Which I/O contract fits the execution model?

Goal & mental model verified

MVC supports an imperative request model; WebFlux uses non-blocking composition and reactive backpressure. Mono represents zero-or-one values and Flux a sequence. Virtual threads provide a separate way to scale blocking waits.

Worked example · design exercise synthesis

A JPA quote API fits MVC naturally. A streaming aggregator with non-blocking downstream clients may fit WebFlux. Wrapping a blocking JPA call in Mono does not make its I/O non-blocking.

Engineering decision synthesis

Choose based on dependency APIs, concurrency pattern and team expertise. MVC with virtual threads can retain readable blocking code; WebFlux adds composition and scheduling discipline.

Pitfall & diagnosis synthesis

Blocking an event loop can stall unrelated requests. A reactive return type alone does not identify the server stack, and virtual threads do not remove database limits.

Improve & validate synthesis

Prototype the same realistic workload with bounded dependencies. Compare memory, tail latency and failure handling; adopt complexity only for a measured benefit.