Flink Window Trigger (触发器)

Trigger 的作用

英文单词 trigger 的意思是触发，作为名词是扳机的意思，例如枪支上的扳机就叫 trigger，所以也有开火的意思。Flink中，window操作需要伴随对窗口中的数据进行处理的逻辑，也就是窗口函数，而 Trigger 的作用就是决定何时触发窗口函数中的逻辑执行。

Trigger 抽象类

Flink中定义了Trigger抽象类，任何trigger必须继承Trigger类，并实现其中的onElement(),onProcessingTime(),onEventTime(),clear()等抽象方法，Flink官方提供了几种常用的trigger实现，同时，用户可以根据需求自定义trigger，以下是Trigger类的部分代码：

public abstract class Trigger<T, W extends Window> implements Serializable {

	private static final long serialVersionUID = -4104633972991191369L;

	/**
	 * Called for every element that gets added to a pane. The result of this will determine
	 * whether the pane is evaluated to emit results.
	 *
	 * @param element The element that arrived.
	 * @param timestamp The timestamp of the element that arrived.
	 * @param window The window to which the element is being added.
	 * @param ctx A context object that can be used to register timer callbacks.
	 */
	public abstract TriggerResult onElement(T element, long timestamp, W window, TriggerContext ctx) throws Exception;

	/**
	 * Called when a processing-time timer that was set using the trigger context fires.
	 *
	 * @param time The timestamp at which the timer fired.
	 * @param window The window for which the timer fired.
	 * @param ctx A context object that can be used to register timer callbacks.
	 */
	public abstract TriggerResult onProcessingTime(long time, W window, TriggerContext ctx) throws Exception;

	/**
	 * Called when an event-time timer that was set using the trigger context fires.
	 *
	 * @param time The timestamp at which the timer fired.
	 * @param window The window for which the timer fired.
	 * @param ctx A context object that can be used to register timer callbacks.
	 */
	public abstract TriggerResult onEventTime(long time, W window, TriggerContext ctx) throws Exception;

	/**
	 * Returns true if this trigger supports merging of trigger state and can therefore
	 * be used with a
	 * {@link org.apache.flink.streaming.api.windowing.assigners.MergingWindowAssigner}.
	 *
	 * <p>If this returns {@code true} you must properly implement
	 * {@link #onMerge(Window, OnMergeContext)}
	 */
	public boolean canMerge() {
		return false;
	}

	/**
	 * Called when several windows have been merged into one window by the
	 * {@link org.apache.flink.streaming.api.windowing.assigners.WindowAssigner}.
	 *
	 * @param window The new window that results from the merge.
	 * @param ctx A context object that can be used to register timer callbacks and access state.
	 */
	public void onMerge(W window, OnMergeContext ctx) throws Exception {
		throw new UnsupportedOperationException("This trigger does not support merging.");
	}

	/**
	 * Clears any state that the trigger might still hold for the given window. This is called
	 * when a window is purged. Timers set using {@link TriggerContext#registerEventTimeTimer(long)}
	 * and {@link TriggerContext#registerProcessingTimeTimer(long)} should be deleted here as
	 * well as state acquired using {@link TriggerContext#getPartitionedState(StateDescriptor)}.
	 */
	public abstract void clear(W window, TriggerContext ctx) throws Exception;
	
}

• onElement() 方法会在窗口中每进入一条数据的时候调用一次
• onProcessingTime() 方法会在一个ProcessingTime定时器触发的时候调用
• onEventTime()方法会在一个EventTime定时器触发的时候调用
• clear()方法会在窗口清除的时候调用