RocketMQ-存储机制-刷盘机制

RocketMQ-存储机制-刷盘机制

在理解RocketMQ刷盘实现之前，先理解一下上图展示的刷盘的2种实现的：

1）直接通过内存映射文件，通过flush刷新到磁盘

2）当异步刷盘且启用了对外内存池的时候，先write到writeBuffer，然后commit到Filechannel，最后flush到磁盘

另外输盘的方式分为异步刷盘 同步刷盘 异步转存刷盘方式。

初始化过程如下

if (FlushDiskType.SYNC_FLUSH == defaultMessageStore.getMessageStoreConfig().getFlushDiskType()) {
            this.flushCommitLogService = new GroupCommitService();  //同步
        } else {
            this.flushCommitLogService = new FlushRealTimeService(); // 异步
        }

        this.commitLogService = new CommitRealTimeService();   异步转存

在commitlog的putMessage方法最后handleDiskFlush，处理了刷盘的操作.

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());
                // 提交同步刷盘请求
                service.putRequest(request);
                //  同步等待
                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 { // 唤醒输盘操作
                service.wakeup();
            }
        }
        // Asynchronous flush   异步刷盘
        else {
            if (!this.defaultMessageStore.getMessageStoreConfig().isTransientStorePoolEnable()) {
                flushCommitLogService.wakeup();   //异步
            } else {
                commitLogService.wakeup();  // 异步转存
            }
        }
    }

同步刷盘GroupCommitService

调用putRequest之后，实际上，会放到一个写容器中，如果当前不在处理，那就唤醒同步刷盘线程立即处理

public synchronized void putRequest(final GroupCommitRequest request) {
            synchronized (this.requestsWrite) {
                this.requestsWrite.add(request);
            }
            if (hasNotified.compareAndSet(false, true)) {
                waitPoint.countDown(); // notify
            }
        }

而此时的同步刷盘线程，如果正好检测到有请求过来就会立即执行任务，如果处在等待状态，则被唤醒，等待处理完，又把通知状态设置为false。

protected void waitForRunning(long interval) {
        // 如果有刷盘请求  则立即返回
        if (hasNotified.compareAndSet(true, false)) {
            this.onWaitEnd();
            return;
        }

        //entry to wait
        waitPoint.reset();

        try {
            // 如果没有输盘请求  await  直到被唤醒
            // 唤醒之后处理do_commit 且当前hasNotified 已设置成true
            waitPoint.await(interval, TimeUnit.MILLISECONDS);
        } catch (InterruptedException e) {
            log.error("Interrupted", e);
        } finally {
            // 最后通知状态设置成false
            hasNotified.set(false);
            this.onWaitEnd();
        }
    }

实际上具体的刷盘逻辑是在doCommit方法中。因为采用的是读写分离的方式，所以在每次执行刷盘逻辑之前，都会交互读写容器中的数据。

private void swapRequests() {
            List<GroupCommitRequest> tmp = this.requestsWrite;
            this.requestsWrite = this.requestsRead;
            this.requestsRead = tmp;
        }

对于刷盘有没有成功的判断是这样的，每一次flush，都会记录好flush的位置，如果发现当前已经flush的位置已经超过了请求flush的位置，那就说明已经刷新成功，而这个过程运行重试2次。刷新完成之后便通知用户

        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;
                        // 确认有没有刷盘成功
                        for (int i = 0; i < 2 && !flushOK; i++) {
                            flushOK = CommitLog.this.mappedFileQueue.getFlushedWhere() >= req.getNextOffset();

                            if (!flushOK) {
                                CommitLog.this.mappedFileQueue.flush(0);
                            }
                        }
                        // 通知请求客户端，返回刷盘结果
                        req.wakeupCustomer(flushOK);
                    }

                    long storeTimestamp = CommitLog.this.mappedFileQueue.getStoreTimestamp();
                    if (storeTimestamp > 0) {
                        // 记录checkoutpoint
                        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);
                }
            }
        }

在深入看一下CommitLog.this.mappedFileQueue.flush(0);中的 mappedFile.flush(flushLeastPages);，实际上就是根据当前flushwhere的点位来找到对应的MappendFile，然后flush当前MapedFile中的writeBuffer或者mappedByteBuffer中的数据到自盘

    public int flush(final int flushLeastPages) {
        if (this.isAbleToFlush(flushLeastPages)) {
            if (this.hold()) {
                int value = getReadPosition();

                try {
                    //We only append data to fileChannel or mappedByteBuffer, never both.
                    if (writeBuffer != null || this.fileChannel.position() != 0) {
                        this.fileChannel.force(false);
                    } else {
                        this.mappedByteBuffer.force();
                    }
                } catch (Throwable e) {
                    log.error("Error occurred when force data to disk.", e);
                }

                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();
    }

异步刷盘FlushRealTimeService

异步刷盘分为定时异步调度刷盘和实时调度刷盘。

 class FlushRealTimeService extends FlushCommitLogService {
        private long lastFlushTimestamp = 0;
        private long printTimes = 0;

        public void run() {
            CommitLog.log.info(this.getServiceName() + " service started");

            while (!this.isStopped()) {
                // 实时刷盘还是调度刷盘
                boolean flushCommitLogTimed = CommitLog.this.defaultMessageStore.getMessageStoreConfig().isFlushCommitLogTimed();

                // 异步刷盘时间间隔  500ms
                int interval = CommitLog.this.defaultMessageStore.getMessageStoreConfig().getFlushIntervalCommitLog();
                // 每次刷新多少的pagecache页
                int flushPhysicQueueLeastPages = CommitLog.this.defaultMessageStore.getMessageStoreConfig().getFlushCommitLogLeastPages();
                // 10s
                int flushPhysicQueueThoroughInterval =
                    CommitLog.this.defaultMessageStore.getMessageStoreConfig().getFlushCommitLogThoroughInterval();

                boolean printFlushProgress = false;

                // Print flush progress
                long currentTimeMillis = System.currentTimeMillis();
                if (currentTimeMillis >= (this.lastFlushTimestamp + flushPhysicQueueThoroughInterval)) {
                    this.lastFlushTimestamp = currentTimeMillis;
                    flushPhysicQueueLeastPages = 0;
                    printFlushProgress = (printTimes++ % 10) == 0;
                }

                try {
                    if (flushCommitLogTimed) {
                        Thread.sleep(interval);  // 异步调度刷盘  还是  实时刷盘？
                    } else {
                        this.waitForRunning(interval);
                    }

                    if (printFlushProgress) {
                        this.printFlushProgress();
                    }

                    long begin = System.currentTimeMillis();
                    CommitLog.this.mappedFileQueue.flush(flushPhysicQueueLeastPages);
                    long storeTimestamp = CommitLog.this.mappedFileQueue.getStoreTimestamp();
                    if (storeTimestamp > 0) {
                        CommitLog.this.defaultMessageStore.getStoreCheckpoint().setPhysicMsgTimestamp(storeTimestamp);
                    }
                    long past = System.currentTimeMillis() - begin;
                    if (past > 500) {
                        log.info("Flush data to disk costs {} ms", past);
                    }
                } catch (Throwable e) {
                    CommitLog.log.warn(this.getServiceName() + " service has exception. ", e);
                    this.printFlushProgress();
                }
            }

            // Normal shutdown, to ensure that all the flush before exit
            boolean result = false;
            for (int i = 0; i < RETRY_TIMES_OVER && !result; i++) {
                result = CommitLog.this.mappedFileQueue.flush(0);
                CommitLog.log.info(this.getServiceName() + " service shutdown, retry " + (i + 1) + " times " + (result ? "OK" : "Not OK"));
            }

            this.printFlushProgress();

            CommitLog.log.info(this.getServiceName() + " service end");
        }

        @Override
        public String getServiceName() {
            return FlushRealTimeService.class.getSimpleName();
        }

        private void printFlushProgress() {
            // CommitLog.log.info("how much disk fall behind memory, "
            // + CommitLog.this.mappedFileQueue.howMuchFallBehind());
        }

        @Override
        public long getJointime() {
            return 1000 * 60 * 5;
        }
    }

这里面会去判断未flush的数据量有没有超过flushLeastPages，处理的方式就是 (flushOffset - writeOffset)  /   os_page_size  >=  flushLeastPages来决定是和否需要flush。

异步转存commit服务CommitRealTimeService

转存服务就是为了把对外内存中的数据写进filechannel。在CommitRealTimeService的run方法中：

boolean result = CommitLog.this.mappedFileQueue.commit(commitDataLeastPages);

    public boolean commit(final int commitLeastPages) {
        boolean result = true;
        // 获取committedWhere对应的mappedFile
        MappedFile mappedFile = this.findMappedFileByOffset(this.committedWhere, false);
        if (mappedFile != null) {
            // commit
            int offset = mappedFile.commit(commitLeastPages);
            long where = mappedFile.getFileFromOffset() + offset;
            result = where == this.committedWhere;
            this.committedWhere = where;
        }

        return result;
    }

protected void commit0(final int commitLeastPages) {
        int writePos = this.wrotePosition.get();
        int lastCommittedPosition = this.committedPosition.get();

        if (writePos - this.committedPosition.get() > 0) {
            try {
                ByteBuffer byteBuffer = writeBuffer.slice();
                byteBuffer.position(lastCommittedPosition);
                byteBuffer.limit(writePos);
                this.fileChannel.position(lastCommittedPosition);
                // commit主要是读取DM中的数据往文件通道写数据，然后交给异步刷盘服务去flush
                this.fileChannel.write(byteBuffer);
                this.committedPosition.set(writePos);
            } catch (Throwable e) {
                log.error("Error occurred when commit data to FileChannel.", e);
            }
        }
    }

 启动异步转存服务，一定是启用了对外内存池，且设置成异步刷盘的方式。