buildDecisionModel

@delta-base/toolkit

---

Function: buildDecisionModel()

buildDecisionModel<State, EventType>(eventStore, options): Promise<DecisionModelResult<State, EventType>>

Build a decision model by reading events via a DCB query and folding them into state. Returns the state and a pre-built append condition for use with EventStore.append().

This is the primary pattern for command handling with Dynamic Consistency Boundaries (DCB). The query filters events by type and tags, and the append condition ensures no conflicting events were appended between the read and the write.

Type Parameters

State

State

EventType

EventType extends Readonly<{ data: EventData; metadata?: PlatformEventMetadata; tags?: string[]; type: string; }> = Readonly<{ data: EventData; metadata?: PlatformEventMetadata; tags?: string[]; type: string; }>

Parameters

eventStore

EventStore

An event store that supports readByQuery (DCB)

options

BuildDecisionModelOptions<State, EventType>

Query, initial state, and evolve function

Returns

Promise<DecisionModelResult<State, EventType>>

The decision model state and an append condition

Example

const { state, appendCondition } = await buildDecisionModel(
  eventStore,
  {
    query: {
      items: [{
        types: ['CourseDefined', 'CourseArchived'],
        tags: [`course:${courseId}`],
      }],
    },
    initialState: () => ({ exists: false }),
    evolve: (state, event) => {
      switch (event.type) {
        case 'CourseDefined': return { exists: true };
        case 'CourseArchived': return { exists: false };
        default: return state;
      }
    },
  }
);

if (!state.exists) throw new Error('Course not found');

await eventStore.append(
  [{ type: 'CourseCapacityChanged', data: { newCapacity: 30 }, tags: [`course:${courseId}`] }],
  appendCondition
);