cratis-chronicle-read-model

Chronicle read models

A read model is derived state built from events. The path is:

event types -> read-model record -> a projection or a reducer -> a query surface

Chronicle owns the events, the projection or reducer, and the stored instance. The query surface is Arc's, and the two halves are separable: a read model that only exists to be read from inside the backend needs no query surface at all.

Verified product sources

This skill is verified against these exact sources:

Package	Version	Purpose
Cratis.Chronicle	16.45.2	Cratis.Chronicle.Events, Cratis.Chronicle.Keys, Cratis.Chronicle.ReadModels
Cratis.Arc.Core	current	Cratis.Arc.Queries.ModelBound.ReadModelAttribute and PathAttribute
Cratis.Arc.MongoDB	current	the Observe family of IMongoCollection<T> extensions

Reverify product sources before claiming support for another version. Confirm the exact Arc package version in the consuming project before citing one.

Step 1 — Define the events

Events are the source of truth. Each is a record carrying Cratis.Chronicle.Events.EventTypeAttribute, named as a past-tense fact.

using Cratis.Chronicle.Events;

/// <summary>Emitted when <description>.</summary>
[EventType]
public record <EntityName><PastTenseVerb>(<ConceptType> <PropertyName>);

• One clear purpose per event; do not mix concerns into one fact.
• Avoid nullable properties. Analyzer CHR0012 warns on them. Model an optional fact as a separate event.
• Prefer strongly typed concept properties over raw primitives.
• An event never carries its own event-source id as a payload property — the id is in the event context.
• [EventType] takes (string id = "", uint generation = 1). Pass neither for a new event: the id defaults to the CLR type name and the generation to 1.

Step 2 — Define the read-model record

using Cratis.Chronicle.Keys;

public record <ReadModelName>(
    [property: Key] <IdType> Id,
    <PropertyType> <PropertyName>);