Spring MVC và Spring WebFlux khác nhau thế nào? Khi nào chọn cái nào?

// MVC
@GetMapping("/user/{id}")
User get(@PathVariable Long id) { return repo.findById(id).orElseThrow(); }

// WebFlux
@GetMapping("/user/{id}")
Mono<User> get(@PathVariable Long id) { return repo.findById(id); }

Khi chọn WebFlux:
- High-concurrency I/O-bound (chat, streaming, SSE).
- Microservice gateway forward nhiều request đồng thời.
- Backpressure cần thiết (stream dữ liệu lớn).

Khi giữ MVC:
- CPU-bound task (WebFlux không giúp).
- Dùng JPA/Hibernate (chưa có reactive equivalent).
- Team chưa quen reactive — debug Mono/Flux khó.

2026 quan trọng: Virtual Threads (Java 21) cho phép Spring MVC đạt concurrency tương đương WebFlux mà giữ code đồng bộ → nhiều team quay về MVC. Bật bằng spring.threads.virtual.enabled=true.

// MVC
@GetMapping("/user/{id}")
User get(@PathVariable Long id) { return repo.findById(id).orElseThrow(); }

// WebFlux
@GetMapping("/user/{id}")
Mono<User> get(@PathVariable Long id) { return repo.findById(id); }

Choose WebFlux when:
- High-concurrency I/O-bound (chat, streaming, SSE).
- Microservice gateways forwarding many requests concurrently.
- You need backpressure (streaming large data).

Stick with MVC when:
- CPU-bound tasks (WebFlux does not help).
- You use JPA/Hibernate (no mature reactive equivalent).
- The team is unfamiliar with reactive — debugging Mono/Flux is hard.

Important in 2026: Virtual Threads (Java 21) let Spring MVC reach WebFlux-like concurrency while keeping synchronous code → many teams return to MVC. Enable with spring.threads.virtual.enabled=true.