How to log request and response bodies in Spring WebFlux

I want to have centralised logging for requests and responses in my REST API on Spring WebFlux with Kotlin. So far I've tried this approaches

@Bean
fun apiRouter() = router {
    (accept(MediaType.APPLICATION_JSON) and "/api").nest {
        "/user".nest {
            GET("/", userHandler::listUsers)
            POST("/{userId}", userHandler::updateUser)
        }
    }
}.filter { request, next ->
    logger.info { "Processing request $request with body ${request.bodyToMono<String>()}" }
    next.handle(request).doOnSuccess { logger.info { "Handling with response $it" } }
}

Here request method and path log successfully but the body is Mono, so how should I log it? Should it be the other way around and I have to subscribe on request body Mono and log it in the callback? Another problem is that ServerResponse interface here doesn't have access to the response body. How can I get it here?

---

Another approach I've tried is using WebFilter

@Bean
fun loggingFilter(): WebFilter =
        WebFilter { exchange, chain ->
            val request = exchange.request
            logger.info { "Processing request method=${request.method} path=${request.path.pathWithinApplication()} params=[${request.queryParams}] body=[${request.body}]"  }

            val result = chain.filter(exchange)

            logger.info { "Handling with response ${exchange.response}" }

            return@WebFilter result
        }

Same problem here: request body is Flux and no response body.

Is there a way to access full request and response for logging from some filters? What don't I understand?

This is more or less similar to the situation in Spring MVC.

In Spring MVC, you can use a AbstractRequestLoggingFilter filter and ContentCachingRequestWrapper and/or ContentCachingResponseWrapper. Many tradeoffs here:

• if you'd like to access servlet request attributes, you need to actually read and parse the request body
• logging the request body means buffering the request body, which can use a significant amount of memory
• if you'd like to access the response body, you need to wrap the response and buffer the response body as it's being written, for later retrieval

ContentCaching*Wrapper classes don't exist in WebFlux but you could create similar ones. But keep in mind other points here:

• buffering data in memory somehow goes against the reactive stack, since we're trying there to be very efficient with the available resources
• you should not tamper with the actual flow of data and flush more/less often than expected, otherwise you'd risk breaking streaming uses cases
• at that level, you only have access to DataBuffer instances, which are (roughly) memory-efficient byte arrays. Those belong to buffer pools and are recycled for other exchanges. If those aren't properly retained/released, memory leaks are created (and buffering data for later consumption certainly fits that scenario)
• again at that level, it's only bytes and you don't have access to any codec to parse the HTTP body. I'd forget about buffering the content if it's not human-readable in the first place

Other answers to your question:

• yes, the WebFilter is probably the best approach
• no, you shouldn't subscribe to the request body otherwise you'd consume data that the handler won't be able to read; you can flatMap on the request and buffer data in doOn operators
• wrapping the response should give you access to the response body as it's being written; don't forget about memory leaks, though

I didn't find a good way to log request/response bodies, but if you are just interested in meta data then you can do it like follows.

import org.springframework.http.HttpHeaders
import org.springframework.http.HttpStatus
import org.springframework.http.server.reactive.ServerHttpResponse
import org.springframework.stereotype.Component
import org.springframework.web.server.ServerWebExchange
import org.springframework.web.server.WebFilter
import org.springframework.web.server.WebFilterChain
import reactor.core.publisher.Mono

@Component
class LoggingFilter(val requestLogger: RequestLogger, val requestIdFactory: RequestIdFactory) : WebFilter {
    val logger = logger()

    override fun filter(exchange: ServerWebExchange, chain: WebFilterChain): Mono<Void> {
        logger.info(requestLogger.getRequestMessage(exchange))
        val filter = chain.filter(exchange)
        exchange.response.beforeCommit {
            logger.info(requestLogger.getResponseMessage(exchange))
            Mono.empty()
        }
        return filter
    }
}

@Component
class RequestLogger {

    fun getRequestMessage(exchange: ServerWebExchange): String {
        val request = exchange.request
        val method = request.method
        val path = request.uri.path
        val acceptableMediaTypes = request.headers.accept
        val contentType = request.headers.contentType
        return ">>> $method $path ${HttpHeaders.ACCEPT}: $acceptableMediaTypes ${HttpHeaders.CONTENT_TYPE}: $contentType"
    }

    fun getResponseMessage(exchange: ServerWebExchange): String {
        val request = exchange.request
        val response = exchange.response
        val method = request.method
        val path = request.uri.path
        val statusCode = getStatus(response)
        val contentType = response.headers.contentType
        return "<<< $method $path HTTP${statusCode.value()} ${statusCode.reasonPhrase} ${HttpHeaders.CONTENT_TYPE}: $contentType"
    }

    private fun getStatus(response: ServerHttpResponse): HttpStatus =
        try {
            response.statusCode
        } catch (ex: Exception) {
            HttpStatus.CONTINUE
        }
}

This is what I came up with for java.

public class RequestResponseLoggingFilter implements WebFilter {
    
    @Override
    public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
        ServerHttpRequest httpRequest = exchange.getRequest();
        final String httpUrl = httpRequest.getURI().toString();

        ServerHttpRequestDecorator loggingServerHttpRequestDecorator = new ServerHttpRequestDecorator(exchange.getRequest()) {
            String requestBody = "";

            @Override
            public Flux<DataBuffer> getBody() {
                return super.getBody().doOnNext(dataBuffer -> {
                    try (ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream()) {
                        Channels.newChannel(byteArrayOutputStream).write(dataBuffer.asByteBuffer().asReadOnlyBuffer());
                        requestBody = IOUtils.toString(byteArrayOutputStream.toByteArray(), "UTF-8");
                        commonLogger.info(LogMessage.builder()
                                .step(httpUrl)
                                .message("log incoming http request")
                                .stringPayload(requestBody)
                                .build());
                    } catch (IOException e) {
                        commonLogger.error(LogMessage.builder()
                                .step("log incoming request for " + httpUrl)
                                .message("fail to log incoming http request")
                                .errorType("IO exception")
                                .stringPayload(requestBody)
                                .build(), e);
                    }
                });
            }
        };

        ServerHttpResponseDecorator loggingServerHttpResponseDecorator = new ServerHttpResponseDecorator(exchange.getResponse()) {
            String responseBody = "";
            @Override
            public Mono<Void> writeWith(Publisher<? extends DataBuffer> body) {
                Mono<DataBuffer> buffer = Mono.from(body);
                return super.writeWith(buffer.doOnNext(dataBuffer -> {
                    try (ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream()) {
                        Channels.newChannel(byteArrayOutputStream).write(dataBuffer.asByteBuffer().asReadOnlyBuffer());
                        responseBody = IOUtils.toString(byteArrayOutputStream.toByteArray(), "UTF-8");
                        commonLogger.info(LogMessage.builder()
                                .step("log outgoing response for " + httpUrl)
                                .message("incoming http request")
                                .stringPayload(responseBody)
                                .build());
                    } catch (Exception e) {
                        commonLogger.error(LogMessage.builder()
                                .step("log outgoing response for " + httpUrl)
                                .message("fail to log http response")
                                .errorType("IO exception")
                                .stringPayload(responseBody)
                                .build(), e);
                    }
                }));
            }
        };
        return chain.filter(exchange.mutate().request(loggingServerHttpRequestDecorator).response(loggingServerHttpResponseDecorator).build());
    }

}

Since Spring Boot 2.2.x, Spring Webflux supports Kotlin coroutines. With coroutines, you can have the advantages of non-blocking calls without having to handle Mono and Flux wrapped objects. It adds extensions to ServerRequest and ServerResponse, adding methods like ServerRequest#awaitBody() and ServerResponse.BodyBuilder.bodyValueAndAwait(body: Any). So you could rewrite you code like this:

@Bean
fun apiRouter() = coRouter {
    (accept(MediaType.APPLICATION_JSON) and "/api").nest {
        "/user".nest {
            /* the handler methods now use ServerRequest and ServerResponse directly
             you just need to add suspend before your function declaration:
             suspend fun listUsers(ServerRequest req, ServerResponse res) */ 
            GET("/", userHandler::listUsers)
            POST("/{userId}", userHandler::updateUser)
        }
    }

    // this filter will be applied to all routes built by this coRouter
    filter { request, next ->
      // using non-blocking request.awayBody<T>()
      logger.info("Processing $request with body ${request.awaitBody<String>()}")
        val res = next(request)
        logger.info("Handling with Content-Type ${res.headers().contentType} and status code ${res.rawStatusCode()}")
        res 
    }
}

In order to create a WebFilter Bean with coRoutines, I think you can use this CoroutineWebFilter interface (I haven't tested it, I don't know if it works).

I am pretty new to Spring WebFlux, and I don't know how to do it in Kotlin, but should be the same as in Java using WebFilter:

public class PayloadLoggingWebFilter implements WebFilter {

    public static final ByteArrayOutputStream EMPTY_BYTE_ARRAY_OUTPUT_STREAM = new ByteArrayOutputStream(0);

    private final Logger logger;
    private final boolean encodeBytes;

    public PayloadLoggingWebFilter(Logger logger) {
        this(logger, false);
    }

    public PayloadLoggingWebFilter(Logger logger, boolean encodeBytes) {
        this.logger = logger;
        this.encodeBytes = encodeBytes;
    }

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
        if (logger.isInfoEnabled()) {
            return chain.filter(decorate(exchange));
        } else {
            return chain.filter(exchange);
        }
    }

    private ServerWebExchange decorate(ServerWebExchange exchange) {
        final ServerHttpRequest decorated = new ServerHttpRequestDecorator(exchange.getRequest()) {

            @Override
            public Flux<DataBuffer> getBody() {

                if (logger.isDebugEnabled()) {
                    final ByteArrayOutputStream baos = new ByteArrayOutputStream();
                    return super.getBody().map(dataBuffer -> {
                        try {
                            Channels.newChannel(baos).write(dataBuffer.asByteBuffer().asReadOnlyBuffer());
                        } catch (IOException e) {
                            logger.error("Unable to log input request due to an error", e);
                        }
                        return dataBuffer;
                    }).doOnComplete(() -> flushLog(baos));

                } else {
                    return super.getBody().doOnComplete(() -> flushLog(EMPTY_BYTE_ARRAY_OUTPUT_STREAM));
                }
            }

        };

        return new ServerWebExchangeDecorator(exchange) {

            @Override
            public ServerHttpRequest getRequest() {
                return decorated;
            }

            private void flushLog(ByteArrayOutputStream baos) {
                ServerHttpRequest request = super.getRequest();
                if (logger.isInfoEnabled()) {
                    StringBuffer data = new StringBuffer();
                    data.append('[').append(request.getMethodValue())
                        .append("] '").append(String.valueOf(request.getURI()))
                        .append("' from ")
                            .append(
                                Optional.ofNullable(request.getRemoteAddress())
                                            .map(addr -> addr.getHostString())
                                        .orElse("null")
                            );
                    if (logger.isDebugEnabled()) {
                        data.append(" with payload [\n");
                        if (encodeBytes) {
                            data.append(new HexBinaryAdapter().marshal(baos.toByteArray()));
                        } else {
                            data.append(baos.toString());
                        }
                        data.append("\n]");
                        logger.debug(data.toString());
                    } else {
                        logger.info(data.toString());
                    }

                }
            }
        };
    }

}

Here some tests on this: github

I think this is what Brian Clozel (@brian-clozel) meant.

You can actually enable DEBUG logging for Netty and Reactor-Netty related to see full picture of what's happening. You could play with the below and see what you want and don't. That was the best I could.

reactor.ipc.netty.channel.ChannelOperationsHandler: DEBUG
reactor.ipc.netty.http.server.HttpServer: DEBUG
reactor.ipc.netty.http.client: DEBUG
io.reactivex.netty.protocol.http.client: DEBUG
io.netty.handler: DEBUG
io.netty.handler.proxy.HttpProxyHandler: DEBUG
io.netty.handler.proxy.ProxyHandler: DEBUG
org.springframework.web.reactive.function.client: DEBUG
reactor.ipc.netty.channel: DEBUG

Here is the GitHub Repo with complete implementation to log both request and response body along with http headers for webflux/java based application...

Assuming we are dealing with a simple JSON or XML response, if debug level for corresponding loggers is not sufficient for some reason, one can use string representation before transforming it to object:

Mono<Response> mono = WebClient.create()
                               .post()
                               .body(Mono.just(request), Request.class)
                               .retrieve()
                               .bodyToMono(String.class)
                               .doOnNext(this::sideEffectWithResponseAsString)
                               .map(this::transformToResponse);

the following are the side-effect and transformation methods:

private void sideEffectWithResponseAsString(String response) { ... }
private Response transformToResponse(String response) { /*use Jackson or JAXB*/ }    

What Brian said. In addition, logging request/response bodies don't make sense for reactive streaming. If you imagine the data flowing through a pipe as a stream, you don't have the full content at any time unless you buffer it, which defeats the whole point. For small request/response, you can get away with buffering, but then why use the reactive model (other than to impress your coworkers :-) )?

The only reason for logging request/response that I could conjure up is debugging, but with the reactive programming model, debugging method has to be modified too. Project Reactor doc has an excellent section on debugging that you can refer to: http://projectreactor.io/docs/core/snapshot/reference/#debugging

If your using controller instead of handler best way is aop with annotating you controller class with @Log annotation.And FYI this takes plain json object as request not mono.

@Target(AnnotationTarget.FUNCTION)
@Retention(AnnotationRetention.RUNTIME)
annotation class Log

@Aspect
@Component
class LogAspect {
    companion object {
        val log = KLogging().logger
    }

    @Around("@annotation(Log)")
    @Throws(Throwable::class)
    fun logAround(joinPoint: ProceedingJoinPoint): Any? {
        val start = System.currentTimeMillis()
        val result = joinPoint.proceed()
        return if (result is Mono<*>) result.doOnSuccess(getConsumer(joinPoint, start)) else result
    }

    fun getConsumer(joinPoint: ProceedingJoinPoint, start: Long): Consumer<Any>? {
        return Consumer {
            var response = ""
            if (Objects.nonNull(it)) response = it.toString()
            log.info(
                "Enter: {}.{}() with argument[s] = {}",
                joinPoint.signature.declaringTypeName, joinPoint.signature.name,
                joinPoint.args
            )
            log.info(
                "Exit: {}.{}() had arguments = {}, with result = {}, Execution time = {} ms",
                joinPoint.signature.declaringTypeName, joinPoint.signature.name,
                joinPoint.args[0],
                response, System.currentTimeMillis() - start
            )
        }
    }
}

I think the appropriate thing to do here is to write the contents of each request to a file in an asynchronous manner (java.nio) and set up an interval that reads those request body files asynchrolusly and writes them to the log in a memory usage aware manner (atleast one file at a time but up too 100 mb at a time) and after logging them removes the files from disk.