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DCB Event Store

A TypeScript/Node.js implementation of the Dynamic Consistency Boundary (DCB) pattern for event sourcing, as specified by Sara Pellegrini and Bastian Waidelich at dcb.events.

DCBs replace the traditional aggregate with a more flexible consistency model: instead of locking an entire aggregate root, you define the exact subset of events (by type and tags) that a command depends on, and the event store enforces consistency only within that boundary. This eliminates false conflicts and allows independent commands to execute concurrently even when they share a stream.

See the full DCB specification for the pattern definition.

Packages

Package npm Description
@dcb-es/event-store @dcb-es/event-store Core abstractions -- EventStore interface, tags, queries, decision models, in-memory implementation
@dcb-es/event-store-postgres @dcb-es/event-store-postgres Production Postgres adapter with advisory locking, pg_notify subscriptions, and handler infrastructure

Documentation

Getting Started

  • Overview -- The DCB pattern, how this library models it, and high-level architecture
  • Getting Started -- Installation, project setup, running tests, and debugging

Core (@dcb-es/event-store)

  • EventStore Interface -- EventStore, DcbEvent, AppendCondition, AppendConditionError, and the read/append contract
  • Tags and Queries -- Tags, Query, SequencePosition, and how tag-based filtering drives the consistency boundary
  • Decision Models -- EventHandler, EventHandlerWithState, buildDecisionModel, and the pattern for deriving command decisions from event history
  • MemoryEventStore -- In-memory implementation, test utilities, and streamAllEventsToArray

Postgres (@dcb-es/event-store-postgres)

  • Design -- How the Postgres adapter implements DCBs: scoped locking, condition checking, append strategies, notifications
  • PostgresEventStore -- Configuration, schema, append strategies, pg_notify subscriptions, and subscribe()
  • Lock Strategies -- Advisory locks, row-level locks, FNV-1a hashing, and choosing a strategy
  • Event Handling -- runHandler, waitUntilProcessed, bookmark management, and building projections

Reference

  • Examples -- Walkthrough of course-manager-cli and course-manager-cli-with-readmodel
  • Internals -- Implementation details, design decisions, and schema layout

Quick Example

import { buildDecisionModel, EventHandlerWithState, Tags } from "@dcb-es/event-store"
import { PostgresEventStore } from "@dcb-es/event-store-postgres"

const CourseExists = (courseId: string): EventHandlerWithState<any, boolean> => ({
  tagFilter: Tags.fromObj({ courseId }),
  init: false,
  when: { courseWasRegistered: () => true }
})

// Read events, derive state, get an append condition
const { state, appendCondition } = await buildDecisionModel(eventStore, {
  courseExists: CourseExists("cs101")
})

if (state.courseExists) throw new Error("Course already exists")

// Append with the condition -- the store rejects if conflicting events
// were written since the decision was made
await eventStore.append({
  events: { type: "courseWasRegistered", tags: Tags.fromObj({ courseId: "cs101" }), data: { title: "CS 101", capacity: 30 }, metadata: {} },
  condition: appendCondition
})

License

MIT