Event-driven architecture (EDA) decouples systems through asynchronous events. Instead of direct calls between services, components publish and subscribe to events. This creates flexible, scalable systems that handle change gracefully.
Core Concepts
Events vs Commands vs Queries
Event:
- Something that happened (past tense)
- "OrderPlaced", "UserRegistered", "PaymentProcessed"
- Immutable fact
- Multiple consumers possible
Command:
- Request to do something (imperative)
- "PlaceOrder", "RegisterUser", "ProcessPayment"
- Single handler
- May be rejected
Query:
- Request for information
- "GetOrder", "FindUser", "ListPayments"
- Returns data
- No side effects
Event Structure
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Event Patterns
Publish-Subscribe
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Event Sourcing
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CQRS (Command Query Responsibility Segregation)
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Message Brokers
Redis Streams
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RabbitMQ
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Error Handling
Dead Letter Queues
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Idempotency
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Testing Event-Driven Systems
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Best Practices
Event Design
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Event Versioning
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Conclusion
Event-driven architecture enables scalable, loosely-coupled systems. Events capture what happened, subscribers react independently, and the system evolves without tight dependencies.
Start simple—basic pub/sub covers most needs. Add event sourcing and CQRS when you need audit trails or complex read models. The key is designing meaningful events that capture business reality.