cqrs-event-sourcing-architect

license	Apache-2.0
name	cqrs-event-sourcing-architect
description	CQRS pattern, event stores, projections, eventual consistency. Activate on: CQRS, event sourcing, event store, read model, projection, aggregate, domain event, command handler. NOT for: message broker setup (use event-driven-architecture-expert), database optimization (use data-warehouse-optimizer).
allowed-tools	Read,Write,Edit,Bash(npm:*,npx:*,docker:*)
category	Backend & Infrastructure
tags	["cqrs","event-sourcing","ddd","projections","eventual-consistency"]
pairs-with	[{"skill":"event-driven-architecture-expert","reason":"Event sourcing depends on reliable event infrastructure"},{"skill":"distributed-transaction-manager","reason":"Saga patterns complement CQRS for cross-aggregate consistency"},{"skill":"schema-evolution-manager","reason":"Event schema evolution is critical for long-lived event stores"}]

CQRS & Event Sourcing Architect

Design systems that separate read/write models and derive state from immutable event logs using EventStoreDB, Marten, or custom stores.

Activation Triggers

Activate on: "CQRS", "event sourcing", "event store", "read model", "projection", "aggregate", "domain event", "command handler", "EventStoreDB", "Marten"

NOT for: Message broker infrastructure → event-driven-architecture-expert | Database query optimization → data-warehouse-optimizer | API design → api-architect

Quick Start

1. Model aggregates — identify consistency boundaries in your domain
2. Define domain events — past-tense facts: OrderPlaced, ItemShipped
3. Implement command handlers — validate, then emit events
4. Build event store — append-only log (EventStoreDB, PostgreSQL, DynamoDB)
5. Create projections — materialize read models from event streams

Core Capabilities

Domain	Technologies
Event Stores	EventStoreDB 24.x, Marten 7.x, custom PostgreSQL
Frameworks	NestJS CQRS, Axon (JVM), Eventuous, Commanded (Elixir)
Projections	In-memory, PostgreSQL views, Elasticsearch, Redis
Serialization	JSON with versioned schemas, Protobuf
Testing	Given-When-Then event test pattern

Architecture Patterns

CQRS Separation

Command Side                    Query Side
─────────────                   ──────────
POST /orders                    GET /orders/:id
     ↓                               ↑
Command Handler                 Read Model (Postgres/ES)
     ↓                               ↑
Aggregate.apply(event)          Projection Handler
     ↓                               ↑
Event Store (append)  ──────→  Event Subscription

Aggregate with Event Replay

class OrderAggregate {
  private :  |  |  = ;
   : [] = [];
   : [] = [];

  (: ):  {
     (. !== )   ();
    .( ({ : command., : command. }));
  }

   (: ):  {
    .(event);           
    ..(event); 
  }

   (: ):  {
     (event  ) {
      . = ;
      . = event.;
    }
  }

   (: []):  {
     agg =  ();
    events.( agg.(e));
     agg;
  }
}