flink 事件概念 flink事件时间

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langrisser 2024-07-19 14:25:12

文章标签 flink 事件概念 flink apache ide 文章分类 架构后端开发

flink在event time处理模式下的watermarks分析。

概念先行

stream processor(event time)需要一种方法来衡量事件时间的进度。例如当使用一小时时间窗口处理数据时，窗口时间结束时需要通知window operator(one hour operator)关闭正在运行的窗口，是否可以关闭运行的窗口，是由watermark和当前event time决定的。
flink衡量event time进度的方式就是watermarks，watermarks是datastream的一部分，总会带有一个时间戳t。Watermark(t)表明event time已经到达了该数据流中的t时间点，流中后续不会再出现带有t’<t的元素。

下图是一个使用逻辑时间轴的steam，图下面是watermark数据。图中的events是按时间升序的，这样的stream中的watermark只是流中的周期性标记。

flink 事件概念 flink事件时间_flink

下面这个例子中的流是无序的，水印对于这种无序流是非常重要的。下图中的事件没有按事件排序。watermark可以理解为stream中的一点：

所有时间戳比这个点小的事件都已经到达了
换句话说，watermark(t)后面不会再出现比t小的事件

当operator读取到watermark会把内部的event time时钟调整到watermark的时间点

flink 事件概念 flink事件时间_flink 事件概念_02

实例分析

code

package com.f3.training;

import com.f3.datatypes.ConnectedCarEvent;
import org.apache.flink.api.common.functions.MapFunction;
import org.apache.flink.api.common.state.ValueState;
import org.apache.flink.api.common.state.ValueStateDescriptor;
import org.apache.flink.api.common.typeinfo.TypeHint;
import org.apache.flink.api.common.typeinfo.TypeInformation;
import org.apache.flink.api.java.utils.ParameterTool;
import org.apache.flink.configuration.Configuration;
import org.apache.flink.streaming.api.TimeCharacteristic;
import org.apache.flink.streaming.api.TimerService;
import org.apache.flink.streaming.api.datastream.DataStream;
import org.apache.flink.streaming.api.environment.StreamExecutionEnvironment;
import org.apache.flink.streaming.api.functions.AssignerWithPunctuatedWatermarks;
import org.apache.flink.streaming.api.functions.KeyedProcessFunction;
import org.apache.flink.streaming.api.functions.ProcessFunction;
import org.apache.flink.streaming.api.watermark.Watermark;
import org.apache.flink.util.Collector;
import org.joda.time.DateTime;

import javax.xml.crypto.Data;
import java.util.PriorityQueue;


public class Lab3CarEventSort {
    public static void main(String[] args) throws Exception {

        ParameterTool params = ParameterTool.fromArgs(args);

        StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();
        env.setStreamTimeCharacteristic(TimeCharacteristic.EventTime);
        env.setParallelism(1);

        DataStream<String> carData = env.readTextFile(TrainingBase.pathToCarOutOfOrder);

        DataStream<ConnectedCarEvent> events = carData
                .map((MapFunction<String, ConnectedCarEvent>) ConnectedCarEvent::fromString)
                .assignTimestampsAndWatermarks(new ConnectedCarAssigner());

        //events.print();

        events.keyBy((ConnectedCarEvent event) -> event.carId)
                .process(new SortFunction())
                .print();

        env.execute();


    }

    public static class ConnectedCarAssigner implements AssignerWithPunctuatedWatermarks<ConnectedCarEvent> {
        @Override
        public long extractTimestamp(ConnectedCarEvent event, long previousElementTimestamp) {

            return event.timestamp;
        }

        @Override
        public Watermark checkAndGetNextWatermark(ConnectedCarEvent event, long extractedTimestamp) {
            // simply emit a watermark with every event
            return new Watermark(extractedTimestamp - 30000);
        }
    }


    public static class SortFunction extends KeyedProcessFunction<String, ConnectedCarEvent, ConnectedCarEvent> {
        private ValueState<PriorityQueue<ConnectedCarEvent>> queueState = null;

        @Override
        public void open(Configuration config) {
            ValueStateDescriptor<PriorityQueue<ConnectedCarEvent>> descriptor = new ValueStateDescriptor<>(
                    "sorted-events",
                    TypeInformation.of(new TypeHint<PriorityQueue<ConnectedCarEvent>>() {}));
            queueState = getRuntimeContext().getState(descriptor);
        }

        @Override
        public void processElement(ConnectedCarEvent event, Context context, Collector<ConnectedCarEvent> out) throws Exception {
            TimerService timerService = context.timerService();
            // [1] ts - currentWatermark > 0
            // [2] pre_ts - 30s = currentWatermark
            // [1-2] ts - (pre_ts - 30s) > 0
            //       pre_ts - ts < 30
            // if true: Ordered within error tolerance
            if (context.timestamp() > timerService.currentWatermark()) {
                PriorityQueue<ConnectedCarEvent> queue = queueState.value();
                if (queue == null) {
                    queue = new PriorityQueue<>(10);
                }
                queue.add(event);
                queueState.update(queue);
                timerService.registerEventTimeTimer(event.timestamp);
            }
        }

        @Override
        public void onTimer(long timestamp, OnTimerContext context, Collector<ConnectedCarEvent> out) throws Exception {
            PriorityQueue<ConnectedCarEvent> queue = queueState.value();
            Long watermark = context.timerService().currentWatermark();
            ConnectedCarEvent head = queue.peek();
            while (head != null && head.timestamp <= watermark) {
                out.collect(head);
                queue.remove(head);
                head = queue.peek();
            }
        }
    }
}

分析

/**
 * ----------> 重要排序实例 <----------
 *
 * id             : String  // a unique id for each event
 * car_id         : String  // a unique id for the car
 * timestamp      : long    // timestamp (milliseconds since the epoch)
 * longitude      : float   // GPS longitude
 * latitude       : float   // GPS latitude
 * consumption    : float   // fuel consumption (liters per hour)
 * speed          : float   // speed (kilometers per hour)
 * throttle       : float   // throttle position (%)
 * engineload     : float   // engine load (%)
 *
 *
 * 看一个乱序的例子（重要）：
 *
 * 1484892913000,2017-01-20T06:15:13+0000 - wm:1484892878000,2017-01-20T14:14:38.000+08:00 = last_ts-300000
 * 1484892928000,2017-01-20T06:15:28+0000 - wm:1484892883000,2017-01-20T14:14:43.000+08:00 = last_ts-300000
 * 1484892918000,2017-01-20T06:15:18+0000 - wm:1484892898000,2017-01-20T14:14:58.000+08:00 = last_ts-300000
 **1484892893000,2017-01-20T06:14:53+0000 - wm:1484892898000,2017-01-20T14:14:58.000+08:00 = last_ts-200000
 * 1484892923000,2017-01-20T06:15:23+0000 - wm:1484892898000,2017-01-20T14:14:58.000+08:00 = last_ts+6
 * 1484892933000,2017-01-20T06:15:33+0000 - wm:1484892898000,2017-01-20T14:14:58.000+08:00 = last_ts-025
 * 1484892938000,2017-01-20T06:15:38+0000 - wm:1484892903000,2017-01-20T14:15:03.000+08:00 = last_ts-300000
 * 1484892943000,2017-01-20T06:15:43+0000 - wm:1484892908000,2017-01-20T14:15:08.000+08:00 = last_ts-300000
 *
 * WM不会减小，乱序的元素的wm还是按前面元素的值计算出来的，所以会由于本身乱序（比如递增数列中减小了）
 * 还使用之前的wm（使用较大的wm）出现一种情况，就是wm>乱序ts的情况，这种情况出现说明乱序已经超过
 * 了WM的容忍范围。
 * 例如上面的1484892893000,2017-01-20T06:14:53+0000时间点的WM>TS，因为乱序的时间戳已经超过了
 * 30000，综上可以通过判断ts是否小于wm来判断是否当前数据超出乱序容忍范围。
 *
 * 排序原理：
 * 1、process准入条件是乱序不能大于30s
 * 2、process压入最小堆
 * 3、process对每个时间点注册Timer
 *
 * 4、Timer启动之后 准备弹出最小堆的数据，
 *    条件是数据的ts<当前的wm，注意是触发点的wm，可能已经在几个ts之后了
 */