Event-Driven Architecture Core Knowledge

Full Reference: See advanced.md for Saga implementations (Node.js, Java), Outbox pattern implementations, Event Sourcing, CQRS, and idempotency patterns.

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: event-driven for comprehensive documentation.

When NOT to Use This Skill

• Monolithic applications - Use simple in-process events

• Simple CRUD operations - Use REST APIs or GraphQL

• Real-time UI updates - Use WebSockets or SSE

• Synchronous workflows - Use traditional transactions

Architecture Patterns Overview

Pattern Purpose Complexity

Pub/Sub Decouple producers from consumers Low

Event Sourcing Store state as event sequence High

CQRS Separate read/write models Medium-High

Saga Distributed transactions High

Outbox Reliable event publishing Medium

Saga Pattern

Manages distributed transactions across services without 2PC.

Choreography Saga

┌─────────┐ event ┌─────────┐ event ┌─────────┐ │Order Svc│────────────▶│Payment │────────────▶│Inventory│ └─────────┘ │ Svc │ │ Svc │ ▲ └─────────┘ └─────────┘ │ compensate │ compensate │ └───────────────────────┴───────────────────────┘

Orchestration Saga

                ┌─────────────┐
                │    Saga     │
                │Orchestrator │
                └─────────────┘
                /      |      \
               ▼       ▼       ▼
        ┌───────┐ ┌───────┐ ┌───────┐
        │Order  │ │Payment│ │Invent.│
        └───────┘ └───────┘ └───────┘

Transactional Outbox Pattern

Ensures reliable event publishing with database transactions.

┌────────────────────────────────────────────┐ │ Database Transaction │ │ ┌──────────────┐ ┌──────────────────┐ │ │ │ Business │ │ Outbox Table │ │ │ │ Table │ │ (messages) │ │ │ └──────────────┘ └──────────────────┘ │ └────────────────────────────────────────────┘ │ Polling Relay / CDC │ ┌─────────────────┐ │ Message Broker │ └─────────────────┘

Outbox Table Schema

CREATE TABLE outbox ( id UUID PRIMARY KEY, aggregate_type VARCHAR(255) NOT NULL, aggregate_id VARCHAR(255) NOT NULL, event_type VARCHAR(255) NOT NULL, payload JSONB NOT NULL, status VARCHAR(50) DEFAULT 'PENDING', created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP );

CQRS Architecture

    Commands                          Queries
        │                                 │
        ▼                                 ▼
┌───────────────┐                ┌───────────────┐
│ Command Model │                │  Query Model  │
│   (Write)     │                │   (Read)      │
└───────────────┘                └───────────────┘
        │                                 ▲
        ▼                                 │
┌───────────────┐     Events     ┌───────────────┐
│  Write Store  │───────────────▶│  Read Store   │
└───────────────┘                └───────────────┘

Anti-Patterns

Anti-Pattern Why It's Bad Solution

Event Soup Too many fine-grained events Design coarse-grained domain events

Missing Idempotency Duplicate processing Add idempotency keys

No Compensation Logic Failed saga can't rollback Implement compensating transactions

No Dead Letter Queue Failed events lost Configure DLQ for error handling

Weak Event Ordering Race conditions Use partitioning or ordered queues

Quick Troubleshooting

Issue Diagnostic Solution

Lost events Check message broker Implement Outbox pattern

Duplicate processing Logs show multiple executions Add idempotency checks

Saga stuck Compensation not triggered Add timeout handling

Growing DLQ Many failed messages Analyze failures, fix consumers

Slow event processing High message lag Scale consumers, optimize handlers

Best Practices

Message Design

• Include correlation ID for tracing

• Version your events for evolution

• Keep payloads small, reference large data

Error Handling

• Implement retry with exponential backoff

• Use Dead Letter Queues for failed messages

• Set up alerting on DLQ growth

Monitoring

• Track message lag

• Monitor consumer group health

• Alert on processing errors

• Trace message flow across services

Reference Documentation

Available topics: patterns , saga , outbox , cqrs