Flink源码阅读之Watermark对齐

watermark的产生

我们知道watermark的生成有两种方式：
1.在sourceFunction中通过emitWatermark方法生成
2.通过assignTimestampsAndWatermarks抽取timestamp并生成watermark

watermark的流转

watermark会像普通的element和stream status一样随着stream流不断的向下游流转

watermark的处理

前面文章写过job的执行过程，描述了数据怎么被处理的，所有流数据被封装成StreamElement对象，其有4个子类

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本文主要讨论waterm对齐的过程
具体处理逻辑在StreamTaskNetworkInput#processElement方法中

private void processElement(StreamElement recordOrMark, DataOutput<T> output) throws Exception {
        if (recordOrMark.isRecord()){
            output.emitRecord(recordOrMark.asRecord());
        } else if (recordOrMark.isWatermark()) {
                        //处理watermark
            statusWatermarkValve.inputWatermark(recordOrMark.asWatermark(), lastChannel);
        } else if (recordOrMark.isLatencyMarker()) {
            output.emitLatencyMarker(recordOrMark.asLatencyMarker());
        } else if (recordOrMark.isStreamStatus()) {
            statusWatermarkValve.inputStreamStatus(recordOrMark.asStreamStatus(), lastChannel);
        } else {
            throw new UnsupportedOperationException("Unknown type of StreamElement");
        }
    }

public void inputWatermark(Watermark watermark, int channelIndex) throws Exception {
        // ignore the input watermark if its input channel, or all input channels are idle (i.e. overall the valve is idle).
        //当前流状态是active，input channel的状态也是active，否则不处理
        if (lastOutputStreamStatus.isActive() && channelStatuses[channelIndex].streamStatus.isActive()) {
            long watermarkMillis = watermark.getTimestamp();

            // if the input watermark's value is less than the last received watermark for its input channel, ignore it also.
            //如果当前水印时间小于等于当前channel的水印时间，则忽略
            if (watermarkMillis > channelStatuses[channelIndex].watermark) {
                channelStatuses[channelIndex].watermark = watermarkMillis;

                // previously unaligned input channels are now aligned if its watermark has caught up
                //如果当前channel是未对齐状态，且当前水印时间大于上次发出的水印时间则任务当前channel已经对齐
                if (!channelStatuses[channelIndex].isWatermarkAligned && watermarkMillis >= lastOutputWatermark) {
                    channelStatuses[channelIndex].isWatermarkAligned = true;
                }

                // now, attempt to find a new min watermark across all aligned channels
                //取所有input channel中最小的watermark，当做最新的watermark发出  
                findAndOutputNewMinWatermarkAcrossAlignedChannels();
            }
        }
    }

flink通过InputChannelStatus数组来维护一个算子的所有input channel

/**
     * An {@code InputChannelStatus} keeps track of an input channel's last watermark, stream
     * status, and whether or not the channel's current watermark is aligned with the overall
     * watermark output from the valve.
     *
     * <p>There are 2 situations where a channel's watermark is not considered aligned:
     * <ul>
     *   <li>the current stream status of the channel is idle
     *   <li>the stream status has resumed to be active, but the watermark of the channel hasn't
     *   caught up to the last output watermark from the valve yet.
     * </ul>
     */
    @VisibleForTesting
    protected static class InputChannelStatus {
        protected long watermark;//当前channel最后一个watermark的时间戳
        protected StreamStatus streamStatus;//流的状态
        protected boolean isWatermarkAligned;//watermark是否对齐
}

上面的doc描述的很清楚，一个InputChannelStatus对象维护一个channel发送的最后一个watermark的时间戳，当前流的状态（active or idle），当前watermark是否和总体输出的watermark对齐。
同时有2种情况下不需要对齐：
1.当前stream状态是idle。
2.stream状态是刚恢复成active，且当前channel的watermark还没赶上总体输出的最新的水印。

上面inputWatermark方法的注释已经大概说明了channel对齐的过程。取所有input channel的最小watermark并作为当前watermark逻辑如下：

private void findAndOutputNewMinWatermarkAcrossAlignedChannels() throws Exception {
        long newMinWatermark = Long.MAX_VALUE;
        boolean hasAlignedChannels = false;

        // determine new overall watermark by considering only watermark-aligned channels across all channels
        //只有已对齐的channel才会参与比较
        for (InputChannelStatus channelStatus : channelStatuses) {
            if (channelStatus.isWatermarkAligned) {
                hasAlignedChannels = true;
                newMinWatermark = Math.min(channelStatus.watermark, newMinWatermark);
            }
        }

        // we acknowledge and output the new overall watermark if it really is aggregated
        // from some remaining aligned channel, and is also larger than the last output watermark
        //从已对其的channel中获取到的最小的watermark，如果大于上次发出的watermark则作为最新的watermark发出。
        if (hasAlignedChannels && newMinWatermark > lastOutputWatermark) {
            lastOutputWatermark = newMinWatermark;
            output.emitWatermark(new Watermark(lastOutputWatermark));
        }
    }

至此watermark对齐取最小的逻辑已分析完毕。