物联网mqtt网关搭建背后的技术原理

前言

物联网是现在比较热门的软件领域，众多物联网厂商都有自己的物联网平台，而物联网平台其中一个核心的模块就是Mqtt网关。这篇文章的目的是手把手教大家写书写一个mqtt网关，后端存储支持Kafka/Pulsar，支持mqtt 连接、断链、发送消息、订阅消息。技术选型:

  • Netty java最流行的网络框架
  • netty-codec-mqtt netty的子项目，mqtt编解码插件
  • Pulsar/Kafka 流行的消息中间件作为后端存储

核心pom依赖如下

        <dependency><groupId>io.netty</groupId><artifactId>netty-codec-mqtt</artifactId></dependency><dependency><groupId>io.netty</groupId><artifactId>netty-common</artifactId></dependency><dependency><groupId>io.netty</groupId><artifactId>netty-transport</artifactId></dependency><dependency><groupId>org.apache.pulsar</groupId><artifactId>pulsar-client-original</artifactId><version>${pulsar.version}</version></dependency><dependency><groupId>org.apache.kafka</groupId><artifactId>kafka-clients</artifactId><version>${kafka.version}</version></dependency><dependency><groupId>org.eclipse.paho</groupId><artifactId>org.eclipse.paho.client.mqttv3</artifactId><version>${mqtt-client.version}</version><scope>test</scope></dependency>

软件参数设计

软件参数可谓是非常常见，复杂的开源项目，参数甚至可以达到上百个、配置文件长达数千行。我们需要的配置有

MqttServer监听的端口

监听端口的配置即使是写demo也非常必要，常常用在单元测试中，由于单元测试跑完之后，即使网络服务器关闭，操作系统也不会立即释放端口，所以单元测试的时候指定随机端口非常关键，在java中，我们可以通过这样的工具类来获取一个空闲的端口。未配置的话，我们就使用mqtt的默认端口1883。

package io.github.protocol.mqtt.broker.util;import java.io.IOException;
import java.io.UncheckedIOException;
import java.net.ServerSocket;public class SocketUtil {public static int getFreePort() {try (ServerSocket serverSocket = new ServerSocket(0)) {return serverSocket.getLocalPort();} catch (IOException e) {throw new UncheckedIOException(e);}}}

后端存储配置

我们的mqtt网关是没有可靠的存储能力的，依赖后端的消息中间件来做持久化处理。后端规划支持Pulsar、Kafka两种类型。定义枚举类如下

public enum ProcessorType {KAFKA,PULSAR,
}

对应的KafkaProcessorConfig、PulsarProcessorConfig比较简单，包含基础的连接地址即可，如果后续要做性能调优、安全，这块还是会有更多的配置项

@Setter
@Getter
public class KafkaProcessorConfig {private String bootstrapServers = "localhost:9092";public KafkaProcessorConfig() {}
}

@Setter
@Getter
public class PulsarProcessorConfig {private String httpUrl = "http://localhost:8080";private String serviceUrl = "pulsar://localhost:6650";public PulsarProcessorConfig() {}
}

启动netty MqttServer

我们通过netty启动一个mqttServer，添加mqtt解码器

package io.github.protocol.mqtt.broker;import io.github.protocol.mqtt.broker.processor.KafkaProcessor;
import io.github.protocol.mqtt.broker.processor.KafkaProcessorConfig;
import io.github.protocol.mqtt.broker.processor.MqttProcessor;
import io.github.protocol.mqtt.broker.processor.PulsarProcessor;
import io.github.protocol.mqtt.broker.processor.PulsarProcessorConfig;
import io.github.protocol.mqtt.broker.util.SocketUtil;
import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import io.netty.handler.codec.mqtt.MqttDecoder;
import io.netty.handler.codec.mqtt.MqttEncoder;
import io.netty.handler.logging.LogLevel;
import io.netty.handler.logging.LoggingHandler;
import lombok.extern.slf4j.Slf4j;@Slf4j
public class MqttServer {private final MqttServerConfig mqttServerConfig;public MqttServer() {this(new MqttServerConfig());}public MqttServer(MqttServerConfig mqttServerConfig) {this.mqttServerConfig = mqttServerConfig;if (mqttServerConfig.getPort() == 0) {mqttServerConfig.setPort(SocketUtil.getFreePort());}}public void start() throws Exception {EventLoopGroup bossGroup = new NioEventLoopGroup(1);EventLoopGroup workerGroup = new NioEventLoopGroup();try {ServerBootstrap b = new ServerBootstrap();b.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class).option(ChannelOption.SO_BACKLOG, 100).handler(new LoggingHandler(LogLevel.INFO)).childHandler(new ChannelInitializer<SocketChannel>() {@Overridepublic void initChannel(SocketChannel ch) throws Exception {ChannelPipeline p = ch.pipeline();// decoderp.addLast(new MqttDecoder());p.addLast(MqttEncoder.INSTANCE);}});// Start the server.ChannelFuture f = b.bind(mqttServerConfig.getPort()).sync();// Wait until the server socket is closed.f.channel().closeFuture().sync();} finally {// Shut down all event loops to terminate all threads.bossGroup.shutdownGracefully();workerGroup.shutdownGracefully();}}private MqttProcessor processor(MqttServerConfig config) {return switch (config.getProcessorType()) {case KAFKA -> new KafkaProcessor(config.getMqttAuth(), config.getKafkaProcessorConfig());case PULSAR -> new PulsarProcessor(config.getMqttAuth(), config.getPulsarProcessorConfig());};}public int getPort() {return mqttServerConfig.getPort();}}

MqttserverStarter.java

我们写一个简单的main函数用来启动mqttServer，方便调测

package io.github.protocol.mqtt.broker;public class MqttServerStarter {public static void main(String[] args) throws Exception {new MqttServer().start();}}

客户端使用eclipse mqtt client进行测试

package io.github.protocol.mqtt;import lombok.extern.log4j.Log4j2;
import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttConnectOptions;
import org.eclipse.paho.client.mqttv3.MqttException;
import org.eclipse.paho.client.mqttv3.MqttMessage;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;@Log4j2
public class MqttClientPublishExample {public static void main(String[] args) throws Exception {String topic = "MQTT Examples";String content = "Message from MqttPublishExample";int qos = 2;String broker = "tcp://127.0.0.1:1883";String clientId = "JavaSample";MemoryPersistence persistence = new MemoryPersistence();try {MqttClient sampleClient = new MqttClient(broker, clientId, persistence);MqttConnectOptions connOpts = new MqttConnectOptions();connOpts.setCleanSession(true);log.info("Connecting to broker: {}", broker);sampleClient.connect(connOpts);log.info("Connected");log.info("Publishing message: {}", content);MqttMessage message = new MqttMessage(content.getBytes());message.setQos(qos);sampleClient.publish(topic, message);log.info("Message published");sampleClient.disconnect();log.info("Disconnected");System.exit(0);} catch (MqttException me) {log.error("reason {} msg {}", me.getReasonCode(), me.getMessage(), me);}}}

然后我们先运行MqttServer，再运行MqttClient，发现MqttClient卡住了

Connecting to broker: tcp://127.0.0.1:1883

这是为什么呢，我们通过抓包发现仅仅只有客户端发送了Mqtt connect信息，服务端并没有响应

image.png

但是根据mqtt标准协议，发送Connect消息，必须要有ConnAck响应

image.png

所以我们需要在接收到Connect后，返回connAck消息。我们创建一个MqttHandler，让他继承ChannelInboundHandlerAdapter, 用来接力MqttDecoder解码完成后的消息，这里要重点继承其中的channelRead方法，以及channelInactive方法，用来释放断链时需要释放的资源

package com.github.shoothzj.mqtt;import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import lombok.extern.slf4j.Slf4j;@Slf4j
public class MqttHandler extends ChannelInboundHandlerAdapter {@Overridepublic void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {super.channelRead(ctx, msg);}}

然后把这个handler加入到netty的职责链中，放到解码器的后面

image.png

在mqtt handler中插入我们的代码

    @Overridepublic void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {super.channelRead(ctx, msg);if (msg instanceof MqttConnectMessage) {handleConnect(ctx, (MqttConnectMessage) msg);} else {log.error("Unsupported type msg [{}]", msg);}}private void handleConnect(ChannelHandlerContext ctx, MqttConnectMessage connectMessage) {log.info("connect msg is [{}]", connectMessage);}

打印出connectMessage如下

[MqttConnectMessage[fixedHeader=MqttFixedHeader[messageType=CONNECT, isDup=false, qosLevel=AT_MOST_ONCE, isRetain=false, remainingLength=22], variableHeader=MqttConnectVariableHeader[name=MQTT, version=4, hasUserName=false, hasPassword=false, isWillRetain=false, isWillFlag=false, isCleanSession=true, keepAliveTimeSeconds=60], payload=MqttConnectPayload[clientIdentifier=JavaSample, willTopic=null, willMessage=null, userName=null, password=null]]]

通常，mqtt connect message中会包含qos、用户名、密码等信息，由于我们启动客户端的时候也没有携带用户名和密码，这里获取到的都为null，我们先不校验这些消息，直接给客户端返回connack消息，代表连接成功

        final MqttConnAckMessage ackMessage = MqttMessageBuilders.connAck().returnCode(CONNECTION_ACCEPTED).build();ctx.channel().writeAndFlush(ackMessage);

我们再运行起Server和Client，随后可以看到已经走过了Connect阶段，进入了publish message过程，接下来我们再实现更多的其他场景

image-20201218204302720

附上此阶段的MqttHandler代码

package com.github.shoothzj.mqtt;import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import io.netty.handler.codec.mqtt.MqttConnAckMessage;
import io.netty.handler.codec.mqtt.MqttConnectMessage;
import io.netty.handler.codec.mqtt.MqttConnectPayload;
import io.netty.handler.codec.mqtt.MqttConnectVariableHeader;
import io.netty.handler.codec.mqtt.MqttFixedHeader;
import io.netty.handler.codec.mqtt.MqttMessageBuilders;
import lombok.extern.slf4j.Slf4j;import static io.netty.handler.codec.mqtt.MqttConnectReturnCode.CONNECTION_ACCEPTED;@Slf4j
public class MqttHandler extends ChannelInboundHandlerAdapter {@Overridepublic void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {super.channelRead(ctx, msg);if (msg instanceof MqttConnectMessage) {handleConnect(ctx, (MqttConnectMessage) msg);} else {log.error("Unsupported type msg [{}]", msg);}}private void handleConnect(ChannelHandlerContext ctx, MqttConnectMessage connectMessage) {log.info("connect msg is [{}]", connectMessage);final MqttFixedHeader fixedHeader = connectMessage.fixedHeader();final MqttConnectVariableHeader variableHeader = connectMessage.variableHeader();final MqttConnectPayload connectPayload = connectMessage.payload();final MqttConnAckMessage ackMessage = MqttMessageBuilders.connAck().returnCode(CONNECTION_ACCEPTED).build();ctx.channel().writeAndFlush(ackMessage);}}

我们当前把所有的逻辑都放在MqttHandler里面，不方便后续的扩展。抽象出一个MqttProcessor接口来处理具体的请求，MqttHandler负责解析MqttMessage的类型并分发。MqttProcess接口设计如下

package io.github.protocol.mqtt.broker.processor;import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.mqtt.MqttConnAckMessage;
import io.netty.handler.codec.mqtt.MqttConnectMessage;
import io.netty.handler.codec.mqtt.MqttMessage;
import io.netty.handler.codec.mqtt.MqttPubAckMessage;
import io.netty.handler.codec.mqtt.MqttPublishMessage;
import io.netty.handler.codec.mqtt.MqttSubAckMessage;
import io.netty.handler.codec.mqtt.MqttSubscribeMessage;
import io.netty.handler.codec.mqtt.MqttUnsubAckMessage;
import io.netty.handler.codec.mqtt.MqttUnsubscribeMessage;public interface MqttProcessor {void processConnect(ChannelHandlerContext ctx, MqttConnectMessage msg) throws Exception;void processConnAck(ChannelHandlerContext ctx, MqttConnAckMessage msg) throws Exception;void processPublish(ChannelHandlerContext ctx, MqttPublishMessage msg) throws Exception;void processPubAck(ChannelHandlerContext ctx, MqttPubAckMessage msg) throws Exception;void processPubRec(ChannelHandlerContext ctx, MqttMessage msg) throws Exception;void processPubRel(ChannelHandlerContext ctx, MqttMessage msg) throws Exception;void processPubComp(ChannelHandlerContext ctx, MqttMessage msg) throws Exception;void processSubscribe(ChannelHandlerContext ctx, MqttSubscribeMessage msg) throws Exception;void processSubAck(ChannelHandlerContext ctx, MqttSubAckMessage msg) throws Exception;void processUnsubscribe(ChannelHandlerContext ctx, MqttUnsubscribeMessage msg) throws Exception;void processUnsubAck(ChannelHandlerContext ctx, MqttUnsubAckMessage msg) throws Exception;void processPingReq(ChannelHandlerContext ctx, MqttMessage msg) throws Exception;void processPingResp(ChannelHandlerContext ctx, MqttMessage msg) throws Exception;void processDisconnect(ChannelHandlerContext ctx) throws Exception;void processAuth(ChannelHandlerContext ctx, MqttMessage msg) throws Exception;}

我们允许这些方法抛出异常，当遇到极难处理的故障时，把mqtt连接断掉（如后端存储故障），等待客户端的重连。

MqttHandler中来调用MqttProcessor，相关MqttHandler代码如下

        Preconditions.checkArgument(message instanceof MqttMessage);MqttMessage msg = (MqttMessage) message;try {if (msg.decoderResult().isFailure()) {Throwable cause = msg.decoderResult().cause();if (cause instanceof MqttUnacceptableProtocolVersionException) {// Unsupported protocol versionMqttConnAckMessage connAckMessage = (MqttConnAckMessage) MqttMessageFactory.newMessage(new MqttFixedHeader(MqttMessageType.CONNACK,false, MqttQoS.AT_MOST_ONCE, false, 0),new MqttConnAckVariableHeader(MqttConnectReturnCode.CONNECTION_REFUSED_UNACCEPTABLE_PROTOCOL_VERSION,false), null);ctx.writeAndFlush(connAckMessage);log.error("connection refused due to invalid protocol, client address [{}]",ctx.channel().remoteAddress());ctx.close();return;} else if (cause instanceof MqttIdentifierRejectedException) {// ineligible clientIdMqttConnAckMessage connAckMessage = (MqttConnAckMessage) MqttMessageFactory.newMessage(new MqttFixedHeader(MqttMessageType.CONNACK,false, MqttQoS.AT_MOST_ONCE, false, 0),new MqttConnAckVariableHeader(MqttConnectReturnCode.CONNECTION_REFUSED_IDENTIFIER_REJECTED,false), null);ctx.writeAndFlush(connAckMessage);log.error("ineligible clientId, client address [{}]", ctx.channel().remoteAddress());ctx.close();return;}throw new IllegalStateException(msg.decoderResult().cause().getMessage());}MqttMessageType messageType = msg.fixedHeader().messageType();if (log.isDebugEnabled()) {log.debug("Processing MQTT Inbound handler message, type={}", messageType);}switch (messageType) {case CONNECT:Preconditions.checkArgument(msg instanceof MqttConnectMessage);processor.processConnect(ctx, (MqttConnectMessage) msg);break;case CONNACK:Preconditions.checkArgument(msg instanceof MqttConnAckMessage);processor.processConnAck(ctx, (MqttConnAckMessage) msg);break;case PUBLISH:Preconditions.checkArgument(msg instanceof MqttPublishMessage);processor.processPublish(ctx, (MqttPublishMessage) msg);break;case PUBACK:Preconditions.checkArgument(msg instanceof MqttPubAckMessage);processor.processPubAck(ctx, (MqttPubAckMessage) msg);break;case PUBREC:processor.processPubRec(ctx, msg);break;case PUBREL:processor.processPubRel(ctx, msg);break;case PUBCOMP:processor.processPubComp(ctx, msg);break;case SUBSCRIBE:Preconditions.checkArgument(msg instanceof MqttSubscribeMessage);processor.processSubscribe(ctx, (MqttSubscribeMessage) msg);break;case SUBACK:Preconditions.checkArgument(msg instanceof MqttSubAckMessage);processor.processSubAck(ctx, (MqttSubAckMessage) msg);break;case UNSUBSCRIBE:Preconditions.checkArgument(msg instanceof MqttUnsubscribeMessage);processor.processUnsubscribe(ctx, (MqttUnsubscribeMessage) msg);break;case UNSUBACK:Preconditions.checkArgument(msg instanceof MqttUnsubAckMessage);processor.processUnsubAck(ctx, (MqttUnsubAckMessage) msg);break;case PINGREQ:processor.processPingReq(ctx, msg);break;case PINGRESP:processor.processPingResp(ctx, msg);break;case DISCONNECT:processor.processDisconnect(ctx);break;case AUTH:processor.processAuth(ctx, msg);break;default:throw new UnsupportedOperationException("Unknown MessageType: " + messageType);}} catch (Throwable ex) {ReferenceCountUtil.safeRelease(msg);log.error("Exception was caught while processing MQTT message, ", ex);ctx.close();}

这里的代码，主要是针对MqttMessage的不同类型，调用MqttProcessor的不同方法，值得一提的有两点

  • 提前判断了一些解码异常，fast fail
  • 全局捕获异常，并进行断链处理

维护MqttSession

维护Mqtt会话的session，主要用来持续跟踪客户端会话信息，跟踪在系统中占用的资源等，考虑到无论是何种后端实现，都需要维护Mqtt的Session，我们构筑一个AbstractMqttProcessor来维护MqttSession

package io.github.protocol.mqtt.broker.processor;import io.github.protocol.mqtt.broker.MqttSessionKey;
import io.github.protocol.mqtt.broker.auth.MqttAuth;
import io.github.protocol.mqtt.broker.util.ChannelUtils;
import io.github.protocol.mqtt.broker.util.MqttMessageUtil;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.mqtt.MqttConnAckMessage;
import io.netty.handler.codec.mqtt.MqttConnAckVariableHeader;
import io.netty.handler.codec.mqtt.MqttConnectMessage;
import io.netty.handler.codec.mqtt.MqttConnectReturnCode;
import io.netty.handler.codec.mqtt.MqttFixedHeader;
import io.netty.handler.codec.mqtt.MqttMessage;
import io.netty.handler.codec.mqtt.MqttMessageFactory;
import io.netty.handler.codec.mqtt.MqttMessageIdVariableHeader;
import io.netty.handler.codec.mqtt.MqttMessageType;
import io.netty.handler.codec.mqtt.MqttPubAckMessage;
import io.netty.handler.codec.mqtt.MqttPublishMessage;
import io.netty.handler.codec.mqtt.MqttQoS;
import io.netty.handler.codec.mqtt.MqttSubAckMessage;
import io.netty.handler.codec.mqtt.MqttSubAckPayload;
import io.netty.handler.codec.mqtt.MqttSubscribeMessage;
import io.netty.handler.codec.mqtt.MqttSubscribePayload;
import io.netty.handler.codec.mqtt.MqttUnsubAckMessage;
import io.netty.handler.codec.mqtt.MqttUnsubscribeMessage;
import lombok.extern.slf4j.Slf4j;
import org.apache.commons.lang3.StringUtils;import java.util.stream.IntStream;@Slf4j
public abstract class AbstractProcessor implements MqttProcessor {protected final MqttAuth mqttAuth;public AbstractProcessor(MqttAuth mqttAuth) {this.mqttAuth = mqttAuth;}@Overridepublic void processConnect(ChannelHandlerContext ctx, MqttConnectMessage msg) throws Exception {String clientId = msg.payload().clientIdentifier();String username = msg.payload().userName();byte[] pwd = msg.payload().passwordInBytes();if (StringUtils.isBlank(clientId) || StringUtils.isBlank(username)) {MqttConnAckMessage connAckMessage = (MqttConnAckMessage) MqttMessageFactory.newMessage(new MqttFixedHeader(MqttMessageType.CONNACK,false, MqttQoS.AT_MOST_ONCE, false, 0),new MqttConnAckVariableHeader(MqttConnectReturnCode.CONNECTION_REFUSED_IDENTIFIER_REJECTED,false), null);ctx.writeAndFlush(connAckMessage);log.error("the clientId username pwd cannot be empty, client address[{}]", ctx.channel().remoteAddress());ctx.close();return;}if (!mqttAuth.connAuth(clientId, username, pwd)) {MqttConnAckMessage connAckMessage = (MqttConnAckMessage) MqttMessageFactory.newMessage(new MqttFixedHeader(MqttMessageType.CONNACK,false, MqttQoS.AT_MOST_ONCE, false, 0),new MqttConnAckVariableHeader(MqttConnectReturnCode.CONNECTION_REFUSED_BAD_USER_NAME_OR_PASSWORD,false), null);ctx.writeAndFlush(connAckMessage);log.error("the clientId username pwd cannot be empty, client address[{}]", ctx.channel().remoteAddress());ctx.close();return;}MqttSessionKey mqttSessionKey = new MqttSessionKey();mqttSessionKey.setUsername(username);mqttSessionKey.setClientId(clientId);ChannelUtils.setMqttSession(ctx.channel(), mqttSessionKey);log.info("username {} clientId {} remote address {} connected",username, clientId, ctx.channel().remoteAddress());onConnect(mqttSessionKey);MqttConnAckMessage mqttConnectMessage = (MqttConnAckMessage) MqttMessageFactory.newMessage(new MqttFixedHeader(MqttMessageType.CONNACK,false, MqttQoS.AT_MOST_ONCE, false, 0),new MqttConnAckVariableHeader(MqttConnectReturnCode.CONNECTION_ACCEPTED, false),null);ctx.writeAndFlush(mqttConnectMessage);}protected void onConnect(MqttSessionKey mqttSessionKey) {}@Overridepublic void processConnAck(ChannelHandlerContext ctx, MqttConnAckMessage msg) throws Exception {MqttSessionKey mqttSession = ChannelUtils.getMqttSession(ctx.channel());if (mqttSession == null) {log.error("conn ack, client address {} not authed", ctx.channel().remoteAddress());ctx.close();}}@Overridepublic void processPublish(ChannelHandlerContext ctx, MqttPublishMessage msg) throws Exception {MqttSessionKey mqttSession = ChannelUtils.getMqttSession(ctx.channel());if (mqttSession == null) {log.error("publish, client address {} not authed", ctx.channel().remoteAddress());ctx.close();return;}if (msg.fixedHeader().qosLevel() == MqttQoS.FAILURE) {log.error("failure. clientId {}, username {} ", mqttSession.getClientId(), mqttSession.getUsername());return;}if (msg.fixedHeader().qosLevel() == MqttQoS.EXACTLY_ONCE) {log.error("does not support QoS2 protocol. clientId {}, username {} ",mqttSession.getClientId(), mqttSession.getUsername());return;}onPublish(ctx, mqttSession, msg);}protected void onPublish(ChannelHandlerContext ctx, MqttSessionKey mqttSessionKey,MqttPublishMessage msg) throws Exception {}@Overridepublic void processPubAck(ChannelHandlerContext ctx, MqttPubAckMessage msg) throws Exception {MqttSessionKey mqttSession = ChannelUtils.getMqttSession(ctx.channel());if (mqttSession == null) {log.error("pub ack, client address {} not authed", ctx.channel().remoteAddress());ctx.close();}}@Overridepublic void processPubRec(ChannelHandlerContext ctx, MqttMessage msg) throws Exception {MqttSessionKey mqttSession = ChannelUtils.getMqttSession(ctx.channel());if (mqttSession == null) {log.error("pub rec, client address {} not authed", ctx.channel().remoteAddress());ctx.close();}}@Overridepublic void processPubRel(ChannelHandlerContext ctx, MqttMessage msg) throws Exception {MqttSessionKey mqttSession = ChannelUtils.getMqttSession(ctx.channel());if (mqttSession == null) {log.error("pub rel, client address {} not authed", ctx.channel().remoteAddress());ctx.close();}}@Overridepublic void processPubComp(ChannelHandlerContext ctx, MqttMessage msg) throws Exception {MqttSessionKey mqttSession = ChannelUtils.getMqttSession(ctx.channel());if (mqttSession == null) {log.error("pub comp, client address {} not authed", ctx.channel().remoteAddress());ctx.close();}}@Overridepublic void processSubscribe(ChannelHandlerContext ctx, MqttSubscribeMessage msg) throws Exception {MqttSessionKey mqttSession = ChannelUtils.getMqttSession(ctx.channel());if (mqttSession == null) {log.error("sub, client address {} not authed", ctx.channel().remoteAddress());ctx.close();}onSubscribe(ctx, mqttSession, msg.payload());MqttFixedHeader fixedHeader = new MqttFixedHeader(MqttMessageType.SUBACK,false, MqttQoS.AT_MOST_ONCE, false, 0);IntStream intStream = msg.payload().topicSubscriptions().stream().mapToInt(s -> s.qualityOfService().value());MqttSubAckPayload payload = new MqttSubAckPayload(intStream.toArray());ctx.writeAndFlush(MqttMessageFactory.newMessage(fixedHeader,MqttMessageIdVariableHeader.from(msg.variableHeader().messageId()),payload));}protected void onSubscribe(ChannelHandlerContext ctx, MqttSessionKey mqttSessionKey,MqttSubscribePayload subscribePayload) throws Exception {}@Overridepublic void processSubAck(ChannelHandlerContext ctx, MqttSubAckMessage msg) throws Exception {MqttSessionKey mqttSession = ChannelUtils.getMqttSession(ctx.channel());if (mqttSession == null) {log.error("sub ack, client address {} not authed", ctx.channel().remoteAddress());ctx.close();}}@Overridepublic void processUnsubscribe(ChannelHandlerContext ctx, MqttUnsubscribeMessage msg) throws Exception {MqttSessionKey mqttSession = ChannelUtils.getMqttSession(ctx.channel());if (mqttSession == null) {log.error("unsub, client address {} not authed", ctx.channel().remoteAddress());ctx.close();}}@Overridepublic void processUnsubAck(ChannelHandlerContext ctx, MqttUnsubAckMessage msg) throws Exception {MqttSessionKey mqttSession = ChannelUtils.getMqttSession(ctx.channel());if (mqttSession == null) {log.error("unsub ack, client address {} not authed", ctx.channel().remoteAddress());ctx.close();}}@Overridepublic void processPingReq(ChannelHandlerContext ctx, MqttMessage msg) throws Exception {ctx.writeAndFlush(MqttMessageUtil.pingResp());}@Overridepublic void processPingResp(ChannelHandlerContext ctx, MqttMessage msg) throws Exception {MqttSessionKey mqttSession = ChannelUtils.getMqttSession(ctx.channel());if (mqttSession == null) {log.error("ping resp, client address {} not authed", ctx.channel().remoteAddress());ctx.close();}}@Overridepublic void processDisconnect(ChannelHandlerContext ctx) throws Exception {MqttSessionKey mqttSession = ChannelUtils.getMqttSession(ctx.channel());if (mqttSession == null) {log.error("disconnect, client address {} not authed", ctx.channel().remoteAddress());}onDisconnect(mqttSession);}protected void onDisconnect(MqttSessionKey mqttSessionKey) {}@Overridepublic void processAuth(ChannelHandlerContext ctx, MqttMessage msg) throws Exception {MqttSessionKey mqttSession = ChannelUtils.getMqttSession(ctx.channel());if (mqttSession == null) {log.error("auth, client address {} not authed", ctx.channel().remoteAddress());ctx.close();}}
}

可以看到，这里的AbstractProcessor主要是维护了MqttSessionKey，校验MqttSessionKey，并拦截publish中不支持的Qos2、Failure。同时，也影响了mqtt心跳请求。同样的，我们允许在onPublish、onSubscribe中抛出异常。

基于消息队列实现的mqtt网关的基础思想也比较简单，简而言之就是，有publish消息的时候向消息队列中生产消息。有订阅的时候就从消息队列中拉取消息。由此延伸出来，我们可能需要维护每个mqtt topic和producer、consumer的对应关系，因为像kafka、pulsar这些消息中间件的消费者都是区分topic的，片段通用代码如下:

    protected final ReentrantReadWriteLock.ReadLock rLock;protected final ReentrantReadWriteLock.WriteLock wLock;protected final Map<MqttSessionKey, List<MqttTopicKey>> sessionProducerMap;protected final Map<MqttSessionKey, List<MqttTopicKey>> sessionConsumerMap;protected final Map<MqttTopicKey, P> producerMap;protected final Map<MqttTopicKey, C> consumerMap;public AbstractMqProcessor(MqttAuth mqttAuth) {super(mqttAuth);ReentrantReadWriteLock lock = new ReentrantReadWriteLock();rLock = lock.readLock();wLock = lock.writeLock();this.sessionProducerMap = new HashMap<>();this.sessionConsumerMap = new HashMap<>();this.producerMap = new HashMap<>();this.consumerMap = new HashMap<>();}@Overrideprotected void onConnect(MqttSessionKey mqttSessionKey) {wLock.lock();try {sessionProducerMap.put(mqttSessionKey, new ArrayList<>());sessionConsumerMap.put(mqttSessionKey, new ArrayList<>());} finally {wLock.unlock();}}@Overrideprotected void onDisconnect(MqttSessionKey mqttSessionKey) {wLock.lock();try {// find producersList<MqttTopicKey> produceTopicKeys = sessionProducerMap.get(mqttSessionKey);if (produceTopicKeys != null) {for (MqttTopicKey mqttTopicKey : produceTopicKeys) {P producer = producerMap.get(mqttTopicKey);if (producer != null) {ClosableUtils.close(producer);producerMap.remove(mqttTopicKey);}}}sessionProducerMap.remove(mqttSessionKey);List<MqttTopicKey> consumeTopicKeys = sessionConsumerMap.get(mqttSessionKey);if (consumeTopicKeys != null) {for (MqttTopicKey mqttTopicKey : consumeTopicKeys) {C consumer = consumerMap.get(mqttTopicKey);if (consumer != null) {ClosableUtils.close(consumer);consumerMap.remove(mqttTopicKey);}}}sessionConsumerMap.remove(mqttSessionKey);} finally {wLock.unlock();}}
}

kafka processor实现

由于kafka producer不区分topic，我们可以在kafka processor中复用producer，在将来单个kafka producer的性能到达上限时，我们可以将kafka producer扩展为kafka producer列表进行轮询处理，消费者由于mqtt协议可能针对每个订阅topic有不同的行为，不合适复用同一个消费者实例。我们在构造函数中启动KafkaProducer

private final KafkaProcessorConfig kafkaProcessorConfig;private final KafkaProducer<String, ByteBuffer> producer;public KafkaProcessor(MqttAuth mqttAuth, KafkaProcessorConfig kafkaProcessorConfig) {super(mqttAuth);this.kafkaProcessorConfig = kafkaProcessorConfig;this.producer = createProducer();}protected KafkaProducer<String, ByteBuffer> createProducer() {Properties properties = new Properties();properties.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, kafkaProcessorConfig.getBootstrapServers());properties.put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, StringSerializer.class);properties.put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, ByteBufferSerializer.class);return new KafkaProducer<>(properties);}

处理MqttPublish消息，MqttPublish消息包含如下几个关键参数

MqttQoS mqttQoS = publishMessage.fixedHeader().qosLevel();
String topic = publishMessage.variableHeader().topicName();
ByteBuffer byteBuffer = publishMessage.payload().nioBuffer();

其中

  • qos代表这条消息的质量级别，0没有任何保障，1代表至少一次，2代表恰好一次。当前仅支持qos0、qos1
  • topicName就是topic的名称
  • ByteBuffer就是消息的内容

根据topic、qos发送消息，代码如下

        String topic = msg.variableHeader().topicName();ProducerRecord<String, ByteBuffer> record = new ProducerRecord<>(topic, msg.payload().nioBuffer());switch (msg.fixedHeader().qosLevel()) {case AT_MOST_ONCE -> producer.send(record, (metadata, exception) -> {if (exception != null) {log.error("mqttSessionKey {} send message to kafka error", mqttSessionKey, exception);return;}log.debug("mqttSessionKey {} send message to kafka success, topic {}, partition {}, offset {}",mqttSessionKey, metadata.topic(), metadata.partition(), metadata.offset());});case AT_LEAST_ONCE -> {try {RecordMetadata recordMetadata = producer.send(record).get();log.info("mqttSessionKey {} send message to kafka success, topic {}, partition {}, offset {}",mqttSessionKey, recordMetadata.topic(),recordMetadata.partition(), recordMetadata.offset());ctx.writeAndFlush(MqttMessageUtil.pubAckMessage(msg.variableHeader().packetId()));} catch (Exception e) {log.error("mqttSessionKey {} send message to kafka error", mqttSessionKey, e);}}case EXACTLY_ONCE, FAILURE -> throw new IllegalStateException(String.format("mqttSessionKey %s can not reach here", mqttSessionKey));}

处理订阅消息，我们暂时仅根据订阅的topic，创建topic进行消费即可，由于kafka原生客户端建议的消费代码模式如下

while (true) {ConsumerRecords<String, byte[]> records = consumer.poll(Duration.ofSeconds(1));for (ConsumerRecord<String, byte[]> record : records) {// do logic}
}

我们需要切换到其他线程对consumer进行消息，书写一个KafkaConsumerListenerWrapper的wrapper，转换为listener异步消费模型

package io.github.protocol.mqtt.broker.processor;import lombok.extern.slf4j.Slf4j;
import org.apache.kafka.clients.admin.AdminClient;
import org.apache.kafka.clients.admin.AdminClientConfig;
import org.apache.kafka.clients.admin.KafkaAdminClient;
import org.apache.kafka.clients.admin.NewTopic;
import org.apache.kafka.clients.admin.TopicDescription;
import org.apache.kafka.clients.consumer.ConsumerConfig;
import org.apache.kafka.clients.consumer.ConsumerRecord;
import org.apache.kafka.clients.consumer.ConsumerRecords;
import org.apache.kafka.clients.consumer.KafkaConsumer;
import org.apache.kafka.common.errors.UnknownTopicOrPartitionException;
import org.apache.kafka.common.errors.WakeupException;
import org.apache.kafka.common.serialization.ByteArrayDeserializer;
import org.apache.kafka.common.serialization.StringDeserializer;import java.time.Duration;
import java.util.Collections;
import java.util.Properties;
import java.util.concurrent.ExecutionException;@Slf4j
public class KafkaConsumerListenerWrapper implements AutoCloseable {private final AdminClient adminClient;private final KafkaConsumer<String, byte[]> consumer;public KafkaConsumerListenerWrapper(KafkaProcessorConfig config, String username) {Properties adminProperties = new Properties();adminProperties.put(AdminClientConfig.BOOTSTRAP_SERVERS_CONFIG, config.getBootstrapServers());this.adminClient = KafkaAdminClient.create(adminProperties);Properties properties = new Properties();properties.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, config.getBootstrapServers());properties.put(ConsumerConfig.GROUP_ID_CONFIG, username);properties.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);properties.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, ByteArrayDeserializer.class);this.consumer = new KafkaConsumer<>(properties);}public void start(String topic, KafkaMessageListener listener) throws Exception {try {TopicDescription topicDescription = adminClient.describeTopics(Collections.singletonList(topic)).values().get(topic).get();log.info("topic info is {}", topicDescription);} catch (ExecutionException ee) {if (ee.getCause() instanceof UnknownTopicOrPartitionException) {log.info("topic {} not exist, create it", topic);adminClient.createTopics(Collections.singletonList(new NewTopic(topic, 1, (short) 1)));} else {log.error("find topic info {} error", topic, ee);}} catch (Exception e) {throw new IllegalStateException("find topic info error", e);}consumer.subscribe(Collections.singletonList(topic));log.info("consumer topic {} start", topic);new Thread(() -> {try {while (true) {ConsumerRecords<String, byte[]> records = consumer.poll(Duration.ofSeconds(1));for (ConsumerRecord<String, byte[]> record : records) {listener.messageReceived(record);}}} catch (WakeupException we) {consumer.close();} catch (Exception e) {log.error("consumer topic {} consume error", topic, e);consumer.close();}}).start();Thread.sleep(5_000);}@Overridepublic void close() throws Exception {log.info("wake up {} consumer", consumer);consumer.wakeup();}
}

    @Overrideprotected void onSubscribe(ChannelHandlerContext ctx, MqttSessionKey mqttSessionKey,MqttSubscribePayload subscribePayload) throws Exception {for (MqttTopicSubscription topicSubscription : subscribePayload.topicSubscriptions()) {KafkaConsumerListenerWrapper consumer = createConsumer(mqttSessionKey, topicSubscription.topicName());subscribe(ctx, consumer, topicSubscription.topicName());}}private KafkaConsumerListenerWrapper createConsumer(MqttSessionKey mqttSessionKey, String topic) {MqttTopicKey mqttTopicKey = new MqttTopicKey();mqttTopicKey.setTopic(topic);mqttTopicKey.setMqttSessionKey(mqttSessionKey);wLock.lock();try {KafkaConsumerListenerWrapper consumer = consumerMap.get(mqttTopicKey);if (consumer == null) {consumer = new KafkaConsumerListenerWrapper(kafkaProcessorConfig, mqttSessionKey.getUsername());sessionConsumerMap.compute(mqttSessionKey, (mqttSessionKey1, mqttTopicKeys) -> {if (mqttTopicKeys == null) {mqttTopicKeys = new ArrayList<>();}mqttTopicKeys.add(mqttTopicKey);return mqttTopicKeys;});consumerMap.put(mqttTopicKey, consumer);}return consumer;} finally {wLock.unlock();}}protected void subscribe(ChannelHandlerContext ctx,KafkaConsumerListenerWrapper consumer, String topic) throws Exception {BoundInt boundInt = new BoundInt(65535);consumer.start(topic, record -> {log.info("receive message from kafka, topic {}, partition {}, offset {}",record.topic(), record.partition(), record.offset());MqttPublishMessage mqttPublishMessage = MqttMessageUtil.publishMessage(MqttQoS.AT_LEAST_ONCE, topic, boundInt.nextVal(), record.value());ctx.writeAndFlush(mqttPublishMessage);});}

在上述的代码中，有一个需要通篇注意的点：日志打印的时候，注意要将关键的信息携带，比如：topic、mqtt username、mqtt clientId等，在写demo的时候没有感觉，但是海量请求下需要定位问题的时候，就知道这些信息的关键之处了。

使用BountInt这个简单的工具类来生成从0~65535的packageId，满足协议的要求

pulsar processor实现

pulsar相比kafka来说，更适合作为mqtt协议的代理。原因有如下几点：

  • pulsar支持百万topic、topic实现更轻量
  • pulsar原生支持listener的消费模式，不需要每个消费者启动一个线程
  • pulsar支持share的消费模式，消费模式更灵活
  • pulsar消费者的subscribe可确保成功创建订阅，相比kafka的消费者没有这样的语义保障
    protected final ReentrantReadWriteLock.ReadLock rLock;protected final ReentrantReadWriteLock.WriteLock wLock;protected final Map<MqttSessionKey, List<MqttTopicKey>> sessionProducerMap;protected final Map<MqttSessionKey, List<MqttTopicKey>> sessionConsumerMap;protected final Map<MqttTopicKey, Producer<byte[]>> producerMap;protected final Map<MqttTopicKey, Consumer<byte[]>> consumerMap;private final PulsarProcessorConfig pulsarProcessorConfig;private final PulsarAdmin pulsarAdmin;private final PulsarClient pulsarClient;public PulsarProcessor(MqttAuth mqttAuth, PulsarProcessorConfig pulsarProcessorConfig) {super(mqttAuth);ReentrantReadWriteLock lock = new ReentrantReadWriteLock();rLock = lock.readLock();wLock = lock.writeLock();this.sessionProducerMap = new HashMap<>();this.sessionConsumerMap = new HashMap<>();this.producerMap = new HashMap<>();this.consumerMap = new HashMap<>();this.pulsarProcessorConfig = pulsarProcessorConfig;try {this.pulsarAdmin = PulsarAdmin.builder().serviceHttpUrl(pulsarProcessorConfig.getHttpUrl()).build();this.pulsarClient = PulsarClient.builder().serviceUrl(pulsarProcessorConfig.getServiceUrl()).build();} catch (Exception e) {throw new IllegalStateException("Failed to create pulsar client", e);}}

处理publish消息

    @Overrideprotected void onPublish(ChannelHandlerContext ctx, MqttSessionKey mqttSessionKey,MqttPublishMessage msg) throws Exception {String topic = msg.variableHeader().topicName();Producer<byte[]> producer = getOrCreateProducer(mqttSessionKey, topic);int len = msg.payload().readableBytes();byte[] messageBytes = new byte[len];msg.payload().getBytes(msg.payload().readerIndex(), messageBytes);switch (msg.fixedHeader().qosLevel()) {case AT_MOST_ONCE -> producer.sendAsync(messageBytes).thenAccept(messageId -> log.info("clientId [{}],"+ " username [{}]. send message to pulsar success messageId: {}",mqttSessionKey.getClientId(), mqttSessionKey.getUsername(), messageId)).exceptionally((e) -> {log.error("clientId [{}], username [{}]. send message to pulsar fail: ",mqttSessionKey.getClientId(), mqttSessionKey.getUsername(), e);return null;});case AT_LEAST_ONCE -> {try {MessageId messageId = producer.send(messageBytes);MqttFixedHeader fixedHeader = new MqttFixedHeader(MqttMessageType.PUBACK,false, MqttQoS.AT_MOST_ONCE, false, 0);MqttPubAckMessage pubAckMessage = (MqttPubAckMessage) MqttMessageFactory.newMessage(fixedHeader,MqttMessageIdVariableHeader.from(msg.variableHeader().packetId()), null);log.info("clientId [{}], username [{}]. send pulsar success. messageId: {}",mqttSessionKey.getClientId(), mqttSessionKey.getUsername(), messageId);ctx.writeAndFlush(pubAckMessage);} catch (PulsarClientException e) {log.error("clientId [{}], username [{}]. send pulsar error: {}",mqttSessionKey.getClientId(), mqttSessionKey.getUsername(), e.getMessage());}}case EXACTLY_ONCE, FAILURE -> throw new IllegalStateException(String.format("mqttSessionKey %s can not reach here", mqttSessionKey));}}private Producer<byte[]> getOrCreateProducer(MqttSessionKey mqttSessionKey, String topic) throws Exception {MqttTopicKey mqttTopicKey = new MqttTopicKey();mqttTopicKey.setTopic(topic);mqttTopicKey.setMqttSessionKey(mqttSessionKey);rLock.lock();try {Producer<byte[]> producer = producerMap.get(mqttTopicKey);if (producer != null) {return producer;}} finally {rLock.unlock();}wLock.lock();try {Producer<byte[]> producer = producerMap.get(mqttTopicKey);if (producer == null) {producer = createProducer(topic);sessionProducerMap.compute(mqttSessionKey, (mqttSessionKey1, mqttTopicKeys) -> {if (mqttTopicKeys == null) {mqttTopicKeys = new ArrayList<>();}mqttTopicKeys.add(mqttTopicKey);return mqttTopicKeys;});producerMap.put(mqttTopicKey, producer);}return producer;} finally {wLock.unlock();}}protected Producer<byte[]> createProducer(String topic) throws Exception {return pulsarClient.newProducer(Schema.BYTES).topic(topic).create();}

处理subscribe消息

    @Overrideprotected void onSubscribe(ChannelHandlerContext ctx, MqttSessionKey mqttSessionKey,MqttSubscribePayload subscribePayload) throws Exception {for (MqttTopicSubscription topicSubscription : subscribePayload.topicSubscriptions()) {subscribe(ctx, mqttSessionKey, topicSubscription.topicName());}}protected void subscribe(ChannelHandlerContext ctx, MqttSessionKey mqttSessionKey,String topic) throws Exception {MqttTopicKey mqttTopicKey = new MqttTopicKey();mqttTopicKey.setTopic(topic);mqttTopicKey.setMqttSessionKey(mqttSessionKey);wLock.lock();try {Consumer<byte[]> consumer = consumerMap.get(mqttTopicKey);if (consumer == null) {consumer = createConsumer(ctx, mqttSessionKey.getUsername(), topic);sessionConsumerMap.compute(mqttSessionKey, (mqttSessionKey1, mqttTopicKeys) -> {if (mqttTopicKeys == null) {mqttTopicKeys = new ArrayList<>();}mqttTopicKeys.add(mqttTopicKey);return mqttTopicKeys;});consumerMap.put(mqttTopicKey, consumer);}} finally {wLock.unlock();}}protected Consumer<byte[]> createConsumer(ChannelHandlerContext ctx, String username,String topic) throws Exception {BoundInt boundInt = new BoundInt(65535);try {PartitionedTopicStats partitionedStats = pulsarAdmin.topics().getPartitionedStats(topic, false);log.info("topic {} partitioned stats {}", topic, partitionedStats);} catch (PulsarAdminException.NotFoundException nfe) {log.info("topic {} not found", topic);pulsarAdmin.topics().createPartitionedTopic(topic, 1);}return pulsarClient.newConsumer(Schema.BYTES).topic(topic).messageListener((consumer, msg) -> {log.info("receive message from pulsar, topic {}, message {}", topic, msg.getMessageId());MqttPublishMessage mqttPublishMessage = MqttMessageUtil.publishMessage(MqttQoS.AT_LEAST_ONCE, topic, boundInt.nextVal(), msg.getData());ctx.writeAndFlush(mqttPublishMessage);}).subscriptionName(username).subscribe();}

测试用例

鲁邦的软件应该有相应的测试用例，这里简单写了两个基础的pubsub用例，实际的production ready的项目，测试用例会更加复杂，涵盖各种异常的场景。有句话说的很好 ”单元测试是对开发人员的即时激励“，我也很认同这句话

kafka

启动kafka测试broker

我们可以通过embedded-kafka-java这个项目来启动用做单元测试的kafka broker。通过如下的group引入依赖

        <dependency><groupId>io.github.embedded-middleware</groupId><artifactId>embedded-kafka-core</artifactId><version>0.0.2</version><scope>test</scope></dependency>

我们就可以通过如下的代码启动基于kafka的mqtt broker

@Slf4j
public class MqttKafkaTestUtil {public static MqttServer setupMqttKafka() throws Exception {EmbeddedKafkaServer embeddedKafkaServer = new EmbeddedKafkaServer();new Thread(() -> {try {embeddedKafkaServer.start();} catch (Exception e) {log.error("kafka broker started exception ", e);}}).start();Thread.sleep(5_000);MqttServerConfig mqttServerConfig = new MqttServerConfig();mqttServerConfig.setPort(0);mqttServerConfig.setProcessorType(ProcessorType.KAFKA);KafkaProcessorConfig kafkaProcessorConfig = new KafkaProcessorConfig();kafkaProcessorConfig.setBootstrapServers(String.format("localhost:%d", embeddedKafkaServer.getKafkaPort()));mqttServerConfig.setKafkaProcessorConfig(kafkaProcessorConfig);MqttServer mqttServer = new MqttServer(mqttServerConfig);new Thread(() -> {try {mqttServer.start();} catch (Exception e) {log.error("mqsar broker started exception ", e);}}).start();Thread.sleep(5000L);return mqttServer;}}

kafka端到端测试用例，比较简单，通过mqtt client publish一条消息，然后消费出来

@Log4j2
public class MqttKafkaPubSubTest {@Testpublic void pubSubTest() throws Exception {MqttServer mqttServer = MqttKafkaTestUtil.setupMqttKafka();String topic = UUID.randomUUID().toString();String content = "test-msg";String broker = String.format("tcp://localhost:%d", mqttServer.getPort());String clientId = UUID.randomUUID().toString();MemoryPersistence persistence = new MemoryPersistence();MqttClient sampleClient = new MqttClient(broker, clientId, persistence);MqttConnectOptions connOpts = new MqttConnectOptions();connOpts.setUserName(UUID.randomUUID().toString());connOpts.setPassword(UUID.randomUUID().toString().toCharArray());connOpts.setCleanSession(true);log.info("Mqtt connecting to broker");sampleClient.connect(connOpts);CompletableFuture<String> future = new CompletableFuture<>();log.info("Mqtt subscribing");sampleClient.subscribe(topic, (s, mqttMessage) -> {log.info("messageArrived");future.complete(mqttMessage.toString());});log.info("Mqtt subscribed");MqttMessage message = new MqttMessage(content.getBytes());message.setQos(1);log.info("Mqtt message publishing");sampleClient.publish(topic, message);log.info("Mqtt message published");TimeUnit.SECONDS.sleep(3);sampleClient.disconnect();String msg = future.get(5, TimeUnit.SECONDS);Assertions.assertEquals(content, msg);}}

pulsar

我们可以通过embedded-pulsar-java这个项目来启动用做单元测试的pulsar broker。通过如下的group引入依赖

        <dependency><groupId>io.github.embedded-middleware</groupId><artifactId>embedded-pulsar-core</artifactId><version>0.0.2</version><scope>test</scope></dependency>

我们就可以通过如下的代码启动基于pulsar的mqtt broker

@Slf4j
public class MqttPulsarTestUtil {public static MqttServer setupMqttPulsar() throws Exception {EmbeddedPulsarServer embeddedPulsarServer = new EmbeddedPulsarServer();embeddedPulsarServer.start();MqttServerConfig mqttServerConfig = new MqttServerConfig();mqttServerConfig.setPort(0);mqttServerConfig.setProcessorType(ProcessorType.PULSAR);PulsarProcessorConfig pulsarProcessorConfig = new PulsarProcessorConfig();pulsarProcessorConfig.setHttpUrl(String.format("http://localhost:%d", embeddedPulsarServer.getWebPort()));pulsarProcessorConfig.setServiceUrl(String.format("pulsar://localhost:%d", embeddedPulsarServer.getTcpPort()));mqttServerConfig.setPulsarProcessorConfig(pulsarProcessorConfig);MqttServer mqttServer = new MqttServer(mqttServerConfig);new Thread(() -> {try {mqttServer.start();} catch (Exception e) {log.error("mqsar broker started exception ", e);}}).start();Thread.sleep(5000L);return mqttServer;}
}

pulsar端到端测试用例，比较简单，通过mqtt client publish一条消息，然后消费出来

@Log4j2
public class MqttPulsarPubSubTest {@Testpublic void pubSubTest() throws Exception {MqttServer mqttServer = MqttPulsarTestUtil.setupMqttPulsar();String topic = UUID.randomUUID().toString();String content = "test-msg";String broker = String.format("tcp://localhost:%d", mqttServer.getPort());String clientId = UUID.randomUUID().toString();MemoryPersistence persistence = new MemoryPersistence();MqttClient sampleClient = new MqttClient(broker, clientId, persistence);MqttConnectOptions connOpts = new MqttConnectOptions();connOpts.setUserName(UUID.randomUUID().toString());connOpts.setPassword(UUID.randomUUID().toString().toCharArray());connOpts.setCleanSession(true);log.info("Mqtt connecting to broker");sampleClient.connect(connOpts);CompletableFuture<String> future = new CompletableFuture<>();log.info("Mqtt subscribing");sampleClient.subscribe(topic, (s, mqttMessage) -> {log.info("messageArrived");future.complete(mqttMessage.toString());});log.info("Mqtt subscribed");MqttMessage message = new MqttMessage(content.getBytes());message.setQos(1);log.info("Mqtt message publishing");sampleClient.publish(topic, message);log.info("Mqtt message published");TimeUnit.SECONDS.sleep(3);sampleClient.disconnect();String msg = future.get(5, TimeUnit.SECONDS);Assertions.assertEquals(content, msg);}
}

性能优化

这里我们简单描述几个性能优化点，像一些调整线程数、buffer大小这类的参数调整就不在这里赘述了，这些需要具体的性能压测来决定参数的设置。

在linux上使用Epoll网络模型

public class EventLoopUtil {/*** @return an EventLoopGroup suitable for the current platform*/public static EventLoopGroup newEventLoopGroup(int nThreads, ThreadFactory threadFactory) {if (Epoll.isAvailable()) {return new EpollEventLoopGroup(nThreads, threadFactory);} else {return new NioEventLoopGroup(nThreads, threadFactory);}}public static Class<? extends ServerSocketChannel> getServerSocketChannelClass(EventLoopGroup eventLoopGroup) {if (eventLoopGroup instanceof EpollEventLoopGroup) {return EpollServerSocketChannel.class;} else {return NioServerSocketChannel.class;}}}

通过Epollo.isAvailable，以及在指定channel类型的时候通过判断group的类型选择对应的channel类型

        EventLoopGroup acceptorGroup = EventLoopUtil.newEventLoopGroup(1,new DefaultThreadFactory("mqtt-acceptor"));EventLoopGroup workerGroup = EventLoopUtil.newEventLoopGroup(1,new DefaultThreadFactory("mqtt-worker"));

                b.group(acceptorGroup, workerGroup)// key point.channel(EventLoopUtil.getServerSocketChannelClass(workerGroup)).option(ChannelOption.SO_BACKLOG, 100).handler(new LoggingHandler(LogLevel.INFO)).childHandler(new ChannelInitializer<SocketChannel>() {@Overridepublic void initChannel(SocketChannel ch) throws Exception {ChannelPipeline p = ch.pipeline();// decoderp.addLast(new MqttDecoder());p.addLast(MqttEncoder.INSTANCE);p.addLast(new MqttHandler(processor(mqttServerConfig)));}});

关闭tcp keepalive

由于mqtt协议本身就有心跳机制，所以可以关闭tcp的keepalive，依赖mqtt协议层的心跳即可，节约海量连接下的性能。配置ChannelOption.SO_KEEPALIVE为false即可

                    .option(ChannelOption.SO_KEEPALIVE, false)

超时时间调短

默认情况下，无论是单元测试中mqtt，还是pulsar producer和kafka producer的生产超时时间，都相对较长（一般为30s），如果在内网环境部署，可以将超时时间调整到5s。来避免无意义的超时等待

使用多个KafkaProducer来优化性能

单个KafkaProducer会达到tcp链路带宽的瓶颈，当有海量请求，而延时在kafka生产比较突出的情况下，可以考虑启动多个KafkaProducer。并根据mqtt协议的特点（链路多，单个链路上qps不高），用mqttSessionKey的哈希值来决定使用那个KafkaProducer发送消息

在KafkaProcessorConfig中添加如下配置，生产者个数，默认为1

        private int producerNum = 1;

在初始化的时候，初始化Producer数组，而不是单个Producer

        this.producerArray = new KafkaProducer[kafkaProcessorConfig.getProducerNum()];for (int i = 0; i < kafkaProcessorConfig.getProducerNum(); i++) {producerArray[i] = createProducer();}

封装一个方法来获取producer

    private Producer<String, ByteBuffer> getProducer(MqttSessionKey mqttSessionKey) {return producerArray[Math.abs(mqttSessionKey.hashCode() % kafkaProcessorConfig.getProducerNum())];}

结语

本文的代码均已上传到github。我们这里仅仅只实现了基础的mqtt 连接、发布、订阅功能，甚至不支持暂停、取消订阅。想要实现一个成熟商用的mqtt网关，我们还需要用户隔离、对协议的更多支持、可靠性、可运维、流控、安全等能力。如有商用生产级别的mqtt需求，又无法快速构筑成熟的mqtt网关的提供稳定可靠的mqtt服务，支持海量设备连接上云、设备和云端消息双向通信能力。

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