Building a Full Reactive Stack Backend with Spring Cloud, WebFlux, and Reactive MongoDB

Spring Cloud traditionally uses synchronous calls (RestTemplate or Feign) for inter‑service communication; with Spring Cloud Finchley you can switch to non‑blocking, reactive calls.

The example builds a reactive microservice system from scratch using WebFlux and Spring Data Reactive. It requires a Eureka service registry, two microservices (account and customer), and a MongoDB instance running in Docker.

docker run -d --name mongo -p 27017:27017 mongo

**Eureka Server**: create a Spring Boot project, add spring-cloud-starter-netflix-eureka-server dependency, configure application.yml (port 8000, service‑url), annotate the main class with @EnableEurekaServer , and start the server.

**Account Service** (cloud‑account): add dependencies for Reactive Web, Reactive MongoDB, Eureka client, and Lombok. Define the Account entity with Lombok annotations, a reactive repository extending ReactiveCrudRepository , and an AccountController exposing /account/customer/{customer} that returns a Flux<Account> and prints a timestamp.

**Customer Service** (cloud‑customer): same dependencies as the account service. Define a Customer entity, a CustomerMongoReactiveRepository , and a CustomerController with standard CRUD endpoints. Add a WebClient.Builder bean annotated with @LoadBalanced for reactive calls to the account service.

**Inter‑service communication**: configure a load‑balanced WebClient in WebClientConfig . Show the correct bean definition ( WebClient.Builder ) and an incorrect one ( WebClient without load‑balancing). Implement GET /customer/{id}/account in CustomerController that calls http://cloud‑account/account/customer/{id} via the load‑balanced WebClient, returning a Flux<Account> .

**Testing**: start two instances of the account service, send repeated requests to http://localhost:8200/customer/5ae15fa640f1687f200d8941/account , and observe the timestamps alternating between the two instances, confirming round‑robin load balancing.

**Conclusion**: the guide demonstrates a full reactive stack backend—service registration, reactive service calls, and reactive data access—showing how reactive programming can be applied throughout a Spring‑based microservice architecture.