Case Study: Monolith to Cloud-Native

The most common enterprise task is not building new apps, but Modernizing old ones. Migrating a 10-year-old Monolith to Microservices without downtime is the ultimate test of a Cloud Architect.

1. The "Strangler Fig" Pattern

Don't try a "Big Bang" migration. It always fails. Instead, use the Strangler Fig Pattern:

1. Put an **API Gateway** in front of the Monolith.
2. Take ONE small feature (e.g., Notifications) and build it as a new Microservice.
3. Route all new notification traffic to the Microservice at the Gateway.
4. Repeat until the Monolith is reduced to a tiny shell and can be turned off.

2. Handling Data Consistency

Moving from a single SQL DB to many Microservice DBs is hard. We use **Event Sourcing**. The Monolith publishes an event (e.g., `UserCreated`) to a Message Queue, and the new Microservices consume that event to stay in sync during the transition period.

4. Interview Mastery

Q: "How do you handle 'Shared Libraries' across microservices?"

Architect Answer: "Avoid them if possible. Shared libraries create **Deployment Coupling**—if you update the library, you have to redeploy 50 services. We prefer 'Duplication over Coupling.' If a tiny bit of logic is needed in two places, we copy it. If it's a massive, complex piece of logic, we turn it into its own **Service** and call it via gRPC. Microservices are about independence, and shared libraries are the enemy of independence."