event-sourcing
Implement event sourcing and CQRS patterns using event stores, aggregates, and projections. Use when building audit trails, temporal queries, or systems requiring full history.
What this skill does
# Event Sourcing
## Table of Contents
- [Overview](#overview)
- [When to Use](#when-to-use)
- [Quick Start](#quick-start)
- [Reference Guides](#reference-guides)
- [Best Practices](#best-practices)
## Overview
Store state changes as a sequence of events rather than the current state, enabling temporal queries, audit trails, and event replay.
## When to Use
- Audit trail requirements
- Temporal queries (state at any point in time)
- Event-driven microservices
- CQRS implementations
- Financial systems
- Complex domain models
- Debugging and analysis
- Compliance and regulation
## Quick Start
Minimal working example:
```typescript
interface DomainEvent {
id: string;
aggregateId: string;
aggregateType: string;
eventType: string;
data: any;
metadata: {
userId?: string;
timestamp: number;
version: number;
};
}
interface Aggregate {
id: string;
version: number;
}
class EventStore {
private events: DomainEvent[] = [];
async appendEvents(
aggregateId: string,
expectedVersion: number,
events: Omit<DomainEvent, "id" | "metadata">[],
// ... (see reference guides for full implementation)
```
## Reference Guides
Detailed implementations in the `references/` directory:
| Guide | Contents |
|---|---|
| [Event Store (TypeScript)](references/event-store-typescript.md) | Event Store (TypeScript) |
| [Projections (Read Models)](references/projections-read-models.md) | Projections (Read Models) |
| [Event Store with PostgreSQL](references/event-store-with-postgresql.md) | Event Store with PostgreSQL |
| [Snapshots for Performance](references/snapshots-for-performance.md) | Snapshots for Performance |
## Best Practices
### ✅ DO
- Store events immutably
- Version your events
- Use optimistic concurrency
- Create snapshots for performance
- Use projections for queries
- Keep events small and focused
- Include metadata (timestamp, user, etc.)
- Handle event versioning/migration
### ❌ DON'T
- Mutate past events
- Store current state only
- Skip concurrency checks
- Query event store for reads
- Make events too large
- Forget about event schema evolution
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