user name is taken.")] string UserName);


[Command]
public record RegisterUser(string UserName)
{
    public UserRegistered Handle() => new(UserName);
}

There is no guard line: the kernel checks the constraint inside the append, for every append.

What holds

Section titled “What holds”
  • A second IssueLibraryCard for the same member is rejected, whoever sends it and whatever the timing. Arc turns the violation into a validation error that carries your message.
  • Two registrations of the same user name cannot both succeed. RegisterUser has no event source id property, so each call creates a new user stream, which is right for a creation command. The constraint, not the stream, makes the name unique.

What does not hold

Section titled “What does not hold”
  • A constraint is a fixed rule. If a user can change their name, release the old value with a removal event; see the constraints documentation.
  • Constraints across namespaces are not checked: tenants in different namespaces can claim the same value.
Section titled “Related”

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