# Webflux Reactive Patterns

> Use when the complete Spring Boot 3 request path is reactive: WebFlux controllers, Reactor pipelines, R2DBC persistence, streaming, backpressure, or cancellation. For defining and registering declarative outbound API interfaces, use http-interface-clients.

- Skill: `rrezartprebreza/webflux-reactive-patterns` (Agent Skill, multi-file: 4 files)
- Install (CLI): `npx skillmds@latest add rrezartprebreza/webflux-reactive-patterns`
- Raw SKILL.md: https://api.skillmd.com/api/skills/rrezartprebreza/webflux-reactive-patterns/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Integrations & APIs
- Author: rrezartprebreza (https://skillmd.com/u/rrezartprebreza)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/rrezartprebreza/webflux-reactive-patterns

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# 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.