周期性触发的自定义触发器

背景

本文我们实现一个周期性触发的自定义触发器，顺便看下实现自定义触发器的一些要点

周期性触发器实现

实现一个每分钟触发一次的自定义事件时间触发器，实现代码和注意事项如下所示

package wikiedits.trigger;

import org.apache.flink.api.common.state.ValueState;
import org.apache.flink.api.common.state.ValueStateDescriptor;
import org.apache.flink.api.common.typeinfo.TypeInformation;
import org.apache.flink.streaming.api.windowing.triggers.Trigger;
import org.apache.flink.streaming.api.windowing.triggers.TriggerResult;
import org.apache.flink.streaming.api.windowing.windows.Window;

public class OneMinuteIntervalTrigger<W extends Window> extends Trigger<Object, W> {
    private static final long serialVersionUID = 1L;

    private final long interval;

    // 触发时间的状态对象
    private final ValueStateDescriptor<Long> stateDesc =
            new ValueStateDescriptor<>("fire-time", TypeInformation.of(Long.class));

    private OneMinuteIntervalTrigger(long interval) {
        this.interval = interval;
    }

    @Override
    public TriggerResult onElement(Object element, long timestamp, W window, TriggerContext ctx) throws Exception {

        if (window.maxTimestamp() <= ctx.getCurrentWatermark()) {// 这里其实不是必要的,取决于窗口结束时间到之后是否要触发一次计算
            // if the watermark is already past the window fire immediately
            return TriggerResult.FIRE;
        } else {// 多次注册也没事，反正是同一个计时器，这表明窗口结束时想要触发一次计算,此外注意getEnd和maxTimestamp方法的区别
            ctx.registerEventTimeTimer(window.maxTimestamp());
        }

        // 仅仅在第一次未注册时注册一次,后续由ontimer触发
        ValueState<Long> fireTimestamp = ctx.getPartitionedState(stateDesc);
        if (fireTimestamp.value() == null) {
            long start = timestamp - (timestamp % interval);
            long nextFireTimestamp = start + interval;
            ctx.registerEventTimeTimer(nextFireTimestamp);
            fireTimestamp.update(nextFireTimestamp);
        }

        return TriggerResult.CONTINUE;
    }

    // 计时器触发的函数
    @Override
    public TriggerResult onEventTime(long time, W window, TriggerContext ctx) throws Exception {

        // 这里窗口结束时触发不是必要的，取决于是否想要在窗口结束是触发一次计算，并且这里如果不处理延迟的消息，可以返回FIRE_AND_PURGE清理窗口状态(但是注意即使返回PURGE，也不会清理触发器的状态)
        if (time == window.maxTimestamp()) {
            return TriggerResult.FIRE;
        }

        ValueState<Long> fireTimestampState = ctx.getPartitionedState(stateDesc);

        Long fireTimestamp = fireTimestampState.value();

        // 继续注册计时器
        if (fireTimestamp != null && fireTimestamp == time) {
            fireTimestampState.update(time + interval);
            ctx.registerEventTimeTimer(time + interval);
            return TriggerResult.FIRE;
        }

        return TriggerResult.CONTINUE;
    }

    @Override
    public TriggerResult onProcessingTime(long time, W window, TriggerContext ctx) throws Exception {
        return TriggerResult.CONTINUE;
    }

    @Override
    public void clear(W window, TriggerContext ctx) throws Exception {
        // 清理触发器状态
        ValueState<Long> fireTimestamp = ctx.getPartitionedState(stateDesc);
        Long timestamp = fireTimestamp.value();
        if (timestamp != null) {
            ctx.deleteEventTimeTimer(timestamp);
            fireTimestamp.clear();
        }
    }

}

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代码里面注解已经比较详细的说明了注意事项，此外对于状态的清理，我们需要看的是WindowOperator，如下