摘要：发送消息阶段，不允许发送重复的消息。虽然不能严格保证不重复，但是正常情况下很少会出现重复发送消费情况，只有网络异常，启停等异常情况下会出现消息重复。
问题列表
Broker 怎么响应Consumer请求?
Broker 怎么维护ConsumeQueue?
Broker 怎么处理事务消息的 ConsumeQueue ?
Broker 怎么处理定时消息的 ConsumeQueue?
Broker 怎么处理回溯消费请求?
Broker 的消息是 at least once还是exactly only once?
怎么响应Consumer请求?
原理:
如上图所示,RocketMQ将所有消息都放在CommitLog里面,消费是维护一个ConsumeQueue帮助Consumer消费.pull操作要读两次,先读ConsumeQueue得到offset，再读CommitLog得到消息内容.
ConsumeQueue有一个长度20的ByteBufferb变量byteBufferIndex,里面维护者消息偏移量,消息发现,tags的hashcode
//消息偏移量
this.byteBufferIndex.putLong(offset);
//消息大小
this.byteBufferIndex.putInt(size);
//tags的HashCode
this.byteBufferIndex.putLong(tagsCode);
BrokerController.initialize方法中会注册PullMessageProcessor来处理pull message 请求
this.remotingServer.registerProcessor(RequestCode.PULL_MESSAGE, this.pullMessageProcessor, this.pullMessageExecutor);
this.pullMessageProcessor.registerConsumeMessageHook(consumeMessageHookList);
在PullMessageProcessor.processRequest中又委托给DefaultMessageStore获取
final GetMessageResult getMessageResult =
this.brokerController.getMessageStore().getMessage(requestHeader.getConsumerGroup(), requestHeader.getTopic(),
requestHeader.getQueueId(), requestHeader.getQueueOffset(), requestHeader.getMaxMsgNums(), messageFilter);
DefaultMessageStore.getMessage
public GetMessageResult getMessage(final String group, final String topic, final int queueId, final long offset, final int maxMsgNums,
final MessageFilter messageFilter) {
if (this.shutdown) {
log.warn("message store has shutdown, so getMessage is forbidden");
return null;
}
if (!this.runningFlags.isReadable()) {
log.warn("message store is not readable, so getMessage is forbidden " + this.runningFlags.getFlagBits());
return null;
}
long beginTime = this.getSystemClock().now();
GetMessageStatus status = GetMessageStatus.NO_MESSAGE_IN_QUEUE;
long nextBeginOffset = offset;
long minOffset = 0;
long maxOffset = 0;
GetMessageResult getResult = new GetMessageResult();
final long maxOffsetPy = this.commitLog.getMaxOffset();
//根据topic和queueId找到ConsumeQueue
ConsumeQueue consumeQueue = findConsumeQueue(topic, queueId);
if (consumeQueue != null) {
minOffset = consumeQueue.getMinOffsetInQueue();
maxOffset = consumeQueue.getMaxOffsetInQueue();
if (maxOffset == 0) {
status = GetMessageStatus.NO_MESSAGE_IN_QUEUE;
nextBeginOffset = nextOffsetCorrection(offset, 0);
} else if (offset < minOffset) {
status = GetMessageStatus.OFFSET_TOO_SMALL;
nextBeginOffset = nextOffsetCorrection(offset, minOffset);
} else if (offset == maxOffset) {
status = GetMessageStatus.OFFSET_OVERFLOW_ONE;
nextBeginOffset = nextOffsetCorrection(offset, offset);
} else if (offset > maxOffset) {
status = GetMessageStatus.OFFSET_OVERFLOW_BADLY;
if (0 == minOffset) {
nextBeginOffset = nextOffsetCorrection(offset, minOffset);
} else {
nextBeginOffset = nextOffsetCorrection(offset, maxOffset);
}
} else {
//前面都在处理异常,这里开始真正获取
//获取consumeQueue offset之后所有可读的offset,consumeQueue也是文件存储
SelectMappedBufferResult bufferConsumeQueue = consumeQueue.getIndexBuffer(offset);
if (bufferConsumeQueue != null) {
try {
status = GetMessageStatus.NO_MATCHED_MESSAGE;
long nextPhyFileStartOffset = Long.MIN_VALUE;
long maxPhyOffsetPulling = 0;
int i = 0;
final int maxFilterMessageCount = Math.max(16000, maxMsgNums * ConsumeQueue.CQ_STORE_UNIT_SIZE);
final boolean diskFallRecorded = this.messageStoreConfig.isDiskFallRecorded();
ConsumeQueueExt.CqExtUnit cqExtUnit = new ConsumeQueueExt.CqExtUnit();
//循环单个获取
for (; i < bufferConsumeQueue.getSize() && i < maxFilterMessageCount; i += ConsumeQueue.CQ_STORE_UNIT_SIZE) {
long offsetPy = bufferConsumeQueue.getByteBuffer().getLong();
int sizePy = bufferConsumeQueue.getByteBuffer().getInt();
long tagsCode = bufferConsumeQueue.getByteBuffer().getLong();
maxPhyOffsetPulling = offsetPy;
if (nextPhyFileStartOffset != Long.MIN_VALUE) {
if (offsetPy < nextPhyFileStartOffset)
continue;
}
boolean isInDisk = checkInDiskByCommitOffset(offsetPy, maxOffsetPy);
if (this.isTheBatchFull(sizePy, maxMsgNums, getResult.getBufferTotalSize(), getResult.getMessageCount(),
isInDisk)) {
break;
}
boolean extRet = false;
if (consumeQueue.isExtAddr(tagsCode)) {
extRet = consumeQueue.getExt(tagsCode, cqExtUnit);
if (extRet) {
tagsCode = cqExtUnit.getTagsCode();
} else {
// can"t find ext content.Client will filter messages by tag also.
log.error("[BUG] can"t find consume queue extend file content!addr={}, offsetPy={}, sizePy={}, topic={}, group={}",
tagsCode, offsetPy, sizePy, topic, group);
}
}
if (messageFilter != null
&& !messageFilter.isMatchedByConsumeQueue(tagsCode, extRet ? cqExtUnit : null)) {
if (getResult.getBufferTotalSize() == 0) {
status = GetMessageStatus.NO_MATCHED_MESSAGE;
}
continue;
}
//根据偏移量和大小从CommitLog获取消息
SelectMappedBufferResult selectResult = this.commitLog.getMessage(offsetPy, sizePy);
if (null == selectResult) {
if (getResult.getBufferTotalSize() == 0) {
status = GetMessageStatus.MESSAGE_WAS_REMOVING;
}
nextPhyFileStartOffset = this.commitLog.rollNextFile(offsetPy);
continue;
}
if (messageFilter != null
&& !messageFilter.isMatchedByCommitLog(selectResult.getByteBuffer().slice(), null)) {
if (getResult.getBufferTotalSize() == 0) {
status = GetMessageStatus.NO_MATCHED_MESSAGE;
}
// release...
selectResult.release();
continue;
}
this.storeStatsService.getGetMessageTransferedMsgCount().incrementAndGet();
//将查到的消息放入返回值
getResult.addMessage(selectResult);
status = GetMessageStatus.FOUND;
nextPhyFileStartOffset = Long.MIN_VALUE;
}
if (diskFallRecorded) {
long fallBehind = maxOffsetPy - maxPhyOffsetPulling;
brokerStatsManager.recordDiskFallBehindSize(group, topic, queueId, fallBehind);
}
nextBeginOffset = offset + (i / ConsumeQueue.CQ_STORE_UNIT_SIZE);
long diff = maxOffsetPy - maxPhyOffsetPulling;
long memory = (long) (StoreUtil.TOTAL_PHYSICAL_MEMORY_SIZE
* (this.messageStoreConfig.getAccessMessageInMemoryMaxRatio() / 100.0));
getResult.setSuggestPullingFromSlave(diff > memory);
} finally {
bufferConsumeQueue.release();
}
} else {
status = GetMessageStatus.OFFSET_FOUND_NULL;
nextBeginOffset = nextOffsetCorrection(offset, consumeQueue.rollNextFile(offset));
log.warn("consumer request topic: " + topic + "offset: " + offset + " minOffset: " + minOffset + " maxOffset: "
+ maxOffset + ", but access logic queue failed.");
}
}
} else {
status = GetMessageStatus.NO_MATCHED_LOGIC_QUEUE;
nextBeginOffset = nextOffsetCorrection(offset, 0);
}
if (GetMessageStatus.FOUND == status) {
this.storeStatsService.getGetMessageTimesTotalFound().incrementAndGet();
} else {
this.storeStatsService.getGetMessageTimesTotalMiss().incrementAndGet();
}
long eclipseTime = this.getSystemClock().now() - beginTime;
this.storeStatsService.setGetMessageEntireTimeMax(eclipseTime);
getResult.setStatus(status);
getResult.setNextBeginOffset(nextBeginOffset);
getResult.setMaxOffset(maxOffset);
getResult.setMinOffset(minOffset);
return getResult;
}
CommitLog.getMessage
public SelectMappedBufferResult getMessage(final long offset, final int size) {
int mappedFileSize = this.defaultMessageStore.getMessageStoreConfig().getMapedFileSizeCommitLog();
//根据offset获取到映射文件
MappedFile mappedFile = this.mappedFileQueue.findMappedFileByOffset(offset, offset == 0);
if (mappedFile != null) {
int pos = (int) (offset % mappedFileSize);
//从映射文件中获取指定位置的数据
return mappedFile.selectMappedBuffer(pos, size);
}
return null;
}
MappedFile.selectMappedBuffer
public SelectMappedBufferResult selectMappedBuffer(int pos, int size) {
int readPosition = getReadPosition();
if ((pos + size) <= readPosition) {
if (this.hold()) {
//Java NIO 获取指定pos数据
ByteBuffer byteBuffer = this.mappedByteBuffer.slice();
byteBuffer.position(pos);
ByteBuffer byteBufferNew = byteBuffer.slice();
byteBufferNew.limit(size);
return new SelectMappedBufferResult(this.fileFromOffset + pos, byteBufferNew, size, this);
} else {
log.warn("matched, but hold failed, request pos: " + pos + ", fileFromOffset: "
+ this.fileFromOffset);
}
} else {
log.warn("selectMappedBuffer request pos invalid, request pos: " + pos + ", size: " + size
+ ", fileFromOffset: " + this.fileFromOffset);
}
return null;
}
消息获取流程明白了,看下怎么获取ConsumeQueue,它是维护在一个以queueId为key的ConcurrentMap中,没有就新建一个.
public ConsumeQueue findConsumeQueue(String topic, int queueId) {
ConcurrentMap map = consumeQueueTable.get(topic);
if (null == map) {
ConcurrentMap newMap = new ConcurrentHashMap(128);
ConcurrentMap oldMap = consumeQueueTable.putIfAbsent(topic, newMap);
if (oldMap != null) {
map = oldMap;
} else {
map = newMap;
}
}
ConsumeQueue logic = map.get(queueId);
if (null == logic) {
//m
ConsumeQueue newLogic = new ConsumeQueue(//
topic, //
queueId, //
StorePathConfigHelper.getStorePathConsumeQueue(this.messageStoreConfig.getStorePathRootDir()), //
this.getMessageStoreConfig().getMapedFileSizeConsumeQueue(), //
this);
ConsumeQueue oldLogic = map.putIfAbsent(queueId, newLogic);
if (oldLogic != null) {
logic = oldLogic;
} else {
logic = newLogic;
}
}
return logic;
}
Broker 怎么维护ConsumeQueue?
很容易的想到,是在存放Message的时候维护了ConsumeQueue.但是相关代码没找到,我们从顶层往上层找
CosumeQueue.putMessagePositionInfo
-> CosumeQueue.putMessagePositionInfoWrapper
-> DefaultMessageStore.putMessagePositionInfo
-> CommitLogDispatcherBuildConsumeQueue.dispatch
-> DefaultMessageStore.doDispatch
-> ReputMessageService.doReput
private void doReput() {
for (boolean doNext = true; this.isCommitLogAvailable() && doNext; ) {
if (DefaultMessageStore.this.getMessageStoreConfig().isDuplicationEnable() //
&& this.reputFromOffset >= DefaultMessageStore.this.getConfirmOffset()) {
break;
}
//从CommitLog中读取上次偏移量之后的新消息
SelectMappedBufferResult result = DefaultMessageStore.this.commitLog.getData(reputFromOffset);
if (result != null) {
try {
this.reputFromOffset = result.getStartOffset();
//顺序读
for (int readSize = 0; readSize < result.getSize() && doNext; ) {
//构建请求去维护consumeQueue的请求
DispatchRequest dispatchRequest =
DefaultMessageStore.this.commitLog.checkMessageAndReturnSize(result.getByteBuffer(), false, false);
int size = dispatchRequest.getMsgSize();
if (dispatchRequest.isSuccess()) {
if (size > 0) {
DefaultMessageStore.this.doDispatch(dispatchRequest);
if (BrokerRole.SLAVE != DefaultMessageStore.this.getMessageStoreConfig().getBrokerRole()
&& DefaultMessageStore.this.brokerConfig.isLongPollingEnable()) {
DefaultMessageStore.this.messageArrivingListener.arriving(dispatchRequest.getTopic(),
dispatchRequest.getQueueId(), dispatchRequest.getConsumeQueueOffset() + 1,
dispatchRequest.getTagsCode(), dispatchRequest.getStoreTimestamp(),
dispatchRequest.getBitMap(), dispatchRequest.getPropertiesMap());
}
// FIXED BUG By shijia
this.reputFromOffset += size;
readSize += size;
if (DefaultMessageStore.this.getMessageStoreConfig().getBrokerRole() == BrokerRole.SLAVE) {
DefaultMessageStore.this.storeStatsService
.getSinglePutMessageTopicTimesTotal(dispatchRequest.getTopic()).incrementAndGet();
DefaultMessageStore.this.storeStatsService
.getSinglePutMessageTopicSizeTotal(dispatchRequest.getTopic())
.addAndGet(dispatchRequest.getMsgSize());
}
} else if (size == 0) {
this.reputFromOffset = DefaultMessageStore.this.commitLog.rollNextFile(this.reputFromOffset);
readSize = result.getSize();
}
} else if (!dispatchRequest.isSuccess()) {
if (size > 0) {
log.error("[BUG]read total count not equals msg total size. reputFromOffset={}", reputFromOffset);
this.reputFromOffset += size;
} else {
doNext = false;
if (DefaultMessageStore.this.brokerConfig.getBrokerId() == MixAll.MASTER_ID) {
log.error("[BUG]the master dispatch message to consume queue error, COMMITLOG OFFSET: {}",
this.reputFromOffset);
this.reputFromOffset += result.getSize() - readSize;
}
}
}
}
} finally {
result.release();
}
} else {
doNext = false;
}
}
}
逻辑:启个线程,不断地查询有没有新的提交,如果有就请求维护consumeQueue.
Broker 怎么处理事务消息的ConsumeQueue?逻辑:
1.prepare消息不会放入ConsumeQueue
2.commit消息才能被ConsumeQueue
CommitLogDispatcherBuildConsumeQueue.dispatch
class CommitLogDispatcherBuildConsumeQueue implements CommitLogDispatcher {
@Override
public void dispatch(DispatchRequest request) {
final int tranType = MessageSysFlag.getTransactionValue(request.getSysFlag());
switch (tranType) {
case MessageSysFlag.TRANSACTION_NOT_TYPE:
// commit操作会放入consumeQueue
case MessageSysFlag.TRANSACTION_COMMIT_TYPE:
DefaultMessageStore.this.putMessagePositionInfo(request);
break;
// prepare消息不会放入ConsumeQueue
case MessageSysFlag.TRANSACTION_PREPARED_TYPE:
case MessageSysFlag.TRANSACTION_ROLLBACK_TYPE:
break;
}
}
}
Broker 怎么处理定时消息的 ConsumeQueue?
Producer那篇有讲,定时消息是吧topic和queueId存储到属性中,真正存储与ScheduleTopic上,时间到了才放入原有topic,故ConsumeQueue无需特殊维护.
Broker 怎么处理回溯消费请求?逻辑:
1.在ConsumeQueue根据时间查询offset
2.请求Consumer Group中所有Consumer重置offset
代码:
AdminBrokerProcessor.processRequest
case RequestCode.INVOKE_BROKER_TO_RESET_OFFSET:
return this.resetOffset(ctx, request);
Broker2Client.resetOffset
public RemotingCommand resetOffset(ChannelHandlerContext ctx, RemotingCommand request) throws RemotingCommandException {
final ResetOffsetRequestHeader requestHeader =
(ResetOffsetRequestHeader) request.decodeCommandCustomHeader(ResetOffsetRequestHeader.class);
log.info("[reset-offset] reset offset started by {}. topic={}, group={}, timestamp={}, isForce={}",
RemotingHelper.parseChannelRemoteAddr(ctx.channel()), requestHeader.getTopic(), requestHeader.getGroup(),
requestHeader.getTimestamp(), requestHeader.isForce());
boolean isC = false;
LanguageCode language = request.getLanguage();
switch (language) {
case CPP:
isC = true;
break;
}
return this.brokerController.getBroker2Client().resetOffset(requestHeader.getTopic(), requestHeader.getGroup(),
requestHeader.getTimestamp(), requestHeader.isForce(), isC);
}
主要逻辑在此方法
Broker2Client.resetOffset
public RemotingCommand resetOffset(String topic, String group, long timeStamp, boolean isForce,
boolean isC) {
final RemotingCommand response = RemotingCommand.createResponseCommand(null);
TopicConfig topicConfig = this.brokerController.getTopicConfigManager().selectTopicConfig(topic);
if (null == topicConfig) {
log.error("[reset-offset] reset offset failed, no topic in this broker. topic={}", topic);
response.setCode(ResponseCode.SYSTEM_ERROR);
response.setRemark("[reset-offset] reset offset failed, no topic in this broker. topic=" + topic);
return response;
}
Map offsetTable = new HashMap();
for (int i = 0; i < topicConfig.getWriteQueueNums(); i++) {
MessageQueue mq = new MessageQueue();
mq.setBrokerName(this.brokerController.getBrokerConfig().getBrokerName());
mq.setTopic(topic);
mq.setQueueId(i);
long consumerOffset =
this.brokerController.getConsumerOffsetManager().queryOffset(group, topic, i);
if (-1 == consumerOffset) {
response.setCode(ResponseCode.SYSTEM_ERROR);
response.setRemark(String.format("THe consumer group <%s> not exist", group));
return response;
}
long timeStampOffset;
if (timeStamp == -1) {
timeStampOffset = this.brokerController.getMessageStore().getMaxOffsetInQueue(topic, i);
} else {
//根据时间找offset
timeStampOffset = this.brokerController.getMessageStore().getOffsetInQueueByTime(topic, i, timeStamp);
}
if (timeStampOffset < 0) {
log.warn("reset offset is invalid. topic={}, queueId={}, timeStampOffset={}", topic, i, timeStampOffset);
timeStampOffset = 0;
}
if (isForce || timeStampOffset < consumerOffset) {
offsetTable.put(mq, timeStampOffset);
} else {
offsetTable.put(mq, consumerOffset);
}
}
ResetOffsetRequestHeader requestHeader = new ResetOffsetRequestHeader();
requestHeader.setTopic(topic);
requestHeader.setGroup(group);
requestHeader.setTimestamp(timeStamp);
//给Consumer发送请求重置offset
RemotingCommand request =
RemotingCommand.createRequestCommand(RequestCode.RESET_CONSUMER_CLIENT_OFFSET, requestHeader);
if (isC) {
// c++ language
ResetOffsetBodyForC body = new ResetOffsetBodyForC();
List offsetList = convertOffsetTable2OffsetList(offsetTable);
body.setOffsetTable(offsetList);
request.setBody(body.encode());
} else {
// other language
ResetOffsetBody body = new ResetOffsetBody();
body.setOffsetTable(offsetTable);
request.setBody(body.encode());
}
ConsumerGroupInfo consumerGroupInfo =
this.brokerController.getConsumerManager().getConsumerGroupInfo(group);
if (consumerGroupInfo != null && !consumerGroupInfo.getAllChannel().isEmpty()) {
ConcurrentMap channelInfoTable =
consumerGroupInfo.getChannelInfoTable();
for (Map.Entry entry : channelInfoTable.entrySet()) {
int version = entry.getValue().getVersion();
if (version >= MQVersion.Version.V3_0_7_SNAPSHOT.ordinal()) {
try {
this.brokerController.getRemotingServer().invokeOneway(entry.getKey(), request, 5000);
log.info("[reset-offset] reset offset success. topic={}, group={}, clientId={}",
topic, group, entry.getValue().getClientId());
} catch (Exception e) {
log.error("[reset-offset] reset offset exception. topic={}, group={}",
new Object[] {topic, group}, e);
}
} else {
response.setCode(ResponseCode.SYSTEM_ERROR);
response.setRemark("the client does not support this feature. version="
+ MQVersion.getVersionDesc(version));
log.warn("[reset-offset] the client does not support this feature. version={}",
RemotingHelper.parseChannelRemoteAddr(entry.getKey()), MQVersion.getVersionDesc(version));
return response;
}
}
} else {
String errorInfo =
String.format("Consumer not online, so can not reset offset, Group: %s Topic: %s Timestamp: %d",
requestHeader.getGroup(),
requestHeader.getTopic(),
requestHeader.getTimestamp());
log.error(errorInfo);
response.setCode(ResponseCode.CONSUMER_NOT_ONLINE);
response.setRemark(errorInfo);
return response;
}
response.setCode(ResponseCode.SUCCESS);
ResetOffsetBody resBody = new ResetOffsetBody();
resBody.setOffsetTable(offsetTable);
response.setBody(resBody.encode());
return response;
}
ConsumeQueue中没有时间戳啊,还是得去CommitLog中找
public long getOffsetInQueueByTime(final long timestamp) {
MappedFile mappedFile = this.mappedFileQueue.getMappedFileByTime(timestamp);
if (mappedFile != null) {
long offset = 0;
int low = minLogicOffset > mappedFile.getFileFromOffset() ? (int) (minLogicOffset - mappedFile.getFileFromOffset()) : 0;
int high = 0;
int midOffset = -1, targetOffset = -1, leftOffset = -1, rightOffset = -1;
long leftIndexValue = -1L, rightIndexValue = -1L;
long minPhysicOffset = this.defaultMessageStore.getMinPhyOffset();
SelectMappedBufferResult sbr = mappedFile.selectMappedBuffer(0);
if (null != sbr) {
ByteBuffer byteBuffer = sbr.getByteBuffer();
high = byteBuffer.limit() - CQ_STORE_UNIT_SIZE;
try {
//二分查找?
while (high >= low) {
midOffset = (low + high) / (2 * CQ_STORE_UNIT_SIZE) * CQ_STORE_UNIT_SIZE;
byteBuffer.position(midOffset);
long phyOffset = byteBuffer.getLong();
int size = byteBuffer.getInt();
if (phyOffset < minPhysicOffset) {
low = midOffset + CQ_STORE_UNIT_SIZE;
leftOffset = midOffset;
continue;
}
//获取指定偏移量的时间戳
long storeTime =
this.defaultMessageStore.getCommitLog().pickupStoreTimestamp(phyOffset, size);
if (storeTime < 0) {
return 0;
} else if (storeTime == timestamp) {
targetOffset = midOffset;
break;
} else if (storeTime > timestamp) {
high = midOffset - CQ_STORE_UNIT_SIZE;
rightOffset = midOffset;
rightIndexValue = storeTime;
} else {
low = midOffset + CQ_STORE_UNIT_SIZE;
leftOffset = midOffset;
leftIndexValue = storeTime;
}
}
if (targetOffset != -1) {
offset = targetOffset;
} else {
if (leftIndexValue == -1) {
offset = rightOffset;
} else if (rightIndexValue == -1) {
offset = leftOffset;
} else {
offset =
Math.abs(timestamp - leftIndexValue) > Math.abs(timestamp
- rightIndexValue) ? rightOffset : leftOffset;
}
}
return (mappedFile.getFileFromOffset() + offset) / CQ_STORE_UNIT_SIZE;
} finally {
sbr.release();
}
}
}
return 0;
}
再来关注下消息存储时存储的额外信息 MessageExt
// 队列IDBroker 的消息是 at least once还是exactly only once?private int queueId; //存储消息大小 private int storeSize; //队列的offset private long queueOffset; //消息标志位 private int sysFlag; //消息在客户端创建时间戳 private long bornTimestamp; //producer 地址 private SocketAddress bornHost; //消息在服务器存储时间戳 private long storeTimestamp; //存储的broker地址 private SocketAddress storeHost; //消息id private String msgId; //消息对应的Commit Log Offset private long commitLogOffset; //消息体CRC private int bodyCRC; // 当前消息被某个订阅组重新消费了几次（订阅组之间独立计数） private int reconsumeTimes;
at least once:
是指每个消息必须投递一次,RocketMQ Consumer 先 pull 消息到本地，消费完成后，才向服务器返回 ack，如果没有消费一定不会ack消息，所以RocketMQ可以很好的支持此特性。
exactly only once:
(1). 发送消息阶段，不允许发送重复的消息。
(2). 消费消息阶段，不允许消费重复的消息。只有以上两个条件都满足情况下，才能认为消息是“Exactly Only Once”，而要实现以上两点，在分布式系统环境下，不可避免要产生巨大的开销。所以 RocketMQ 为了追求高性能，并不保证此特性，要求在业务上进行去重， 也就是说消费消息要做到幂等性。RocketMQ 虽然不能严格保证不重复，但是正常情况下很少会出现重复发送、消 费情况，只有网络异常，Consumer 启停等异常情况下会出现消息重复。此问题的本质原因是网络调用存在不确定性，即既不成功也不失败的第三种状态，所以才产生了消息重复性问 题。
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摘要：完全无状态，可集群部署与集群中的其中一个节点随机选择建立长连接，定期从取路由信息，并向提供服务的建立长连接，且定时向发送心跳。既可以从订阅消息，也可以从订阅消息，订阅规则由配置决定。 问题列表: Name Server 的作用是什么? Name Server 存储了Broker的什么信息? Name Server 为Producer的提供些什么信息? Name Server 为Co...
摘要：如果干净页不足，此时写入会被阻塞，系统尝试刷盘部分数据，大约每次尝试个，异步刷盘消息收到后返回同时调用消息存储过程异常情况下可能造成少量数据丢失来找出更多干净。刷盘线程刷盘后，唤醒前端等待线程，可能是一批线程。 这次源码学习的方法是带着问题学习源码实现,问题列表如下 Broker 怎么接收消息的? Broker 异常情况下怎么保证数据可靠性? Broker 怎么保证存储高吞吐量? B...
摘要：每个与集群中的所有节点建立长连接，定时注册信息到所有。完全无状态，可集群部署。本系列源码解析主要参照原理简介来追寻其代码实现虽然版本不太一致但这也是能找到的最详细的资料了接下来根据其模块来源码阅读目录如下 为什么选择读RocketMQ? 对MQ的理解一直不深,上周看了,还是觉得不够深入,找个成熟的产品来学习吧,RabbitMQ是erLang写的,Kafka是Scala写的,非Java写...
摘要：的都是从消息来消费，但是为了能做到实时收消息，使用长轮询方式，可以保证消息实时性同方式一致。这种情况建议应用，再消费下一条消息，这样可以减轻重试消息的压力。逻辑请求按参数返回按照重置消费从而实现回溯消费 这次源码学习的方法是带着问题学习源码实现,问题列表如下 Consumer Group的概念是什么? Consumer pull过程是怎样的? Consumer 支持push吗? C...
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