消息中间件 RocketMQ 高级功能和源码分析（七）
一、 消息中间件 RocketMQ 源码分析：消息存储核心类介绍
1、消息存储在 store 模块中。消息存储核心类 DefaultMessageStore.java
2、消息存储核心类介绍
private final MessageStoreConfig messageStoreConfig; //消息配置属性
private final CommitLog commitLog; //CommitLog文件存储的实现类
private final ConcurrentMap<String/* topic */, ConcurrentMap<Integer/* queueId */, ConsumeQueue>> consumeQueueTable; //消息队列存储缓存表,按照消息主题分组
private final FlushConsumeQueueService flushConsumeQueueService; //消息队列文件刷盘线程
private final CleanCommitLogService cleanCommitLogService; //清除CommitLog文件服务
private final CleanConsumeQueueService cleanConsumeQueueService; //清除ConsumerQueue队列文件服务
private final IndexService indexService; //索引实现类
private final AllocateMappedFileService allocateMappedFileService; //MappedFile分配服务
private final ReputMessageService reputMessageService;//CommitLog消息分发,根据CommitLog文件构建ConsumerQueue、IndexFile文件
private final HAService haService; //存储HA机制
private final ScheduleMessageService scheduleMessageService; //消息服务调度线程
private final StoreStatsService storeStatsService; //消息存储服务
private final TransientStorePool transientStorePool; //消息堆外内存缓存
private final BrokerStatsManager brokerStatsManager; //Broker状态管理器
private final MessageArrivingListener messageArrivingListener; //消息拉取长轮询模式消息达到监听器
private final BrokerConfig brokerConfig; //Broker配置类
private StoreCheckpoint storeCheckpoint; //文件刷盘监测点
private final LinkedList<CommitLogDispatcher> dispatcherList; //CommitLog文件转发请求
二、 消息中间件 RocketMQ 源码分析：消息存储流程
1、消息存储流程 示例图：
2、 消息存储入口：DefaultMessageStore#putMessage
//判断Broker角色如果是从节点,则无需写入
if (BrokerRole.SLAVE == this.messageStoreConfig.getBrokerRole()) {
long value = this.printTimes.getAndIncrement();
if ((value % 50000) == 0) {
log.warn("message store is slave mode, so putMessage is forbidden ");
}
return new PutMessageResult(PutMessageStatus.SERVICE_NOT_AVAILABLE, null);
}
//判断当前写入状态如果是正在写入,则不能继续
if (!this.runningFlags.isWriteable()) {
long value = this.printTimes.getAndIncrement();
return new PutMessageResult(PutMessageStatus.SERVICE_NOT_AVAILABLE, null);
} else {
this.printTimes.set(0);
}
//判断消息主题长度是否超过最大限制
if (msg.getTopic().length() > Byte.MAX_VALUE) {
log.warn("putMessage message topic length too long " + msg.getTopic().length());
return new PutMessageResult(PutMessageStatus.MESSAGE_ILLEGAL, null);
}
//判断消息属性长度是否超过限制
if (msg.getPropertiesString() != null && msg.getPropertiesString().length() > Short.MAX_VALUE) {
log.warn("putMessage message properties length too long " + msg.getPropertiesString().length());
return new PutMessageResult(PutMessageStatus.PROPERTIES_SIZE_EXCEEDED, null);
}
//判断系统PageCache缓存去是否占用
if (this.isOSPageCacheBusy()) {
return new PutMessageResult(PutMessageStatus.OS_PAGECACHE_BUSY, null);
}
//将消息写入CommitLog文件
PutMessageResult result = this.commitLog.putMessage(msg);
3、 代码：CommitLog#putMessage
//记录消息存储时间
msg.setStoreTimestamp(beginLockTimestamp);
//判断如果mappedFile如果为空或者已满,创建新的mappedFile文件
if (null == mappedFile || mappedFile.isFull()) {
mappedFile = this.mappedFileQueue.getLastMappedFile(0);
}
//如果创建失败,直接返回
if (null == mappedFile) {
log.error("create mapped file1 error, topic: " + msg.getTopic() + " clientAddr: " + msg.getBornHostString());
beginTimeInLock = 0;
return new PutMessageResult(PutMessageStatus.CREATE_MAPEDFILE_FAILED, null);
}
//写入消息到mappedFile中
result = mappedFile.appendMessage(msg, this.appendMessageCallback);
4、 代码：MappedFile#appendMessagesInner
//获得文件的写入指针
int currentPos = this.wrotePosition.get();
//如果指针大于文件大小则直接返回
if (currentPos < this.fileSize) {
//通过writeBuffer.slice()创建一个与MappedFile共享的内存区,并设置position为当前指针
ByteBuffer byteBuffer = writeBuffer != null ? writeBuffer.slice() : this.mappedByteBuffer.slice();
byteBuffer.position(currentPos);
AppendMessageResult result = null;
if (messageExt instanceof MessageExtBrokerInner) {
//通过回调方法写入
result = cb.doAppend(this.getFileFromOffset(), byteBuffer, this.fileSize - currentPos, (MessageExtBrokerInner) messageExt);
} else if (messageExt instanceof MessageExtBatch) {
result = cb.doAppend(this.getFileFromOffset(), byteBuffer, this.fileSize - currentPos, (MessageExtBatch) messageExt);
} else {
return new AppendMessageResult(AppendMessageStatus.UNKNOWN_ERROR);
}
this.wrotePosition.addAndGet(result.getWroteBytes());
this.storeTimestamp = result.getStoreTimestamp();
return result;
}
5、 代码：CommitLog#doAppend
//文件写入位置
long wroteOffset = fileFromOffset + byteBuffer.position();
//设置消息ID
this.resetByteBuffer(hostHolder, 8);
String msgId = MessageDecoder.createMessageId(this.msgIdMemory, msgInner.getStoreHostBytes(hostHolder), wroteOffset);
//获得该消息在消息队列中的偏移量
keyBuilder.setLength(0);
keyBuilder.append(msgInner.getTopic());
keyBuilder.append('-');
keyBuilder.append(msgInner.getQueueId());
String key = keyBuilder.toString();
Long queueOffset = CommitLog.this.topicQueueTable.get(key);
if (null == queueOffset) {
queueOffset = 0L;
CommitLog.this.topicQueueTable.put(key, queueOffset);
}
//获得消息属性长度
final byte[] propertiesData =msgInner.getPropertiesString() == null ? null : msgInner.getPropertiesString().getBytes(MessageDecoder.CHARSET_UTF8);
final int propertiesLength = propertiesData == null ? 0 : propertiesData.length;
if (propertiesLength > Short.MAX_VALUE) {
log.warn("putMessage message properties length too long. length={}", propertiesData.length);
return new AppendMessageResult(AppendMessageStatus.PROPERTIES_SIZE_EXCEEDED);
}
//获得消息主题大小
final byte[] topicData = msgInner.getTopic().getBytes(MessageDecoder.CHARSET_UTF8);
final int topicLength = topicData.length;
//获得消息体大小
final int bodyLength = msgInner.getBody() == null ? 0 : msgInner.getBody().length;
//计算消息总长度
final int msgLen = calMsgLength(bodyLength, topicLength, propertiesLength);
6、 代码：CommitLog#calMsgLength
protected static int calMsgLength(int bodyLength, int topicLength, int propertiesLength) {
final int msgLen = 4 //TOTALSIZE
+ 4 //MAGICCODE
+ 4 //BODYCRC
+ 4 //QUEUEID
+ 4 //FLAG
+ 8 //QUEUEOFFSET
+ 8 //PHYSICALOFFSET
+ 4 //SYSFLAG
+ 8 //BORNTIMESTAMP
+ 8 //BORNHOST
+ 8 //STORETIMESTAMP
+ 8 //STOREHOSTADDRESS
+ 4 //RECONSUMETIMES
+ 8 //Prepared Transaction Offset
+ 4 + (bodyLength > 0 ? bodyLength : 0) //BODY
+ 1 + topicLength //TOPIC
+ 2 + (propertiesLength > 0 ? propertiesLength : 0) //propertiesLength
+ 0;
return msgLen;
}
7、 代码：CommitLog#doAppend
//消息长度不能超过4M
if (msgLen > this.maxMessageSize) {
CommitLog.log.warn("message size exceeded, msg total size: " + msgLen + ", msg body size: " + bodyLength
+ ", maxMessageSize: " + this.maxMessageSize);
return new AppendMessageResult(AppendMessageStatus.MESSAGE_SIZE_EXCEEDED);
}
//消息是如果没有足够的存储空间则新创建CommitLog文件
if ((msgLen + END_FILE_MIN_BLANK_LENGTH) > maxBlank) {
this.resetByteBuffer(this.msgStoreItemMemory, maxBlank);
// 1 TOTALSIZE
this.msgStoreItemMemory.putInt(maxBlank);
// 2 MAGICCODE
this.msgStoreItemMemory.putInt(CommitLog.BLANK_MAGIC_CODE);
// 3 The remaining space may be any value
// Here the length of the specially set maxBlank
final long beginTimeMills = CommitLog.this.defaultMessageStore.now();
byteBuffer.put(this.msgStoreItemMemory.array(), 0, maxBlank);
return new AppendMessageResult(AppendMessageStatus.END_OF_FILE, wroteOffset, maxBlank, msgId, msgInner.getStoreTimestamp(),
queueOffset, CommitLog.this.defaultMessageStore.now() - beginTimeMills);
}
//将消息存储到ByteBuffer中,返回AppendMessageResult
final long beginTimeMills = CommitLog.this.defaultMessageStore.now();
// Write messages to the queue buffer
byteBuffer.put(this.msgStoreItemMemory.array(), 0, msgLen);
AppendMessageResult result = new AppendMessageResult(AppendMessageStatus.PUT_OK, wroteOffset,
msgLen, msgId,msgInner.getStoreTimestamp(),
queueOffset,
CommitLog.this.defaultMessageStore.now()
-beginTimeMills);
switch (tranType) {
case MessageSysFlag.TRANSACTION_PREPARED_TYPE:
case MessageSysFlag.TRANSACTION_ROLLBACK_TYPE:
break;
case MessageSysFlag.TRANSACTION_NOT_TYPE:
case MessageSysFlag.TRANSACTION_COMMIT_TYPE:
//更新消息队列偏移量
CommitLog.this.topicQueueTable.put(key, ++queueOffset);
break;
default:
break;
}
8、 代码：CommitLog#putMessage
//释放锁
putMessageLock.unlock();
//刷盘
handleDiskFlush(result, putMessageResult, msg);
//执行HA主从同步
handleHA(result, putMessageResult, msg);
三、 消息中间件 RocketMQ 源码分析：消息存储文件介绍
1、消息存储文件结构图：
2、消息存储文件介绍：
- commitLog：消息存储目录
- config：运行期间一些配置信息
- consumerqueue：消息消费队列存储目录
- index：消息索引文件存储目录
- abort：如果存在改文件寿命 Broker 非正常关闭
- checkpoint：文件检查点，存储 CommitLog 文件最后一次刷盘时间戳、consumerquueue 最后一次刷盘时间，index 索引文件最后一次刷盘时间戳。
四、 消息中间件 RocketMQ 源码分析：存储文件内存映射-MappedFileQueue
1、消息 存储文件内存映射
RocketMQ 通过使用内存映射文件提高 IO 访问性能，无论是 CommitLog、ConsumerQueue 还是 IndexFile，单个文件都被设计为固定长度，如果一个文件写满以后再创建一个新文件，文件名就为该文件第一条消息对应的全局物理偏移量。
2、消息 存储文件内存映射-MappedFileQueue
String storePath; //存储目录
int mappedFileSize; // 单个文件大小
CopyOnWriteArrayList<MappedFile> mappedFiles; //MappedFile文件集合
AllocateMappedFileService allocateMappedFileService; //创建MapFile服务类
long flushedWhere = 0; //当前刷盘指针
long committedWhere = 0; //当前数据提交指针,内存中ByteBuffer当前的写指针,该值大于等于flushWhere
3、根据存储时间查询 MappedFile
public MappedFile getMappedFileByTime(final long timestamp) {
Object[] mfs = this.copyMappedFiles(0);
if (null == mfs)
return null;
//遍历MappedFile文件数组
for (int i = 0; i < mfs.length; i++) {
MappedFile mappedFile = (MappedFile) mfs[i];
//MappedFile文件的最后修改时间大于指定时间戳则返回该文件
if (mappedFile.getLastModifiedTimestamp() >= timestamp) {
return mappedFile;
}
}
return (MappedFile) mfs[mfs.length - 1];
}
4、根据消息偏移量 offset 查找 MappedFile
public MappedFile findMappedFileByOffset(final long offset, final boolean returnFirstOnNotFound) {
try {
//获得第一个MappedFile文件
MappedFile firstMappedFile = this.getFirstMappedFile();
//获得最后一个MappedFile文件
MappedFile lastMappedFile = this.getLastMappedFile();
//第一个文件和最后一个文件均不为空,则进行处理
if (firstMappedFile != null && lastMappedFile != null) {
if (offset < firstMappedFile.getFileFromOffset() ||
offset >= lastMappedFile.getFileFromOffset() + this.mappedFileSize) {
} else {
//获得文件索引
int index = (int) ((offset / this.mappedFileSize)
- (firstMappedFile.getFileFromOffset() / this.mappedFileSize));
MappedFile targetFile = null;
try {
//根据索引返回目标文件
targetFile = this.mappedFiles.get(index);
} catch (Exception ignored) {
}
if (targetFile != null && offset >= targetFile.getFileFromOffset()
&& offset < targetFile.getFileFromOffset() + this.mappedFileSize) {
return targetFile;
}
for (MappedFile tmpMappedFile : this.mappedFiles) {
if (offset >= tmpMappedFile.getFileFromOffset()
&& offset < tmpMappedFile.getFileFromOffset() + this.mappedFileSize) {
return tmpMappedFile;
}
}
}
if (returnFirstOnNotFound) {
return firstMappedFile;
}
}
} catch (Exception e) {
log.error("findMappedFileByOffset Exception", e);
}
return null;
}
5、获取存储文件最小偏移量
public long getMinOffset() {
if (!this.mappedFiles.isEmpty()) {
try {
return this.mappedFiles.get(0).getFileFromOffset();
} catch (IndexOutOfBoundsException e) {
//continue;
} catch (Exception e) {
log.error("getMinOffset has exception.", e);
}
}
return -1;
}
6、获取存储文件最大偏移量
public long getMaxOffset() {
MappedFile mappedFile = getLastMappedFile();
if (mappedFile != null) {
return mappedFile.getFileFromOffset() + mappedFile.getReadPosition();
}
return 0;
}
7、返回存储文件当前写指针
public long getMaxWrotePosition() {
MappedFile mappedFile = getLastMappedFile();
if (mappedFile != null) {
return mappedFile.getFileFromOffset() + mappedFile.getWrotePosition();
}
return 0;
}
五、 消息中间件 RocketMQ 源码分析：存储文件内存映射-MappedFile
1、消息 存储文件内存映射-MappedFile
MappedFile 示例图：
2、消息 存储文件内存映射-MappedFile 文件介绍：
int OS_PAGE_SIZE = 1024 * 4; //操作系统每页大小,默认4K
AtomicLong TOTAL_MAPPED_VIRTUAL_MEMORY = new AtomicLong(0); //当前JVM实例中MappedFile虚拟内存
AtomicInteger TOTAL_MAPPED_FILES = new AtomicInteger(0); //当前JVM实例中MappedFile对象个数
AtomicInteger wrotePosition = new AtomicInteger(0); //当前文件的写指针
AtomicInteger committedPosition = new AtomicInteger(0); //当前文件的提交指针
AtomicInteger flushedPosition = new AtomicInteger(0); //刷写到磁盘指针
int fileSize; //文件大小
FileChannel fileChannel; //文件通道
ByteBuffer writeBuffer = null; //堆外内存ByteBuffer
TransientStorePool transientStorePool = null; //堆外内存池
String fileName; //文件名称
long fileFromOffset; //该文件的处理偏移量
File file; //物理文件
MappedByteBuffer mappedByteBuffer; //物理文件对应的内存映射Buffer
volatile long storeTimestamp = 0; //文件最后一次内容写入时间
boolean firstCreateInQueue = false; //是否是MappedFileQueue队列中第一个文件
3、 MappedFile 初始化
- 未开启
transientStorePoolEnable。transientStorePoolEnable=true为true表示数据先存储到堆外内存，然后通过Commit线程将数据提交到内存映射Buffer中，再通过Flush线程将内存映射Buffer中数据持久化磁盘。
private void init(final String fileName, final int fileSize) throws IOException {
this.fileName = fileName;
this.fileSize = fileSize;
this.file = new File(fileName);
this.fileFromOffset = Long.parseLong(this.file.getName());
boolean ok = false;
ensureDirOK(this.file.getParent());
try {
this.fileChannel = new RandomAccessFile(this.file, "rw").getChannel();
this.mappedByteBuffer = this.fileChannel.map(MapMode.READ_WRITE, 0, fileSize);
TOTAL_MAPPED_VIRTUAL_MEMORY.addAndGet(fileSize);
TOTAL_MAPPED_FILES.incrementAndGet();
ok = true;
} catch (FileNotFoundException e) {
log.error("create file channel " + this.fileName + " Failed. ", e);
throw e;
} catch (IOException e) {
log.error("map file " + this.fileName + " Failed. ", e);
throw e;
} finally {
if (!ok && this.fileChannel != null) {
this.fileChannel.close();
}
}
}
4、 开启transientStorePoolEnable
public void init(final String fileName, final int fileSize,
final TransientStorePool transientStorePool) throws IOException {
init(fileName, fileSize);
this.writeBuffer = transientStorePool.borrowBuffer(); //初始化writeBuffer
this.transientStorePool = transientStorePool;
}
5、 MappedFile 提交
提交数据到 FileChannel，commitLeastPages 为本次提交最小的页数，如果待提交数据不满 commitLeastPages，则不执行本次提交操作。如果 writeBuffer 如果为空，直接返回 writePosition 指针，无需执行 commit 操作，表名 commit 操作主体是 writeBuffer。
public int commit(final int commitLeastPages) {
if (writeBuffer == null) {
//no need to commit data to file channel, so just regard wrotePosition as committedPosition.
return this.wrotePosition.get();
}
//判断是否满足提交条件
if (this.isAbleToCommit(commitLeastPages)) {
if (this.hold()) {
commit0(commitLeastPages);
this.release();
} else {
log.warn("in commit, hold failed, commit offset = " + this.committedPosition.get());
}
}
// 所有数据提交后,清空缓冲区
if (writeBuffer != null && this.transientStorePool != null && this.fileSize == this.committedPosition.get()) {
this.transientStorePool.returnBuffer(writeBuffer);
this.writeBuffer = null;
}
return this.committedPosition.get();
}
6、 MappedFile#isAbleToCommit
判断是否执行 commit 操作，如果文件已满返回 true；如果 commitLeastpages 大于 0，则比较 writePosition 与上一次提交的指针 commitPosition 的差值，除以 OS_PAGE_SIZE 得到当前脏页的数量，如果大于 commitLeastPages 则返回 true，如果 commitLeastpages 小于0表示只要存在脏页就提交。
protected boolean isAbleToCommit(final int commitLeastPages) {
//已经刷盘指针
int flush = this.committedPosition.get();
//文件写指针
int write = this.wrotePosition.get();
//写满刷盘
if (this.isFull()) {
return true;
}
if (commitLeastPages > 0) {
//文件内容达到commitLeastPages页数,则刷盘
return ((write / OS_PAGE_SIZE) - (flush / OS_PAGE_SIZE)) >= commitLeastPages;
}
return write > flush;
}
7、 MappedFile#commit0
具体提交的实现，首先创建 WriteBuffer 区共享缓存区，然后将新创建的 position 回退到上一次提交的位置（commitPosition），设置 limit 为 wrotePosition（当前最大有效数据指针），然后把 commitPosition 到 wrotePosition 的数据写入到 FileChannel 中，然后更新 committedPosition 指针为 wrotePosition。commit 的作用就是将 MappedFile 的 writeBuffer 中数据提交到文件通道 FileChannel 中。
protected void commit0(final int commitLeastPages) {
//写指针
int writePos = this.wrotePosition.get();
//上次提交指针
int lastCommittedPosition = this.committedPosition.get();
if (writePos - this.committedPosition.get() > 0) {
try {
//复制共享内存区域
ByteBuffer byteBuffer = writeBuffer.slice();
//设置提交位置是上次提交位置
byteBuffer.position(lastCommittedPosition);
//最大提交数量
byteBuffer.limit(writePos);
//设置fileChannel位置为上次提交位置
this.fileChannel.position(lastCommittedPosition);
//将lastCommittedPosition到writePos的数据复制到FileChannel中
this.fileChannel.write(byteBuffer);
//重置提交位置
this.committedPosition.set(writePos);
} catch (Throwable e) {
log.error("Error occurred when commit data to FileChannel.", e);
}
}
}
8、 MappedFile#flush
刷写磁盘，直接调用 MappedByteBuffer 或 fileChannel 的 force 方法将内存中的数据持久化到磁盘，那么 flushedPosition 应该等于 MappedByteBuffer 中的写指针；如果 writeBuffer不为空，则 flushPosition 应该等于上一次的 commit指针；因为上一次提交的数据就是进入到 MappedByteBuffer 中的数据；如果 writeBuffer 为空，数据时直接进入到 MappedByteBuffer，wrotePosition 代表的是 MappedByteBuffer 中的指针，故设置 flushPosition 为 wrotePosition。
public int flush(final int flushLeastPages) {
//数据达到刷盘条件
if (this.isAbleToFlush(flushLeastPages)) {
//加锁，同步刷盘
if (this.hold()) {
//获得读指针
int value = getReadPosition();
try {
//数据从writeBuffer提交数据到fileChannel再刷新到磁盘
if (writeBuffer != null || this.fileChannel.position() != 0) {
this.fileChannel.force(false);
} else {
//从mmap刷新数据到磁盘
this.mappedByteBuffer.force();
}
} catch (Throwable e) {
log.error("Error occurred when force data to disk.", e);
}
//更新刷盘位置
this.flushedPosition.set(value);
this.release();
} else {
log.warn("in flush, hold failed, flush offset = " + this.flushedPosition.get());
this.flushedPosition.set(getReadPosition());
}
}
return this.getFlushedPosition();
}
9、 MappedFile#getReadPosition
获取当前文件最大可读指针。如果 writeBuffer 为空，则直接返回当前的写指针；如果 writeBuffer 不为空，则返回上一次提交的指针。在 MappedFile 设置中,只有提交了的数据（写入到 MappedByteBuffer 或 FileChannel 中的数据）才是安全的数据
public int getReadPosition() {
//如果writeBuffer为空,刷盘的位置就是应该等于上次commit的位置,如果为空则为mmap的写指针
return this.writeBuffer == null ? this.wrotePosition.get() : this.committedPosition.get();
}
10、 MappedFile#selectMappedBuffer
查找 pos 到当前最大可读之间的数据，由于在整个写入期间都未曾改 MappedByteBuffer 的指针，如果 mappedByteBuffer.slice()方法返回的共享缓存区空间为整个 MappedFile，然后通过设置 ByteBuffer 的position 为待查找的值，读取字节长度当前可读最大长度，最终返回的 ByteBuffer的limit 为 size。整个共享缓存区的容量为（MappedFile#fileSize-pos）。故在操作 SelectMappedBufferResult 不能对包含在里面的 ByteBuffer 调用 filp 方法。
public SelectMappedBufferResult selectMappedBuffer(int pos) {
//获得最大可读指针
int readPosition = getReadPosition();
//pos小于最大可读指针,并且大于0
if (pos < readPosition && pos >= 0) {
if (this.hold()) {
//复制mappedByteBuffer读共享区
ByteBuffer byteBuffer = this.mappedByteBuffer.slice();
//设置读指针位置
byteBuffer.position(pos);
//获得可读范围
int size = readPosition - pos;
//设置最大刻度范围
ByteBuffer byteBufferNew = byteBuffer.slice();
byteBufferNew.limit(size);
return new SelectMappedBufferResult(this.fileFromOffset + pos, byteBufferNew, size, this);
}
}
return null;
}
11、 MappedFile#shutdown
MappedFile 文件销毁的实现方法为 public boolean destory(long intervalForcibly)，intervalForcibly 表示拒绝被销毁的最大存活时间。
public void shutdown(final long intervalForcibly) {
if (this.available) {
//关闭MapedFile
this.available = false;
//设置当前关闭时间戳
this.firstShutdownTimestamp = System.currentTimeMillis();
//释放资源
this.release();
} else if (this.getRefCount() > 0) {
if ((System.currentTimeMillis() - this.firstShutdownTimestamp) >= intervalForcibly) {
this.refCount.set(-1000 - this.getRefCount());
this.release();
}
}
}
六、 消息中间件 RocketMQ 源码分析：存储文件内存映射-TransientStorePool
1、消息 存储文件内存映射-TransientStorePool 介绍：
短暂的存储池。RocketMQ 单独创建一个 MappedByteBuffer 内存缓存池，用来临时存储数据，数据先写入该内存映射中，然后由 commit 线程定时将数据从该内存复制到与目标物理文件对应的内存映射中。RocketMQ 引入该机制主要的原因是提供一种内存锁定，将当前堆外内存一直锁定在内存中，避免被进程将内存交换到磁盘。
2、 TransientStorePool 示例图：
3、 TransientStorePool 代码：
private final int poolSize; //availableBuffers个数
private final int fileSize; //每隔ByteBuffer大小
private final Deque<ByteBuffer> availableBuffers; //ByteBuffer容器。双端队列
4、 TransientStorePool 初始化
public void init() {
//创建poolSize个堆外内存
for (int i = 0; i < poolSize; i++) {
ByteBuffer byteBuffer = ByteBuffer.allocateDirect(fileSize);
final long address = ((DirectBuffer) byteBuffer).address();
Pointer pointer = new Pointer(address);
//使用com.sun.jna.Library类库将该批内存锁定,避免被置换到交换区,提高存储性能
LibC.INSTANCE.mlock(pointer, new NativeLong(fileSize));
availableBuffers.offer(byteBuffer);
}
}
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# 消息中间件 RocketMQ 高级功能和源码分析（六）
