Event-Driven Architecture

Opening Scenario

You're building an e-commerce platform. When a user places an order, you need to: deduct inventory, charge the payment method, send a confirmation email, notify the warehouse, and update the analytics dashboard. Your first implementation puts all of this in a single placeOrder() function that calls each service sequentially. It works — until the email service goes down for 10 minutes and every order fails. Then the warehouse integration is slow and order latency jumps to 8 seconds. Then marketing wants to add loyalty point accrual and you have to deploy the order service again.

Every new requirement makes the order service more fragile and more coupled.

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Question Progression

1. What's the fundamental coupling problem?

Question: When placeOrder() calls sendEmail() directly, what happens if sendEmail() is slow or unavailable?

Reasoning: In a direct call model, the caller waits for the callee to respond. The order service's success is bound…