Expert guidance for Project Reactor (reactor-core 3.6+, BOM 2024.0+) on the JVM. Triggers when the user writes, reviews, or debugs reactive code involving Mono, Flux, Publisher, Subscriber, Sinks, or reactive pipelines: operator selection (flatMap, concatMap, switchMap, switchIfEmpty, handle, expand, zip, merge, cache, share, replay), Schedulers and threading (publishOn, subscribeOn, boundedElastic, parallel), backpressure (onBackpressureBuffer/Drop/Latest, limitRate, prefetch), Reactor Context and tracing/MDC/security propagation, blocking-to-reactive bridging (fromCallable, fromFuture, BlockHound), Kotlin coroutine/Flow interop, Spring WebFlux, R2DBC, reactor-netty WebClient, StepVerifier/TestPublisher/PublisherProbe/virtual time, debugging (checkpoint, log, Hooks.onOperatorDebug, ReactorDebugAgent), and choosing between Reactor and JDK 21 virtual threads. Does not apply to non-reactive Spring MVC or plain JDBC/imperative code unless they bridge into a reactive pipeline.
2026-05-18