Spring Cloud gateway send response in filter - spring-boot

Question

I am using spring cloud gateway as edge server.
This is the flow
If request has a header named 'x-foo' then find the header value, get a string from another server and send that string as response instead of actually proxying the request.
Here is code for Filter DSL
#Bean
public RouteLocator routes(RouteLocatorBuilder builder) {
return builder.routes()
.route("foo-filter", r -> r.header('x-foo').and().header("x-intercepted").negate()
.filters(f -> f.filter(fooFilter))
.uri("http://localhost:8081")) // 8081 is self port, there are other proxy related configurations too
.build();
}
Code for Foo filter
#Component
#Slf4j
public class FooFilter implements GatewayFilter {
#Autowired
private ReactiveRedisOperations<String, String> redisOps;
#Value("${header-name}")
private String headerName;
#Override
public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
ServerHttpRequest request = exchange.getRequest();
var foo = request.getHeaders().getFirst(headerName);
return redisOps.opsForHash()
.get("foo:" + foo, "response")
.doOnSuccess(s -> {
log.info("data on success");
log.info(s.toString()); // I am getting proper response here
if (s != null) {
ServerHttpResponse response = exchange.getResponse();
response.setStatusCode(HttpStatus.OK);
response.getHeaders().set("x-intercepted", "true");
byte[] bytes = s.toString().getBytes(StandardCharsets.UTF_8);
DataBuffer buffer = response.bufferFactory().wrap(bytes);
response.writeWith(Mono.just(buffer));
response.setComplete();
}
})
.then(chain.filter(exchange));
}
}
The problem is, the response has the response is getting proper 200 code, the injected header is present on response but the data is not available in response.

Answer 1

This is how I got working.
Use flatMap instead of doOnSuccess
don't use then or switchIfEmpty instead use onErrorResume
Return the response.writeWith
#Component
#Slf4j
public class FooFilter implements GatewayFilter {
#Autowired
private ReactiveRedisOperations<String, String> redisOps;
#Value("${header-name}")
private String headerName;
#Override
public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
ServerHttpRequest request = exchange.getRequest();
var foo = request.getHeaders().getFirst(headerName);
return redisOps.opsForHash()
.get("foo:" + foo, "response")
.flatMap(s -> {
log.info("data on success");
log.info(s.toString()); // I am getting proper response here
if (s != null) {
ServerHttpResponse response = exchange.getResponse();
response.setStatusCode(HttpStatus.OK);
response.getHeaders().set("x-intercepted", "true");
byte[] bytes = s.toString().getBytes(StandardCharsets.UTF_8);
DataBuffer buffer = response.bufferFactory().wrap(bytes);
return response.writeWith(Mono.just(buffer));
}else{ return chain.filter(exchange).then(Mono.fromRunnable(() -> {log.info("It was empty")} }
})
.onErrorResume(chain.filter(exchange));
}
}