Validate and update Models atomically

Let only one customer reserve a seat, even when requests arrive together.

Store whether the seat is reserved, and put the availability rule on the command that reserves it.

Define the seat and its commands

Java

@Model
public record Seat(@EntityId SeatId seatId, boolean reserved) {}

public final class SeatId extends Id<Seat> {
    public SeatId(String id) { super(id); }
}

public record CreateSeat(SeatId seatId) {
    @Apply
    Seat apply() { return new Seat(seatId, false); }
}

public record ReserveSeat(SeatId seatId) {
    @AssertLegal
    void check(Seat seat) {
        if (seat.reserved()) throw new IllegalCommandException("Seat already reserved");
    }

    @Apply
    Seat apply(Seat seat) { return new Seat(seatId, true); }
}

Kotlin

@Model
data class Seat(@EntityId val seatId: SeatId, val reserved: Boolean)

class SeatId(id: String) : Id<Seat>(id)

data class CreateSeat(val seatId: SeatId) {
    @Apply
    fun apply() = Seat(seatId, false)
}

data class ReserveSeat(val seatId: SeatId) {
    @AssertLegal
    fun check(seat: Seat) {
        if (seat.reserved) throw IllegalCommandException("Seat already reserved")
    }

    @Apply
    fun apply(seat: Seat) = Seat(seatId, true)
}

Send ReserveSeat through the command gateway. Fluxzero loads the seat, checks the rule and commits the change. If a competing change invalidates the read, the decision is retried against the newer state. A second reservation then meets the same “already reserved” rule.

Register the command types in your application; Spring discovers them in its scan. The fixture discovers them from the supplied commands.

Check the rule

Java

var seatId = new SeatId("A1");
TestFixture.create()
    .givenCommands(new CreateSeat(seatId), new ReserveSeat(seatId))
    .whenCommand(new ReserveSeat(seatId))
    .expectExceptionalResult(IllegalCommandException.class);

Kotlin

val seatId = SeatId("A1")
TestFixture.create()
    .givenCommands(CreateSeat(seatId), ReserveSeat(seatId))
    .whenCommand(ReserveSeat(seatId))
    .expectExceptionalResult(IllegalCommandException::class.java)

The check and the change belong to the same Model operation. Checking availability in a separate query before sending an unconditional update would leave a race between those two calls.

See Model updates for the annotations and atomic changes across Models when a booking must create a reservation as well.