通过源码分析RocketMQ主从复制原理-京东云开发者社区

**作者：京东物流 宫丙来**

## 一、主从复制概述

- RocketMQ Broker的主从复制主要包括两部分内容：CommitLog的消息复制和Broker元数据的复制。

- CommitLog的消息复制是发生在消息写入时，当消息写完Broker Master时，会通过单独的线程，将消息写入到从服务器，在写入的时候支持同步写入、异步写入两种方式。

- Broker元数据的写入，则是Broker从服务器通过单独的线程每隔10s从主Broker上获取，然后更新从的配置，并持久化到相应的配置文件中。

- RocketMQ主从同步一个重要的特征：主从同步不具备主从切换功能，即当主节点宕机后，从不会接管消息发送，但可以提供消息读取。

## 二、CommitLog消息复制

### 2.1、整体概述  CommitLog主从复制的流程如下：

1.Producer发送消息到Broker Master，Broker进行消息存储，并调用handleHA进行主从同步； 2.如果是同步复制的话，参考2.6章节的同步复制； 3.如果是异步复制的话，流程如下：

- Broker Master启动，并在指定端口监听；
- Broker Slave启动，主动连接Broker Master，通过Java NIO建立TCP连接；
- Broker Slave以每隔5s的间隔时间向服务端拉取消息，如果是第一次拉取的话，先获取本地CommitLog文件中最大的偏移量，以该偏移量向服务端拉取消息
- Broker Master 解析请求，并返回数据给Broker Slave；
5.Broker Slave收到一批消息后，将消息写入本地CommitLog文件中，然后向Master汇报拉取进度，并更新下一次待拉取偏移量；

我们先看下异步复制的整体流程，最后再看下同步复制的流程，异步复制的入口为HAService.start();

```java
public void start() throws Exception {
 //broker master启动，接收slave请求，并处理
    this.acceptSocketService.beginAccept();
    this.acceptSocketService.start();
 //同步复制线程启动
    this.groupTransferService.start();
 //broker slave启动
    this.haClient.start();
}

```

下面分别对上面的每一步做详细说明。

### 2.2、HAService Master启动

```java
public void beginAccept() throws Exception {
    this.serverSocketChannel = ServerSocketChannel.open();
    this.selector = RemotingUtil.openSelector();
    this.serverSocketChannel.socket().setReuseAddress(true);
    this.serverSocketChannel.socket().bind(this.socketAddressListen);
    this.serverSocketChannel.configureBlocking(false);
    this.serverSocketChannel.register(this.selector, SelectionKey.OP_ACCEPT);
}
```

在beginAccept方法中主要创建了ServerSocketChannel、Selector、设置TCP reuseAddress、绑定监听端口、设置为非阻塞模式，并注册OP_ACCEPT(连接事件)。可以看到在这里是通过Java原生的NIO来实现的，并没有通过Netty框架来实现。

acceptSocketService.start()启动方法代码如下：

```java
while (!this.isStopped()) {
    try {
   //获取事件
        this.selector.select(1000);
        Set<SelectionKey> selected = this.selector.selectedKeys();
        if (selected != null) {
            for (SelectionKey k : selected) {
//处理OP_ACCEPT事件，并创建HAConnection
                if ((k.readyOps() & SelectionKey.OP_ACCEPT) != 0) {
                    SocketChannel sc = ((ServerSocketChannel) k.channel()).accept();
                    if (sc != null) {
                       HAConnection conn = new HAConnection(HAService.this, sc);
                       //主要是启动readSocketService，writeSocketService这两个线程
 conn.start();
                       HAService.this.addConnection(conn);
                    }
                }
            }
            selected.clear();
        }
    } catch (Exception e) {
        log.error(this.getServiceName() + " service has exception.", e);
    }
}
```

选择器每1s处理一次处理一次连接就绪事件。连接事件就绪后，调用ServerSocketChannel的accept()方法创建SocketChannel，与服务端数据传输的通道。然后为每一个连接创建一个HAConnection对象，该HAConnection将负责Master-Slave数据同步逻辑。HAConnection.start方法如下：

```java
public void start() {
	this.readSocketService.start();
	this.writeSocketService.start();
}

```

### 2.3、HAClient启动

```java
while (!this.isStopped()) {
	try {
		//和broker master建立连接，通过java nio来实现
		if (this.connectMaster()) {
			//在心跳的同时，上报offset
			if (this.isTimeToReportOffset()) {
				//上报offset
				boolean result = this.reportSlaveMaxOffset(this.currentReportedOffset);
				if (!result) {
					this.closeMaster();
				}
			}
			this.selector.select(1000);
			//处理网络读请求，也就是处理从Master传回的消息数据
			boolean ok = this.processReadEvent();
			if (!ok) {
				this.closeMaster();
			}
			if (!reportSlaveMaxOffsetPlus()) {
				continue;
			}
			long interval =
				HAService.this.getDefaultMessageStore().getSystemClock().now()
					- this.lastWriteTimestamp;
			if (interval > HAService.this.getDefaultMessageStore().getMessageStoreConfig()
				.getHaHousekeepingInterval()) {
				log.warn("HAClient, housekeeping, found this connection[" + this.masterAddress
					+ "] expired, " + interval);
				this.closeMaster();
				log.warn("HAClient, master not response some time, so close connection");
			}
		} else {
			this.waitForRunning(1000 * 5);
		}
	} catch (Exception e) {
		log.warn(this.getServiceName() + " service has exception. ", e);
		this.waitForRunning(1000 * 5);
	}
}
```

#### 2.3.1、HAService主从建立连接

如果socketChannel为空，则尝试连接Master，如果Master地址为空，返回false。

```java
private boolean connectMaster() throws ClosedChannelException {
	if (null == socketChannel) {
		String addr = this.masterAddress.get();
		if (addr != null) {
			SocketAddress socketAddress = RemotingUtil.string2SocketAddress(addr);
			if (socketAddress != null) {
				this.socketChannel = RemotingUtil.connect(socketAddress);
				if (this.socketChannel != null) {
					//注册读事件，监听broker master返回的数据
					this.socketChannel.register(this.selector, SelectionKey.OP_READ);
				}
			}
		}
		//获取当前的offset
		this.currentReportedOffset = HAService.this.defaultMessageStore.getMaxPhyOffset();
		this.lastWriteTimestamp = System.currentTimeMillis();
	}
	return this.socketChannel != null;
}
```

-  Broker 主从连接
Broker Slave通过NIO来进行Broker Master连接，代码如下：

```java
SocketChannel sc = null;
sc = SocketChannel.open();
sc.configureBlocking(true);
sc.socket().setSoLinger(false, -1);
sc.socket().setTcpNoDelay(true);
sc.socket().setReceiveBufferSize(1024 * 64);
sc.socket().setSendBufferSize(1024 * 64);
sc.socket().connect(remote, timeoutMillis);
sc.configureBlocking(false);
```

-  Slave获取当前offset

```java
public long getMaxPhyOffset() {
	return this.commitLog.getMaxOffset();
}
public long getMaxOffset() {
	return this.mappedFileQueue.getMaxOffset();
}
public long getMaxOffset() {
	MappedFile mappedFile = getLastMappedFile();
	if (mappedFile != null) {
		return mappedFile.getFileFromOffset() + mappedFile.getReadPosition();
	}
	return 0;
}
```

可以看到最终还是通过读取MappedFile的position来获取从的offset。

#### 2.3.2、上报offset时间判断

```java
private boolean isTimeToReportOffset() {
	//当前时间-上次写的时间
	long interval =
		HAService.this.defaultMessageStore.getSystemClock().now() - this.lastWriteTimestamp;
	boolean needHeart = interval > HAService.this.defaultMessageStore.getMessageStoreConfig()
		.getHaSendHeartbeatInterval();

return needHeart;
}
```

判断逻辑为当前时间-上次写的时间>haSendHeartbeatInterval时，则进行心跳和offset的上报。haSendHeartbeatInterval默认为5s，可配置。

#### 2.3.3、上报offset

```java
private boolean reportSlaveMaxOffset(final long maxOffset) {
	this.reportOffset.position(0);
	this.reportOffset.limit(8);
	this.reportOffset.putLong(maxOffset);
	this.reportOffset.position(0);
	this.reportOffset.limit(8);
	//最多发送三次，reportOffset是否有剩余
	for (int i = 0; i < 3 && this.reportOffset.hasRemaining(); i++) {
		try {
			this.socketChannel.write(this.reportOffset);
		} catch (IOException e) {
			log.error(this.getServiceName()
				+ "reportSlaveMaxOffset this.socketChannel.write exception", e);
			return false;
		}
	}
	return !this.reportOffset.hasRemaining();
}
```

主要还是通过NIO发送请求。

### 2.4、Broker Master处理请求

在主从建立连接时创建了HAConnection对象，该对象主要包含了如下两个重要的线程服务类：

```java
//负责写，将commitlog数据发送到从
private WriteSocketService writeSocketService;
//负责读，读取从上报的offset，并根据offset从Broker Master读取commitlog
private ReadSocketService readSocketService;
```

#### 2.4.1、ReadSocketService接收读请求

readSocketService.run方法如下：

```java
while (!this.isStopped()) {
	try {
		this.selector.select(1000);
		//处理读事件
		boolean ok = this.processReadEvent();
		if (!ok) {
			HAConnection.log.error("processReadEvent error");
			break;
		}
		long interval = HAConnection.this.haService.getDefaultMessageStore().getSystemClock().now() - this.lastReadTimestamp;
		if (interval > HAConnection.this.haService.getDefaultMessageStore().getMessageStoreConfig().getHaHousekeepingInterval()) {
			log.warn("ha housekeeping, found this connection[" + HAConnection.this.clientAddr + "] expired, " + interval);
			break;
		}
	} catch (Exception e) {
		HAConnection.log.error(this.getServiceName() + " service has exception.", e);
		break;
	}
}
```

processReadEvent的逻辑如下：

```java
int readSize = this.socketChannel.read(this.byteBufferRead);
if (readSize > 0) {
	readSizeZeroTimes = 0;
	this.lastReadTimestamp = HAConnection.this.haService.getDefaultMessageStore().getSystemClock().now();
	if ((this.byteBufferRead.position() - this.processPostion) >= 8) {
		int pos = this.byteBufferRead.position() - (this.byteBufferRead.position() % 8);
		//获取slave 请求的offset
		long readOffset = this.byteBufferRead.getLong(pos - 8);
		this.processPostion = pos;

HAConnection.this.slaveAckOffset = readOffset;
		if (HAConnection.this.slaveRequestOffset < 0) {
			HAConnection.this.slaveRequestOffset = readOffset;
			log.info("slave[" + HAConnection.this.clientAddr + "] request offset " + readOffset);
		}
		//如果是同步复制的话，判断请求的offset是否push2SlaveMaxOffset相同，相同的话则唤醒master GroupTransferService
		HAConnection.this.haService.notifyTransferSome(HAConnection.this.slaveAckOffset);
	}
}
```

可以看到processReadEvent逻辑很简单，就是从ByteBuffer中解析出offset，然后设置HAConnection.this.slaveRequestOffset；

#### 2.4.2、WriteSocketService进行写处理

Broker Master通过HAConnection.WriteSocketService进行CommitLog的读取，run方法主逻辑如下：

```java
this.selector.select(1000);
//nextTransferFromWhere下次传输commitLog的起始位置
if (-1 == this.nextTransferFromWhere) {
	if (0 == HAConnection.this.slaveRequestOffset) {
		long masterOffset = HAConnection.this.haService.getDefaultMessageStore().getCommitLog().getMaxOffset();
		masterOffset =
			masterOffset
				- (masterOffset % HAConnection.this.haService.getDefaultMessageStore().getMessageStoreConfig()
				.getMapedFileSizeCommitLog());

if (masterOffset < 0) {
			masterOffset = 0;
		}

this.nextTransferFromWhere = masterOffset;
	} else {
		this.nextTransferFromWhere = HAConnection.this.slaveRequestOffset;
	}

log.info("master transfer data from " + this.nextTransferFromWhere + " to slave[" + HAConnection.this.clientAddr
		+ "], and slave request " + HAConnection.this.slaveRequestOffset);
}

//获取commitLog数据
SelectMappedBufferResult selectResult = HAConnection.this.haService.getDefaultMessageStore().getCommitLogData(this.nextTransferFromWhere);
//获取commitLog数据
SelectMappedBufferResult selectResult =
	HAConnection.this.haService.getDefaultMessageStore().getCommitLogData(this.nextTransferFromWhere);
if (selectResult != null) {
	int size = selectResult.getSize();
	if (size > HAConnection.this.haService.getDefaultMessageStore().getMessageStoreConfig().getHaTransferBatchSize()) {
		size = HAConnection.this.haService.getDefaultMessageStore().getMessageStoreConfig().getHaTransferBatchSize();
	}

long thisOffset = this.nextTransferFromWhere;
	this.nextTransferFromWhere += size;

selectResult.getByteBuffer().limit(size);
	this.selectMappedBufferResult = selectResult;

// Build Header
	this.byteBufferHeader.position(0);
	this.byteBufferHeader.limit(headerSize);
	this.byteBufferHeader.putLong(thisOffset);
	this.byteBufferHeader.putInt(size);
	this.byteBufferHeader.flip();
	//nio发送commitlog
	this.lastWriteOver = this.transferData();
} else {
    //如果没有获取到commitLog数据，等待100ms
	HAConnection.this.haService.getWaitNotifyObject().allWaitForRunning(100);
}
```

这里面主要包括获取CommitLog数据、发送CommitLog数据这两个步骤。

##### 2.4.2.1、获取CommitLog数据

```java
public SelectMappedBufferResult getData(final long offset, final boolean returnFirstOnNotFound) {
	int mappedFileSize = this.defaultMessageStore.getMessageStoreConfig().getMapedFileSizeCommitLog();
	MappedFile mappedFile = this.mappedFileQueue.findMappedFileByOffset(offset, returnFirstOnNotFound);
	if (mappedFile != null) {
		int pos = (int) (offset % mappedFileSize);
		SelectMappedBufferResult result = mappedFile.selectMappedBuffer(pos);
		return result;
	}
	return null;
}
public SelectMappedBufferResult selectMappedBuffer(int pos) {
	int readPosition = getReadPosition();
	if (pos < readPosition && pos >= 0) {
		if (this.hold()) {
			ByteBuffer byteBuffer = this.mappedByteBuffer.slice();
			byteBuffer.position(pos);
			int size = readPosition - pos;
			ByteBuffer byteBufferNew = byteBuffer.slice();
			byteBufferNew.limit(size);
			return new SelectMappedBufferResult(this.fileFromOffset + pos, byteBufferNew, size, this);
		}
	}
	return null;
}
```

可以看到最终还是根据offset从MappedFile读取数据。

##### 2.4.2.2、发送CommitLog数据

数据主要包括header、body两部分，数据发送的话还是通过NIO来实现，主要代码如下：

```java
// Build Header
this.byteBufferHeader.position(0);
this.byteBufferHeader.limit(headerSize);
this.byteBufferHeader.putLong(thisOffset);
this.byteBufferHeader.putInt(size);
this.byteBufferHeader.flip();

int writeSize = this.socketChannel.write(this.byteBufferHeader);
// Write Body
if (!this.byteBufferHeader.hasRemaining()) {
	while (this.selectMappedBufferResult.getByteBuffer().hasRemaining()) {
		int writeSize = this.socketChannel.write(this.selectMappedBufferResult.getByteBuffer());
		if (writeSize > 0) {
			writeSizeZeroTimes = 0;
			this.lastWriteTimestamp = HAConnection.this.haService.getDefaultMessageStore().getSystemClock().now();
		} else if (writeSize == 0) {
			if (++writeSizeZeroTimes >= 3) {
				break;
			}
		} else {
			throw new Exception("ha master write body error < 0");
		}
	}
}
```

CommitLog主从发送完成后，Broker Slave则会监听读事件、获取CommitLog数据，并进行CommitLog的写入。

### 2.5、HAClient processReadEvent

在主从建立连接后，从注册了可读事件，目的就是读取从Broker Master返回的CommitLog数据，对应的方法为HAClient.processReadEvent：

```java
int readSize = this.socketChannel.read(this.byteBufferRead);
if (readSize > 0) {
	lastWriteTimestamp = HAService.this.defaultMessageStore.getSystemClock().now();
	readSizeZeroTimes = 0;
	boolean result = this.dispatchReadRequest();
	if (!result) {
		log.error("HAClient, dispatchReadRequest error");
		return false;
	}
} 
```

dispatchReadRequest方法如下：

```java
 //读取返回的body data
byte[] bodyData = new byte[bodySize];
this.byteBufferRead.position(this.dispatchPostion + msgHeaderSize);
this.byteBufferRead.get(bodyData);

HAService.this.defaultMessageStore.appendToCommitLog(masterPhyOffset, bodyData);

this.byteBufferRead.position(readSocketPos);
this.dispatchPostion += msgHeaderSize + bodySize;

//上报从的offset
if (!reportSlaveMaxOffsetPlus()) {
	return false;
}
```

里面的核心逻辑主要包括如下三个步骤：

1. 从byteBufferRead中读取CommitLog数据；

3. 调用defaultMessageStore.appendToCommitLog方法，将数据写入到MappedFile文件，写入方法如下：

```java
public boolean appendToCommitLog(long startOffset, byte[] data) {
	//将数据写到commitlog，同普通消息的存储
	boolean result = this.commitLog.appendData(startOffset, data);
	//唤醒reputMessageService，构建consumeQueue,index
	this.reputMessageService.wakeup();
	return result;
}
```

上报从新的offset，也是读取MappedFile的offset，然后上报Broker Master；

### 2.6、同步复制

上面主要介绍了Broker的异步复制，下面再来看下Broker的同步复制的实现。同步复制的整体流程图如下：

![](//img1.jcloudcs.com/developer.jdcloud.com/cb5fd841-4623-4115-90cf-b488275305ae20230316203319.jpg)

大概说明如下：

1. producer发送消息到broker，broker进行消息的存储，将消息写入到commitLog；

3. broker master写消息线程唤醒WriteSocketService线程，查询commitLog数据，然后发送到从。在WriteSocketService获取commitLog时，如果没有获取到commitLog数据，会等待100ms。所以当commitLog新写入数据的时候，会唤醒WriteSocketService，然后查询commitLog数据，发送到从。

5. broker master创建GroupCommitRequest，同步等待主从复制完成；

7. 从接受新的commitLog数据，然后写commitLog数据，并返回新的slave offset到主；

9. 主更新push2SlaveMaxOffset，并判断push2SlaveMaxOffset是否大于等于主从复制请求的offset，如果大于等于的话，则认为主从复制完成，返回commitLog.handleHA方法成功，从而返回消息保存成功。

对应的代码入口为CommitLog.handleHA方法。

```java
public void handleHA(AppendMessageResult result, PutMessageResult putMessageResult, MessageExt messageExt) {
	//如果是broker主，并且是同步复制的话
	if (BrokerRole.SYNC_MASTER == this.defaultMessageStore.getMessageStoreConfig().getBrokerRole()) {
		//获取HAService
		HAService service = this.defaultMessageStore.getHaService();
		//获取Message上的MessageConst.PROPERTY_WAIT_STORE_MSG_OK，默认是需要等待主从复制完成
		if (messageExt.isWaitStoreMsgOK()) {
			/**
			 * 判断从是否可用，判断的逻辑是：（主offset-push2SlaveMaxOffset<1024 * 1024 * 256），也就是如果主从的offset差的太多，
			 * 则认为从不可用， Tell the producer, slave not available
			 * 这里的result = mappedFile.appendMessage(msg, this.appendMessageCallback);
			 */
			if (service.isSlaveOK(result.getWroteOffset() + result.getWroteBytes())) {
				//组装GroupCommitRequest，nextOffset=result.getWroteOffset() + result.getWroteBytes()，这里的nextOffset指的就是从要写到的offset
				GroupCommitRequest request = new GroupCommitRequest(result.getWroteOffset() + result.getWroteBytes());
				                    /**
                * 调用的是this.groupTransferService.putRequest(request);将request放到requestsWrite list中。
                  * HAService持有GroupTransferService groupTransferService引用;
                */
				service.putRequest(request);
				 /**
                     * 唤醒的是WriteSocketService，查询commitLog数据，然后发送到从。
                     * 在WriteSocketService获取commitLog时，如果没有获取到commitLog数据，等待100ms
                     * HAConnection.this.haService.getWaitNotifyObject().allWaitForRunning(100);
                     * 所以当commitLog新写入数据的时候，会唤醒WriteSocketService，然后查询commitLog数据，发送到从。
                     */
				service.getWaitNotifyObject().wakeupAll();

//等待同步复制完成，判断逻辑是： HAService.this.push2SlaveMaxOffset.get() >= req.getNextOffset();
				boolean flushOK =
					request.waitForFlush(this.defaultMessageStore.getMessageStoreConfig().getSyncFlushTimeout());

//如果同步复制失败的话，设置putMessageResult中的状态为同步从超时
				if (!flushOK) {
					log.error("do sync transfer other node, wait return, but failed, topic: " + messageExt.getTopic() + " tags: "
						+ messageExt.getTags() + " client address: " + messageExt.getBornHostNameString());
					putMessageResult.setPutMessageStatus(PutMessageStatus.FLUSH_SLAVE_TIMEOUT);
				}
			}
			// Slave problem
			else {
				// Tell the producer, slave not available
				putMessageResult.setPutMessageStatus(PutMessageStatus.SLAVE_NOT_AVAILABLE);
			}
		}
	}
}
```

#### 2.6.1、GroupTransferService启动

在HAService启动的时候，启动了GroupTransferService线程，代码如下：

```java
public void run() {
	while (!this.isStopped()) {
		this.waitForRunning(10);
		this.doWaitTransfer();
	}
}
private void doWaitTransfer() {
	synchronized (this.requestsRead) {
		if (!this.requestsRead.isEmpty()) {
			for (CommitLog.GroupCommitRequest req : this.requestsRead) {
				/**
				 * req.getNextOffset：result.getWroteOffset() + result.getWroteBytes()
				 * push2SlaveMaxOffset：
				 */
				boolean transferOK = HAService.this.push2SlaveMaxOffset.get() >= req.getNextOffset();
				//在这循环5次，最多等待5s，因为slave 心跳间隔默认5s
				for (int i = 0; !transferOK && i < 5; i++) {
					this.notifyTransferObject.waitForRunning(1000);
					transferOK = HAService.this.push2SlaveMaxOffset.get() >= req.getNextOffset();
				}
				if (!transferOK) {
					log.warn("transfer messsage to slave timeout, " + req.getNextOffset());
				}
				//主从复制完成，唤醒handleHA后续操作
                req.wakeupCustomer(transferOK);
			}
			this.requestsRead.clear();
		}
	}
}

```

wakeupCustomer：

```java
public void wakeupCustomer(final boolean flushOK) {
    this.flushOK = flushOK;
    this.countDownLatch.countDown();
}
```

#### 2.6.2、唤醒WriteSocketService

service.getWaitNotifyObject().wakeupAll();

唤醒的是WriteSocketService，查询commitLog数据，然后发送到从。在WriteSocketService获取commitLog时，如果没有获取到commitLog数据，等待100ms。HAConnection.this.haService.getWaitNotifyObject().allWaitForRunning(100);所以当commitLog新写入数据的时候，会唤醒WriteSocketService，然后查询commitLog数据，发送到从。

#### 2.6.3、同步等待，直到复制完成

```java
boolean flushOK =
	request.waitForFlush(this.defaultMessageStore.getMessageStoreConfig().getSyncFlushTimeout());

public boolean waitForFlush(long timeout) {
	try {
		//等待同步复制完成
		this.countDownLatch.await(timeout, TimeUnit.MILLISECONDS);
		return this.flushOK;
	} catch (InterruptedException e) {
		log.error("Interrupted", e);
		return false;
	}
}
}
```

## 三、元数据的复制

broker元数据的复制，主要包括topicConfig、consumerOffset、delayOffset、subscriptionGroup这几部分，整体流程图如下：

![](//img1.jcloudcs.com/developer.jdcloud.com/07305543-c37a-40bd-9ba7-53f1c70ba29220230316203554.png)

从broker通过单独的线程，每隔10s进行一次元数据的复制 ，代码入口为：BrokerController.start -> SlaveSynchronize.syncAll：

```java
slaveSyncFuture = this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {
    @Override
    public void run() {
        try {
            //10s 进行一次主从同步
            BrokerController.this.slaveSynchronize.syncAll();
        }
        catch (Throwable e) {
            log.error("ScheduledTask SlaveSynchronize syncAll error.", e);
        }
    }
}, 1000 * 3, 1000 * 10, TimeUnit.MILLISECONDS);

public void syncAll() {
    this.syncTopicConfig();
    this.syncConsumerOffset();
    this.syncDelayOffset();
    this.syncSubscriptionGroupConfig();
}
```

### 3.1、syncTopicConfig

```java
//从Master获取TopicConfig信息，最终调用的是AdminBrokerProcessor.getAllTopicConfig
TopicConfigSerializeWrapper topicWrapper =
    this.brokerController.getBrokerOuterAPI().getAllTopicConfig(masterAddrBak);
if (!this.brokerController.getTopicConfigManager().getDataVersion()
    .equals(topicWrapper.getDataVersion())) {
    this.brokerController.getTopicConfigManager().getDataVersion()
        .assignNewOne(topicWrapper.getDataVersion());
    this.brokerController.getTopicConfigManager().getTopicConfigTable().clear();
    this.brokerController.getTopicConfigManager().getTopicConfigTable()
        .putAll(topicWrapper.getTopicConfigTable());
 //将topicConfig进行持久化，对应的文件为topics.json
    this.brokerController.getTopicConfigManager().persist();
    log.info("Update slave topic config from master, {}", masterAddrBak)
```

### 3.2、syncConsumerOffset

```java
//从"主Broker"获取ConsumerOffset
ConsumerOffsetSerializeWrapper offsetWrapper =
        this.brokerController.getBrokerOuterAPI().getAllConsumerOffset(masterAddrBak);
//设置从的offsetTable
this.brokerController.getConsumerOffsetManager().getOffsetTable()
                    .putAll(offsetWrapper.getOffsetTable());
//并持久化到从的consumerOffset.json文件中
this.brokerController.getConsumerOffsetManager().persist(); 
```

### 3.3、syncDelayOffset

```java
String delayOffset = this.brokerController.getBrokerOuterAPI().getAllDelayOffset(masterAddrBak);
String fileName = StorePathConfigHelper.getDelayOffsetStorePath(this.brokerController
.getMessageStoreConfig().getStorePathRootDir());
 MixAll.string2File(delayOffset, fileName);
```

### 3.4、syncSubscriptionGroupConfig

```java
SubscriptionGroupWrapper subscriptionWrapper =this.brokerController.getBrokerOuterAPI().getAllSubscriptionGroupConfig(masterAddrBak);
SubscriptionGroupManager subscriptionGroupManager =this.brokerController.getSubscriptionGroupManager();
subscriptionGroupManager.getDataVersion().assignNewOne(subscriptionWrapper.getDataVersion());
subscriptionGroupManager.getSubscriptionGroupTable().clear();
subscriptionGroupManager.getSubscriptionGroupTable().putAll(subscriptionWrapper.getSubscriptionGroupTable());
subscriptionGroupManager.persist();
```

## 四、思考与收获

通过上面的分享，我们基本上了解了RocketMQ的主从复制原理，其中有些思想我们可以后续借鉴下：

1. 在功能设计的时候将元数据、程序数据分开管理；

3. 主从复制的时候，基本思想都是从请求主，请求时带上offset，然后主查询数据返回从，从再执行；mysql的主从复制、redis的主从复制基本也是这样；

5. 主从复制包括异步复制、同步复制两种方式，可以通过配置来决定使用哪种同步方式，这个需要根据实际业务场景来决定；

7. 主从复制线程尽量和消息写线程或者主线程分开；

由于时间、精力有限，难免会有纰漏、考虑不到之处，如有问题欢迎沟通、交流。