RocketMQ存储篇三：追加消息

第一篇介绍了存储的概览，定位到消息追加消息处细看一下
获取mappedFile ——> 追加消息 ——> 处理结果

@DefaultMappedFile#appendMsgInner()

public AppendMessageResult appendMessagesInner(final MessageExt messageExt, final AppendMessageCallback cb,
        PutMessageContext putMessageContext) {
        assert messageExt != null;
        assert cb != null;

        int currentPos = WROTE_POSITION_UPDATER.get(this);

        if (currentPos < this.fileSize) {
            ByteBuffer byteBuffer = appendMessageBuffer().slice();
            byteBuffer.position(currentPos);
            AppendMessageResult result;
            if (messageExt instanceof MessageExtBatch && !((MessageExtBatch) messageExt).isInnerBatch()) {
                // traditional batch message
                result = cb.doAppend(this.getFileFromOffset(), byteBuffer, this.fileSize - currentPos,
                    (MessageExtBatch) messageExt, putMessageContext);
            } else if (messageExt instanceof MessageExtBrokerInner) {
                // traditional single message or newly introduced inner-batch message
				// 使用回调函数来处理append消息的过程
                result = cb.doAppend(this.getFileFromOffset(), byteBuffer, this.fileSize - currentPos,
                    (MessageExtBrokerInner) messageExt, putMessageContext);
            } else {
                return new AppendMessageResult(AppendMessageStatus.UNKNOWN_ERROR);
            }
            WROTE_POSITION_UPDATER.addAndGet(this, result.getWroteBytes());
            this.storeTimestamp = result.getStoreTimestamp();
            return result;
        }
        log.error("MappedFile.appendMessage return null, wrotePosition: {} fileSize: {}", currentPos, this.fileSize);
        return new AppendMessageResult(AppendMessageStatus.UNKNOWN_ERROR);
    }

@DefaultAppendMessageCallback#doAppend()
处理逻辑

1. 根据bytebuffer中的wroteposition以及mappedfile开头获取物理offset
   long wroteOffset = fileFromOffset + byteBuffer.position();
2. 构造msgId
3. 判断mappedfile是否有足够的空间写msg, 如果没有，就返回一个mappedFile用完的状态，重新分配mappedfile并写入，如果空间足够，就修改encode阶段消息中的一些假值并返回PUT_OK状态
   if ((msgLen + END_FILE_MIN_BLANK_LENGTH) > maxBlank)
4. 根据put的状态来做处理（刷盘以及ha）

那么，消息到底追加到了哪里呢？
如果开启了trasientPoolEnable，就是writeBuffer，否则是mappedByteBuffer
writeBuffer是从trasientPool中borrow的，应用了池化的思想，在刷盘时有作用，后面叙述
mappedByteBuffer就是commitlog文件的映射

    protected ByteBuffer appendMessageBuffer() {
        this.mappedByteBufferAccessCountSinceLastSwap++;
        return writeBuffer != null ? writeBuffer : this.mappedByteBuffer;
    }