Why Three‑Tier Architecture Fails and How Microservices Solve Scaling Challenges

Three‑Tier Architecture

Since university, we have been taught three‑tier architecture, which works well for small projects with simple business logic and enables rapid iteration. As projects grow, the monolithic three‑tier design becomes high‑cohesion, high‑coupling, making maintenance difficult due to widespread references, package and namespace conflicts.

Microservice Architecture

To reduce business complexity, services are split into microservices. This also eases team management, as each microservice team can operate independently.

Microservice granularity can be fine‑grained or coarse‑grained. Fine‑grained services often forbid cross‑database access, forcing the business logic layer to assemble many RPC/REST calls, which hinders fast iteration. Coarse‑grained services allow cross‑database access, resulting in high cohesion and low coupling, enabling teams to develop independently.

Microservices also improve resource utilization and lower costs.

High Concurrency

When concurrency increases, optimization follows the same pattern for both three‑tier and microservice systems: read/write separation, caching, and sharding (database partitioning). The diagram shows using middleware such as Sharding‑JDBC, Macat, Atlas to achieve sharding.

Refactoring

For a project that may face high concurrency, a possible refactoring approach is illustrated: two packages (Product and Membership) where ProductController depends on Membership's UserService. By prohibiting cross‑package dependencies, each package can be packaged as an independent JAR, facilitating migration from three‑tier to the microservice‑style architecture without affecting future sharding.

If business complexity grows further, a coarse‑grained architecture may encounter issues such as difficulty tracking inventory changes across services. In such cases, reverting to a fine‑grained design with domain experts defining flexible interfaces may be necessary.