cratis-chronicle-reducer

Chronicle reducers

A reducer builds a read model as "current state plus event produces next state", with real C# control flow. It is the last resort on the projection ladder, not a default. A reducer that could have been a projection is a projection that was never written, and it will be flagged for conversion in review.

Verified product sources

This skill is verified against this exact source:

Package	Version	Purpose
Cratis.Chronicle	16.45.2	Cratis.Chronicle.Reducers, Cratis.Chronicle.ReadModels, Cratis.Chronicle.Events

Reverify product sources before claiming support for another version.

Admission test — all three must hold

1. Model-bound attributes cannot express it. Not [FromEvent<T>], [SetFrom<T>], [SetValue<T>], [SetFromContext<T>], [AddFrom<T>], [SubtractFrom<T>], [ChildrenFrom<T>], [Join<T>], [RemovedWith<T>], [RemovedWithJoin<T>], [Nested], [ClearWith<T>], [Increment<T>], [Decrement<T>], [Count<T>], [FromAll], or [FromEvery].
2. The fluent IProjectionFor<T> builder cannot express it. Not UsingKey, UsingKeyFromContext, UsingParentKey, UsingCompositeKey, UsingConstantKey, Join, Children, Nested, WithInitialValues, NotRewindable, or a Set(...).To(...) form.
3. The remainder needs real control flow — branching on prior state, loops, or an accumulation the declarative APIs have no operator for.

If all three hold, write the reducer and carry an inline justification naming the specific limitation that ruled out a projection.

using Cratis.Chronicle.Events;
using Cratis.Chronicle.Reducers;

// Reducer required: the next value depends on the prior value through a
// per-event rule that no counter or projection operator expresses.
public class <ReadModelName>Reducer : IReducerFor<<ReadModelName>>
{
    public <ReadModelName> <Opened>(<OpenedEvent> @event, <ReadModelName>? current, EventContext context) =>
        new(<initialValue>, context.Occurred);

    public <ReadModelName> <Changed>(<ChangedEvent> @event, <ReadModelName>? current) =>
        current is null
            ? new(<initialValue>, default)
            : current with { <Property> = <computedFrom(current, @event)> };
}