Week 5 Day 4: Event-Driven Architecture - Reacting to Change

In a Monolith, Service A calls function B. In Event-Driven, Service A shouts “Something happened!” and B reacts.

Request-Driven (REST/gRPC)

• Order Svc calls Inventory Svc.
• Inventory Svc calls Shipping Svc.
• Tight Coupling: If Shipping is down, Order fails.

Event-Driven

• Order Svc emits OrderPlaced.
• Inventory Svc consumes OrderPlaced, emits InventoryReserved.
• Shipping Svc consumes InventoryReserved, emits PackageSent.
• Loose Coupling: Services don’t know each other exists.

Choreography vs Orchestration

1. Choreography: Dancers knowing their own steps. (Services react to events independently).
   • Hard to visualize the whole flow.
   • Flexible.
2. Orchestration: A Conductor (Orchestrator Service) tells everyone what to do.
   • “Inventory, reserve items.” -> “Done”.
   • “Shipping, ship items.” -> “Done”.
   • Easier to debug.

Pros & Cons

• Pros: Scalable, Resilient, Easy to add new features.
• Cons: Complexity. Hard to debug “Why didn’t the shipping happen?” (Traceability).

Tomorrow: Mini Project. We build a robust Job Queue system! 👷

Next Step

Next: Mini Project →