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Event Sourcing
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.
SKILL.md
Full skill instructions
Event Sourcing
Table of Contents
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:
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) | Event Store (TypeScript) |
| Projections (Read Models) | Projections (Read Models) |
| Event Store with PostgreSQL | Event Store with PostgreSQL |
| Snapshots for Performance | 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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