当broker为slave且有master的情况下，通过HAService，slave从master同步commitlog数据，并通过ReputMessageService异步构建consumequeue。通过SlaveSynchronize定时每分钟从master同步config目录下的四个文件。默认，master监听broker端口+1。
2. 启动
slave可以通过配置指定master地址，也可以从namesrv获取master地址。broker启动，在BrokerController#initialize方法：
if (BrokerRole.SLAVE == this.messageStoreConfig.getBrokerRole()) {
if (this.messageStoreConfig.getHaMasterAddress() != null && this.messageStoreConfig.getHaMasterAddress().length() >= 6) {
this.messageStore.updateHaMasterAddress(this.messageStoreConfig.getHaMasterAddress());
this.updateMasterHAServerAddrPeriodically = false;
} else {
this.updateMasterHAServerAddrPeriodically = true;
}
this.scheduledExecutorService.scheduleAtFixedRate(new Runnable() {
@Override
public void run() {
try {
BrokerController.this.slaveSynchronize.syncAll();
} catch (Throwable e) {
log.error("ScheduledTask syncAll slave exception", e);
}
}
}, 1000 * 10, 1000 * 60, TimeUnit.MILLISECONDS);
}
如果未指定master address，则updateMasterHAServerAddrPeriodically = true：
if (registerBrokerResult != null) {
if (this.updateMasterHAServerAddrPeriodically && registerBrokerResult.getHaServerAddr() != null) {
this.messageStore.updateHaMasterAddress(registerBrokerResult.getHaServerAddr());
}
this.slaveSynchronize.setMasterAddr(registerBrokerResult.getMasterAddr());
if (checkOrderConfig) {
this.getTopicConfigManager().updateOrderTopicConfig(registerBrokerResult.getKvTable());
}
}
HAService的内部类HAClient负责连接master，从其run方法可以看出，当master address不为空，且当前broker为slave，就尝试连接master：
public void run() {
log.info(this.getServiceName() + " service started");
while (!this.isStoped()) {
try {
if (this.connectMaster()) {
if (this.isTimeToReportOffset()) {
boolean result = this.reportSlaveMaxOffset(this.currentReportedOffset);
if (!result) {
this.closeMaster();
}
}
this.selector.select(1000);
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);
}
}
log.info(this.getServiceName() + " service end");
}
slave将commitlog的最大offset上报master，如果为首次连接，那么offset为0， master收到为0的offset后，从其最大offset开始同步，否则从指定的offset开始同步数据给slave，每次最大同步32k数据。
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);
}
SelectMapedBufferResult 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.selectMapedBufferResult = selectResult;
// Build Header
this.byteBufferHeader.position(0);
this.byteBufferHeader.limit(HEADER_SIZE);
this.byteBufferHeader.putLong(thisOffset);
this.byteBufferHeader.putInt(size);
this.byteBufferHeader.flip();
this.lastWriteOver = this.transferData();
} else {
HAConnection.this.haService.getWaitNotifyObject().allWaitForRunning(100);
}
slave收到master消息后，存储到commitlog然后立即上报ackOffset。
if (diff >= (MSG_HEADER_SIZE + bodySize)) {
byte[] bodyData = new byte[bodySize];
this.byteBufferRead.position(this.dispatchPostion + MSG_HEADER_SIZE);
this.byteBufferRead.get(bodyData);
HAService.this.defaultMessageStore.appendToCommitLog(masterPhyOffset, bodyData);
this.byteBufferRead.position(readSocketPos);
this.dispatchPostion += MSG_HEADER_SIZE + bodySize;
if (!reportSlaveMaxOffsetPlus()) {
return false;
}
continue;
}
HAService的内部类AcceptSocketService作为master broker的监听服务，HAConnection的内部类ReadSocketService负责处理slave的offset，WriteSocketService将commitlog发送给slave。slave和master之间每5s发送心跳维护连接。
3. 同步复制和异步复制
从commitlog的putMessage方法可以看出：
if (BrokerRole.SYNC_MASTER == this.defaultMessageStore.getMessageStoreConfig().getBrokerRole()) {
HAService service = this.defaultMessageStore.getHaService();
if (msg.isWaitStoreMsgOK()) {
// Determine whether to wait
if (service.isSlaveOK(result.getWroteOffset() + result.getWroteBytes())) {
if (null == request) {
request = new GroupCommitRequest(result.getWroteOffset() + result.getWroteBytes());
}
service.putRequest(request);
service.getWaitNotifyObject().wakeupAll();
boolean flushOK =
// TODO
request.waitForFlush(this.defaultMessageStore.getMessageStoreConfig().getSyncFlushTimeout());
if (!flushOK) {
log.error("do sync transfer other node, wait return, but failed, topic: " + msg.getTopic() + " tags: "
+ msg.getTags() + " client address: " + msg.getBornHostString());
putMessageResult.setPutMessageStatus(PutMessageStatus.FLUSH_SLAVE_TIMEOUT);
}
}
// Slave problem
else {
// Tell the producer, slave not available
putMessageResult.setPutMessageStatus(PutMessageStatus.SLAVE_NOT_AVAILABLE);
}
}
}
如果为同步复制，当slave落后master 256M数据后，直接报SLAVE_NOT_AVAILABLE异常。这将导致producer内部发送失败，而broker是在将数据写入commitlog文件后才同步给slave，所以可能会在业务逻辑中遇到这种异常而重发导致重复数据产生。
所以，如果将MQ部署为MS结构，最好选择异步复制的方式。