? ? ? ? rocketmq的消息存储是先写入内存，再根据不同的刷盘策略进行刷盘，rocketmq支持两种刷盘模式：
同步刷盘：消息写入后等待刷盘成功返回
异步刷盘：消息写入内存后直接返回，由后台线程去定时刷盘
broker配置文件中设置flushDiskType字段，可选择ASYNC_FLUSH（异步刷盘）和 SYNC_FLUSH（同步刷盘）
前面”Broker-消息存储流程”中，CommitLog.putMessage方法的最后会调用CommitLog.handleDiskFlush方法进行刷盘。
public void handleDiskFlush(AppendMessageResult result, PutMessageResult putMessageResult, MessageExt messageExt) { // Synchronization flush if (FlushDiskType.SYNC_FLUSH == this.defaultMessageStore.getMessageStoreConfig().getFlushDiskType()) { final GroupCommitService service = (GroupCommitService) this.flushCommitLogService; if (messageExt.isWaitStoreMsgOK()) { GroupCommitRequest request = new GroupCommitRequest(result.getWroteOffset() + result.getWroteBytes()); service.putRequest(request); boolean flushOK = request.waitForFlush(this.defaultMessageStore.getMessageStoreConfig().getSyncFlushTimeout()); if (!flushOK) { log.error(“do groupcommit, wait for flush failed, topic: ” + messageExt.getTopic() + ” tags: ” + messageExt.getTags() + ” client address: ” + messageExt.getBornHostString()); putMessageResult.setPutMessageStatus(PutMessageStatus.FLUSH_DISK_TIMEOUT); } } else { service.wakeup(); } } // Asynchronous flush else { if (!this.defaultMessageStore.getMessageStoreConfig().isTransientStorePoolEnable()) { flushCommitLogService.wakeup(); } else { commitLogService.wakeup(); } } }
该方法中，首先判断当前刷盘模式是同步刷盘还是异步刷盘。
对于同步刷盘，构造一个GroupCommitRequest对象，并提交到flushCommitLogService中，线程阻塞等待刷盘完成。
对于异步刷盘，直接激活刷盘线程并返回，线程并不阻塞。
看一下GroupCommitRequest类图
nextOffset：刷盘点偏移量
countDownLatch：刷盘控制器
flushOk：刷盘结果
public boolean waitForFlush(long timeout) { try { this.countDownLatch.await(timeout, TimeUnit.MILLISECONDS); return this.flushOK; } catch (InterruptedException e) { log.error(“Interrupted”, e); return false; } }
提交请求后在countDownLatch上等待，默认超时时间5s，flushCommitLogService线程处理完刷盘后，调用GroupCommitRequest.wakeupCustomer方法通知等待线程
public void wakeupCustomer(final boolean flushOK) { this.flushOK = flushOK; this.countDownLatch.countDown(); }
看下刷盘线程处理逻辑，默认的刷盘线程是GroupCommitService
requestsWrite：写请求缓冲池
requestsRead：处理的缓冲池
主循环不断读取requestsRead处理，然后将两个缓冲池交换，处理另一个缓冲池。
private void swapRequests() { List<GroupCommitRequest> tmp = this.requestsWrite; this.requestsWrite = this.requestsRead; this.requestsRead = tmp; }
?请求一直往requestsWrite缓冲池插入。
public synchronized void putRequest(final GroupCommitRequest request) { //将请求加入requestsWrite缓冲池 synchronized (this.requestsWrite) { this.requestsWrite.add(request); } //如果线程处于等待状态将其唤醒 if (hasNotified.compareAndSet(false, true)) { waitPoint.countDown(); // notify } }
线程不断循环处理刷盘请求，每次处理完其中一个缓冲池，如果另一个缓冲池没有数据会sleep10毫秒
public void run() { CommitLog.log.info(this.getServiceName() + ” service started”); while (!this.isStopped()) { try { this.waitForRunning(10); this.doCommit(); } catch (Exception e) { CommitLog.log.warn(this.getServiceName() + ” service has exception. “, e); } }。。。}
刷盘逻辑在doCommit中处理，最终调用mappedFileQueue.flush进行刷盘。
private void doCommit() { synchronized (this.requestsRead) { if (!this.requestsRead.isEmpty()) { for (GroupCommitRequest req : this.requestsRead) { // There may be a message in the next file, so a maximum of // two times the flush boolean flushOK = false; for (int i = 0; i < 2 && !flushOK; i++) { flushOK = CommitLog.this.mappedFileQueue.getFlushedWhere() >= req.getNextOffset(); if (!flushOK) { CommitLog.this.mappedFileQueue.flush(0); } } req.wakeupCustomer(flushOK); } long storeTimestamp = CommitLog.this.mappedFileQueue.getStoreTimestamp(); if (storeTimestamp > 0) { CommitLog.this.defaultMessageStore.getStoreCheckpoint().setPhysicMsgTimestamp(storeTimestamp); } this.requestsRead.clear(); } else { // Because of individual messages is set to not sync flush, it // will come to this process CommitLog.this.mappedFileQueue.flush(0); } } }
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对于异步刷盘直接激活刷盘线程
// Asynchronous flush else { if (!this.defaultMessageStore.getMessageStoreConfig().isTransientStorePoolEnable()) { flushCommitLogService.wakeup(); } else { commitLogService.wakeup(); } }
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