CorrelationIdFilter.java

package com.checkout.paymentgw.gateway.filter;

import org.slf4j.MDC;
import org.springframework.cloud.gateway.filter.GlobalFilter;
import org.springframework.core.Ordered;
import org.springframework.http.server.reactive.ServerHttpRequest;
import org.springframework.stereotype.Component;
import reactor.core.publisher.Mono;

import java.util.UUID;

/**
 * Inject (or echo back) an {@code X-Correlation-Id} header on every request and add
 * it to the MDC so downstream logs can be tied together.
 *
 * <p>Real cross-service correlation uses W3C Trace Context ({@code traceparent}),
 * but {@code X-Correlation-Id} survives where tracing is not yet wired and works
 * with merchants' own logs.
 */
@Component
public class CorrelationIdFilter implements GlobalFilter, Ordered {

    public static final String HEADER = "X-Correlation-Id";

    @Override
    public Mono<Void> filter(org.springframework.web.server.ServerWebExchange exchange,
                             org.springframework.cloud.gateway.filter.GatewayFilterChain chain) {
        ServerHttpRequest req = exchange.getRequest();
        String cid = req.getHeaders().getFirst(HEADER);
        if (cid == null || cid.isBlank()) {
            cid = UUID.randomUUID().toString();
        }

        ServerHttpRequest mutated = req.mutate().header(HEADER, cid).build();
        MDC.put("correlationId", cid);
        try {
            return chain.filter(exchange.mutate().request(mutated).build());
        } finally {
            MDC.remove("correlationId");
        }
    }

    @Override
    public int getOrder() {
        return Ordered.HIGHEST_PRECEDENCE;
    }
}