エラーハンドリングとフォールバック

Resilient Error Handling Intro

Default Gateway Errors

Customizing Error Responses

Custom Error Handler Example

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.gateway.route.RouteLocator;
import org.springframework.cloud.gateway.route.builder.RouteLocatorBuilder;
import org.springframework.context.annotation.Bean;
import org.springframework.boot.web.reactive.error.ErrorWebExceptionHandler;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.annotation.Order;
import org.springframework.core.io.buffer.DataBuffer;
import org.springframework.http.HttpStatus;
import org.springframework.http.MediaType;
import org.springframework.http.server.reactive.ServerHttpResponse;
import org.springframework.web.server.ServerWebExchange;
import reactor.core.publisher.Mono;
import java.nio.charset.StandardCharsets;

@SpringBootApplication
public class GatewayApplication {
    public static void main(String[] args) {
        SpringApplication.run(GatewayApplication.class, args);
    }

    @Bean
    public RouteLocator routes(RouteLocatorBuilder builder) {
        return builder.routes()
            .route("nonexistent_service", r -> r.path("/fail/**")
                .uri("http://localhost:9999")) // Route to a non-existent service
            .build();
    }
}

@Configuration
@Order(-1) // Ensure this handler runs before default ones
class CustomJsonErrorWebExceptionHandler implements ErrorWebExceptionHandler {

    @Override
    public Mono<Void> handle(ServerWebExchange exchange, Throwable ex) {
        ServerHttpResponse response = exchange.getResponse();
        response.setStatusCode(HttpStatus.INTERNAL_SERVER_ERROR);
        response.getHeaders().setContentType(MediaType.APPLICATION_JSON);

        String errorBody = "{\"status\":500, \"message\":\"Our service is temporarily unavailable. Please try again!\"}";
        DataBuffer buffer = response.bufferFactory().wrap(errorBody.getBytes(StandardCharsets.UTF_8));
        return response.writeWith(Mono.just(buffer));
    }
}

Understanding Fallback Mechanisms

Gateway Fallback Strategies

Implementing a Simple Fallback Route

Configuring a Fallback Route

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.gateway.route.RouteLocator;
import org.springframework.cloud.gateway.route.builder.RouteLocatorBuilder;
import org.springframework.context.annotation.Bean;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@SpringBootApplication
@RestController // For our simple fallback endpoint
public class GatewayApplication {
    public static void main(String[] args) {
        SpringApplication.run(GatewayApplication.class, args);
    }

    @Bean
    public RouteLocator routes(RouteLocatorBuilder builder) {
        return builder.routes()
            .route("backend_service", r -> r.path("/api/**")
                .filters(f -> f.circuitBreaker(config -> config
                    .setName("myCircuitBreaker")
                    .setFallbackUri("forward:/fallback"))) // Fallback URI
                .uri("http://localhost:9000")) // A service that might fail
            .build();
    }

    // This method acts as the fallback service
    @GetMapping("/fallback")
    public String fallback() {
        return "Service is currently unavailable. Please try again later!";
    }
}

Fallback to a Dedicated Service

Check Your Knowledge

Lesson Summary