WebFlux Reactive Patterns

Use WebFlux when the complete request path can benefit from non-blocking I/O.

Pipeline rules

• Return Mono or Flux; do not call subscribe() in request-handling code.
• Compose work with map, flatMap, concatMap, and zip according to ordering requirements.
• Keep blocking JDBC, filesystem, and legacy SDK calls out of event-loop threads.
• Isolate unavoidable blocking calls on boundedElastic at the narrow adapter boundary.
• Apply timeouts at remote-call boundaries and preserve cancellation.

Context and errors

• Put request-scoped metadata in Reactor Context, not ThreadLocal.
• Translate domain failures centrally without swallowing cancellation or infrastructure errors.
• Use onErrorResume only when a defined fallback exists.
• Avoid onErrorContinue; it makes partial processing hard to reason about.

Persistence and streaming

• Use R2DBC for reactive database access; JDBC makes the pipeline blocking.
• Bound concurrency for fan-out operations.
• Define backpressure and buffering limits for streaming endpoints.
• Avoid collecting an unbounded Flux into memory.

Testing

• Use StepVerifier for publisher behavior and WebTestClient for HTTP contracts.
• Test cancellation, timeout, empty results, errors, ordering, and backpressure.
• Enable blocking-call detection in tests when the project supports it.

Examples

• See examples/good-reactive-service.java and examples/bad-reactive-service.java.

Gotchas

• Agent calls block() in a controller or service - keep the full request path non-blocking.
• Agent calls subscribe() manually - the web runtime owns subscription.
• Agent stores tenant or security data in ThreadLocal - use Reactor Context.
• Agent uses unbounded flatMap - set concurrency according to downstream capacity.
• Agent wraps JDBC in Mono.just - that still blocks the caller thread.