上次我们讲到了 Flink Checkpoint Barrier 全流程 还有 Flink 消费消息的全流程
分类
Flink 处理 Barrier 分两种：
- barrier 对齐
- barrier 不对齐 对应的类
我们就以 BarrierBuffer ( barrier 对齐 ) 为例。
正文
关键就是 getNextNonBlocked 方法
代码语言：javascript复制 @Override
// 从 ResultSubPartition 中获取数据并处理 barrier
public BufferOrEvent getNextNonBlocked() throws Exception {
while (true) {
// process buffered BufferOrEvents before grabbing new ones
Optional<BufferOrEvent> next;
//barrier block 解除后 currentBuffered 不为 null，其他情况都是 null 了
if (currentBuffered == null) {
// 如果当前有堆积的消息，直接从 InputGate 中获取，否则从缓存中获取(通过 CachedBufferBlocker 缓存的数据)
// 通过 inputGate 中的 inputChannel 来获取 ResultSubPartition 中的数据
next = inputGate.getNextBufferOrEvent();
}else {
//barrier block 解除后 next 中的 value 不为 null
next = Optional.ofNullable(currentBuffered.getNext());
if (!next.isPresent()) {
//完成缓冲数据的消费
completeBufferedSequence();
return getNextNonBlocked();
}
}
if (!next.isPresent()) {
if (!endOfStream) {
// end of input stream. stream continues with the buffered data
endOfStream = true;
releaseBlocksAndResetBarriers();
return getNextNonBlocked();
}
else {
// final end of both input and buffered data
return null;
}
}
//当 barrier 全部对齐之后，先消费 bufferBlocker 中的 ArrayDeque<BufferOrEvent> currentBuffers 的数据
BufferOrEvent bufferOrEvent = next.get();
if (isBlocked(bufferOrEvent.getChannelIndex())) {
// if the channel is blocked, we just store the BufferOrEvent
// barrier 对齐 缓存数据
bufferBlocker.add(bufferOrEvent);
checkSizeLimit();
}
else if (bufferOrEvent.isBuffer()) {
return bufferOrEvent;
}
// 处理 barrier
else if (bufferOrEvent.getEvent().getClass() == CheckpointBarrier.class) {
if (!endOfStream) {
// process barriers only if there is a chance of the checkpoint completing
//除 trigger task 外的 operator 都是在这里做的 checkpoint 只有通过 processInput 消费到才表示 barrier 经过了上游算子
processBarrier((CheckpointBarrier) bufferOrEvent.getEvent(), bufferOrEvent.getChannelIndex());
}
}
else if (bufferOrEvent.getEvent().getClass() == CancelCheckpointMarker.class) {
processCancellationBarrier((CancelCheckpointMarker) bufferOrEvent.getEvent());
}
else {
if (bufferOrEvent.getEvent().getClass() == EndOfPartitionEvent.class) {
processEndOfPartition();
}
return bufferOrEvent;
}
}
}当没有发生 barrier 对齐完成 这个动作时，currentBuffered == null，currentBuffered 就是当前要处理的 buffer，当 buffer 是数据的时候它就正常消费数据走 Flink 消费消息的全流程，当遇到 barrier 时，开始处理 barrier
代码语言：javascript复制// 一个 opertor 必须收到从每个 inputchannel 发过来的同一序号的 barrier 之后才能发起本节点的 checkpoint，
// 如果有的 channel 的数据处理的快了，那该 barrier 后的数据还需要缓存起来，
// 如果有的 inputchannel 被关闭了，那它就不会再发送 barrier 过来了
private void processBarrier(CheckpointBarrier receivedBarrier, int channelIndex) throws Exception {
final long barrierId = receivedBarrier.getId();
// fast path for single channel cases
if (totalNumberOfInputChannels == 1) {
if (barrierId > currentCheckpointId) {
// new checkpoint
currentCheckpointId = barrierId;
// 触发 checkpoint
notifyCheckpoint(receivedBarrier);
}
return;
}
// -- general code path for multiple input channels --
// 大于等于第二次处理 barrier 的时候
if (numBarriersReceived > 0) {
// this is only true if some alignment is already progress and was not canceled
if (barrierId == currentCheckpointId) {
// regular case
//阻塞 channelIndex 对应的 channel 其实就是 blockedChannels[channelIndex] = true;
onBarrier(channelIndex);
}else if (barrierId > currentCheckpointId) {
// we did not complete the current checkpoint, another started before
LOG.warn("{}: Received checkpoint barrier for checkpoint {} before completing current checkpoint {}. "
"Skipping current checkpoint.",
inputGate.getOwningTaskName(),
barrierId,
currentCheckpointId);
// let the task know we are not completing this
notifyAbort(currentCheckpointId, new CheckpointDeclineSubsumedException(barrierId));
// abort the current checkpoint
releaseBlocksAndResetBarriers();
// begin a the new checkpoint
beginNewAlignment(barrierId, channelIndex);
}else {
// ignore trailing barrier from an earlier checkpoint (obsolete now)
return;
}
}else if (barrierId > currentCheckpointId) {
// first barrier of a new checkpoint
beginNewAlignment(barrierId, channelIndex);
}else {
// either the current checkpoint was canceled (numBarriers == 0) or
// this barrier is from an old subsumed checkpoint
return;
}
// check if we have all barriers - since canceled checkpoints always have zero barriers
// this can only happen on a non canceled checkpoint
if (numBarriersReceived numClosedChannels == totalNumberOfInputChannels) {
// actually trigger checkpoint
if (LOG.isDebugEnabled()) {
LOG.debug("{}: Received all barriers, triggering checkpoint {} at {}.",
inputGate.getOwningTaskName(),
receivedBarrier.getId(),
receivedBarrier.getTimestamp());
}
releaseBlocksAndResetBarriers();
// 当收到全部的 barrier 之后，就会触发 notifyCheckpoint()，
// 该方法又会调用 StreamTask 的 triggerCheckpoint ，和之前的operator是一样的
notifyCheckpoint(receivedBarrier);
}
}numBarriersReceived 的默认值是0，所以第一个 barrier 进来后，会进入 beginNewAlignment 方法
代码语言：javascript复制private void beginNewAlignment(long checkpointId, int channelIndex) throws IOException {
currentCheckpointId = checkpointId;
//numBarriersReceived 并设置 channelIndex 对应的 channel 为 block channel
onBarrier(channelIndex);
startOfAlignmentTimestamp = System.nanoTime();
if (LOG.isDebugEnabled()) {
LOG.debug("{}: Starting stream alignment for checkpoint {}.",
inputGate.getOwningTaskName(),
checkpointId);
}
}当再有其他相同的 barrier 进入时，barrierId == currentCheckpointId 为 true，直到 numBarriersReceived numClosedChannels == totalNumberOfInputChannels 时，触发 notifyCheckpoint，并报告 alignment buffer 以及 alignment time。(彩蛋： 稍后会更新 checkpoint 全流程欢迎关注 )。
如果其他的 channel 中的 barrier 延迟了，即 numBarriersReceived numClosedChannels != totalNumberOfInputChannels，已经 receive barrier 对应的 channel 数据会进入 bufferBlocker。
bufferBlocker 是通过 ArrayDeque currentBuffers 来存储数据的，也就是说默认情况下 bufferBlocker.currentBuffers 会无限增大。
当 numBarriersReceived numClosedChannels == totalNumberOfInputChannels 时，会先进行 releaseBlocksAndResetBarriers() 在进行 notifyCheckpoint。
releaseBlocksAndResetBarriers 主要的目的是要先消费已加入缓存中的数据。
代码语言：javascript复制/**
* Releases the blocks on all channels and resets the barrier count.
* Makes sure the just written data is the next to be consumed.
*/
// 将 bufferBlocker 里面缓存的数据 bufferOrEvent 赋值给 currentBuffered
private void releaseBlocksAndResetBarriers() throws IOException {
LOG.debug("{}: End of stream alignment, feeding buffered data back.",
inputGate.getOwningTaskName());
for (int i = 0; i < blockedChannels.length; i ) {
blockedChannels[i] = false;
}
if (currentBuffered == null) {
// common case: no more buffered data
currentBuffered = bufferBlocker.rollOverReusingResources();
if (currentBuffered != null) {
currentBuffered.open();
}
}else {
// uncommon case: buffered data pending
// push back the pending data, if we have any
LOG.debug("{}: Checkpoint skipped via buffered data:"
"Pushing back current alignment buffers and feeding back new alignment data first.",
inputGate.getOwningTaskName());
// since we did not fully drain the previous sequence, we need to allocate a new buffer for this one
BufferOrEventSequence bufferedNow = bufferBlocker.rollOverWithoutReusingResources();
if (bufferedNow != null) {
bufferedNow.open();
queuedBuffered.addFirst(currentBuffered);
numQueuedBytes = currentBuffered.size();
currentBuffered = bufferedNow;
}
}
if (LOG.isDebugEnabled()) {
LOG.debug("{}: Size of buffered data: {} bytes",
inputGate.getOwningTaskName(),
currentBuffered == null ? 0L : currentBuffered.size());
}
// the next barrier that comes must assume it is the first
numBarriersReceived = 0;
if (startOfAlignmentTimestamp > 0) {
latestAlignmentDurationNanos = System.nanoTime() - startOfAlignmentTimestamp;
startOfAlignmentTimestamp = 0;
}
}当执行完 releaseBlocksAndResetBarriers 方法时，currentBuffered!=null 了，会进入
代码语言：javascript复制//barrier block 解除后 next 中的 value 不为 null
next = Optional.ofNullable(currentBuffered.getNext());然后直接消费数据
代码语言：javascript复制//当 barrier 全部对齐之后，先消费 bufferBlocker 中的 ArrayDeque<BufferOrEvent> currentBuffers 的数据
BufferOrEvent bufferOrEvent = next.get();
if (isBlocked(bufferOrEvent.getChannelIndex())) {
// if the channel is blocked, we just store the BufferOrEvent
// barrier 对齐 缓存数据
bufferBlocker.add(bufferOrEvent);
checkSizeLimit();
}
else if (bufferOrEvent.isBuffer()) {
return bufferOrEvent;
}一直消费缓存中的数据( 此过程会阻塞不会继续消费 inputGate 中的数据)，直至消耗完成
代码语言：javascript复制next = Optional.ofNullable(currentBuffered.getNext());
if (!next.isPresent()) {
//完成缓冲数据的消费
completeBufferedSequence();
return getNextNonBlocked();
}完成了之后，就跟程序第一次运行至此一样，循环往复。
总结
