Problem Root Cause

Spring Cloud Gateway relies on a fully reactive, non-blocking thread pool. Attempting to use standard OpenFeign— which executes synchronous, blocking I/O calls—triggers an IllegalStateException with a message indicating block()/blockFirst()/blockLast() operations are disallowed on reactor-http-nio threads.

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Alternative Blocking Load Balancer Override Fix

This approach replaces the default BlockingLoadBalancerClient to shift load-balancing blocking work off the reactive thread pool.

Step 1: Implement Custom Reactive-Friendly Blocking Load Balancer

Create a subclass wrapping reactive load balancer selection logic in a background thread pool exectuor:

import org.springframework.cloud.client.ServiceInstance;
import org.springframework.cloud.client.loadbalancer.LoadBalancerProperties;
import org.springframework.cloud.client.loadbalancer.Request;
import org.springframework.cloud.client.loadbalancer.Response;
import org.springframework.cloud.client.loadbalancer.reactive.ReactiveLoadBalancer;
import org.springframework.cloud.loadbalancer.blocking.client.BlockingLoadBalancerClient;
import org.springframework.cloud.loadbalancer.support.LoadBalancerClientFactory;
import reactor.core.publisher.Mono;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class OffloadBlockingLoadBalancerClient extends BlockingLoadBalancerClient {
    private final ReactiveLoadBalancer.Factory<ServiceInstance> reactiveFactory;
    private final ExecutorService backgroundPool;

    public OffloadBlockingLoadBalancerClient(LoadBalancerClientFactory factory, LoadBalancerProperties properties) {
        super(factory, properties);
        this.reactiveFactory = factory;
        this.backgroundPool = Executors.newFixedThreadPool(4);
    }

    @Override
    public <T> Response<ServiceInstance> choose(String serviceId, Request<T> request) {
        CompletableFuture<Response<ServiceInstance>> future = CompletableFuture.supplyAsync(
            () -> Mono.from(reactiveFactory.getInstance(serviceId).choose(request)).block(),
            backgroundPool
        );
        try {
            return future.join();
        } catch (Exception e) {
            return super.choose(serviceId, request);
        }
    }
}

Step 2: Register Custom Bean

Add this configuration class to your gateway project to override the default client:

import org.springframework.cloud.client.loadbalancer.LoadBalancerProperties;
import org.springframework.cloud.loadbalancer.support.LoadBalancerClientFactory;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;

@Configuration
public class FeignBlockingWorkaroundConfig {

    @Bean
    @Primary
    public OffloadBlockingLoadBalancerClient offloadLoadBalancerClient(
            LoadBalancerClientFactory factory, LoadBalancerProperties properties) {
        return new OffloadBlockingLoadBalancerClient(factory, properties);
    }
}