在flink中使用event time时，一般需要自定义Timestamp Extractors / Watermark Emitters，实现AssignerWithPeriodicWatermarks 或者 AssignerWithPunctuatedWatermarks 这两个接口，取决于你的业务中是需要一个周期性的watermark，还是一个基于stream中某个特殊元素或者元素中某个特殊字段值生成一个watermark。
而flink中提供了一些已经实现好的Timestamp Extractors/Watermark Emiters，下面详细学习下：
AscendingTimestampExtractor-递增时间戳的分配器
老规矩，先看代码
package org.apache.flink.streaming.api.functions.timestamps;
import org.apache.flink.annotation.PublicEvolving;
import org.apache.flink.streaming.api.functions.AssignerWithPeriodicWatermarks;
import org.apache.flink.streaming.api.watermark.Watermark;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import static java.util.Objects.requireNonNull;
/**
* A timestamp assigner and watermark generator for streams where timestamps are monotonously
* ascending. In this case, the local watermarks for the streams are easy to generate, because
* they strictly follow the timestamps.
*
* @param <T> The type of the elements that this function can extract timestamps from
*/
@PublicEvolving
public abstract class AscendingTimestampExtractor<T> implements AssignerWithPeriodicWatermarks<T> {
private static final long serialVersionUID = 1L;
/** The current timestamp. */
private long currentTimestamp = Long.MIN_VALUE;
/** Handler that is called when timestamp monotony is violated. */
private MonotonyViolationHandler violationHandler = new LoggingHandler();
/**
* Extracts the timestamp from the given element. The timestamp must be monotonically increasing.
*
* @param element The element that the timestamp is extracted from.
* @return The new timestamp.
*/
public abstract long extractAscendingTimestamp(T element);
/**
* Sets the handler for violations to the ascending timestamp order.
*
* @param handler The violation handler to use.
* @return This extractor.
*/
public AscendingTimestampExtractor<T> withViolationHandler(MonotonyViolationHandler handler) {
this.violationHandler = requireNonNull(handler);
return this;
}
// ------------------------------------------------------------------------
@Override
public final long extractTimestamp(T element, long elementPrevTimestamp) {
final long newTimestamp = extractAscendingTimestamp(element);
if (newTimestamp >= this.currentTimestamp) {
this.currentTimestamp = newTimestamp;
return newTimestamp;
} else {
violationHandler.handleViolation(newTimestamp, this.currentTimestamp);
return newTimestamp;
}
}
@Override
public final Watermark getCurrentWatermark() {
return new Watermark(currentTimestamp == Long.MIN_VALUE ? Long.MIN_VALUE : currentTimestamp - 1);
}
// ------------------------------------------------------------------------
// Handling violations of monotonous timestamps
// ------------------------------------------------------------------------
/**
* Interface for handlers that handle violations of the monotonous ascending timestamps
* property.
*/
public interface MonotonyViolationHandler extends java.io.Serializable {
/**
* Called when the property of monotonously ascending timestamps is violated, i.e.,
* when {@code elementTimestamp < lastTimestamp}.
*
* @param elementTimestamp The timestamp of the current element.
* @param lastTimestamp The last timestamp.
*/
void handleViolation(long elementTimestamp, long lastTimestamp);
}
/**
* Handler that does nothing when timestamp monotony is violated.
*/
public static final class IgnoringHandler implements MonotonyViolationHandler {
private static final long serialVersionUID = 1L;
@Override
public void handleViolation(long elementTimestamp, long lastTimestamp) {}
}
/**
* Handler that fails the program when timestamp monotony is violated.
*/
public static final class FailingHandler implements MonotonyViolationHandler {
private static final long serialVersionUID = 1L;
@Override
public void handleViolation(long elementTimestamp, long lastTimestamp) {
throw new RuntimeException("Ascending timestamps condition violated. Element timestamp "
+ elementTimestamp + " is smaller than last timestamp " + lastTimestamp);
}
}
/**
* Handler that only logs violations of timestamp monotony, on WARN log level.
*/
public static final class LoggingHandler implements MonotonyViolationHandler {
private static final long serialVersionUID = 1L;
private static final Logger LOG = LoggerFactory.getLogger(AscendingTimestampExtractor.class);
@Override
public void handleViolation(long elementTimestamp, long lastTimestamp) {
LOG.warn("Timestamp monotony violated: {} < {}", elementTimestamp, lastTimestamp);
}
}
}
1、是抽象类，实现AssignerWithPeriodicWatermarks接口，是周期性生成watermark。同样的，周期间隔时间一样通过ExecutionConfig设置。
2、在source中，每个元素的timestamp必须是递增产生的；当然，到达flink中的顺序可能错乱
3、已经实现了AssignerWithPeriodicWatermarks中定义的extractTimestamp、getCurrentWatermark方法，其中extractTimestamp指定了新的timestamp，数据来源于自定义的抽象方法extractAscendingTimestamp；而getCurrentWatermark方法则使用了currentTimestamp - 1的值
4、使用此类需要实现extractAscendingTimestamp，用于指定一个递增的时间戳。
5、至于MonotonyViolationHandler，其三个子类或者ignore、或者fail抛出异常、或者只是记录log。只有抛出异常才会打断整个过程。
使用方式如下（来源于）：
DataStream<MyEvent> stream = ...
DataStream<MyEvent> withTimestampsAndWatermarks =
stream.assignTimestampsAndWatermarks(new AscendingTimestampExtractor<MyEvent>() {
@Override
public long extractAscendingTimestamp(MyEvent element) {
return element.getCreationTime();
}
});
BoundedOutOfOrdernessTimestampExtractor--允许固定时间延迟的timestamp分配器
这个类实例构造函数需要传入一个时间，比如Time.second(30)，意思是30秒内到达的数据，可以允许在timewindow内处理。
先看代码
package org.apache.flink.streaming.api.functions.timestamps;
import org.apache.flink.streaming.api.functions.AssignerWithPeriodicWatermarks;
import org.apache.flink.streaming.api.watermark.Watermark;
import org.apache.flink.streaming.api.windowing.time.Time;
/**
* This is a {@link AssignerWithPeriodicWatermarks} used to emit Watermarks that lag behind the element with
* the maximum timestamp (in event time) seen so far by a fixed amount of time, <code>t_late</code>. This can
* help reduce the number of elements that are ignored due to lateness when computing the final result for a
* given window, in the case where we know that elements arrive no later than <code>t_late</code> units of time
* after the watermark that signals that the system event-time has advanced past their (event-time) timestamp.
* */
public abstract class BoundedOutOfOrdernessTimestampExtractor<T> implements AssignerWithPeriodicWatermarks<T> {
private static final long serialVersionUID = 1L;
/** The current maximum timestamp seen so far. */
private long currentMaxTimestamp;
/** The timestamp of the last emitted watermark. */
private long lastEmittedWatermark = Long.MIN_VALUE;
/**
* The (fixed) interval between the maximum seen timestamp seen in the records
* and that of the watermark to be emitted.
*/
private final long maxOutOfOrderness;
public BoundedOutOfOrdernessTimestampExtractor(Time maxOutOfOrderness) {
if (maxOutOfOrderness.toMilliseconds() < 0) {
throw new RuntimeException("Tried to set the maximum allowed " +
"lateness to " + maxOutOfOrderness + ". This parameter cannot be negative.");
}
this.maxOutOfOrderness = maxOutOfOrderness.toMilliseconds();
this.currentMaxTimestamp = Long.MIN_VALUE + this.maxOutOfOrderness;
}
public long getMaxOutOfOrdernessInMillis() {
return maxOutOfOrderness;
}
/**
* Extracts the timestamp from the given element.
*
* @param element The element that the timestamp is extracted from.
* @return The new timestamp.
*/
public abstract long extractTimestamp(T element);
@Override
public final Watermark getCurrentWatermark() {
// this guarantees that the watermark never goes backwards.
long potentialWM = currentMaxTimestamp - maxOutOfOrderness;
if (potentialWM >= lastEmittedWatermark) {
lastEmittedWatermark = potentialWM;
}
return new Watermark(lastEmittedWatermark);
}
@Override
public final long extractTimestamp(T element, long previousElementTimestamp) {
long timestamp = extractTimestamp(element);
if (timestamp > currentMaxTimestamp) {
currentMaxTimestamp = timestamp;
}
return timestamp;
}
}
1、可以看到，依然是实现AssignerWithPeriodicWatermarks，也就是依然是周期性
2、有一个带参数的构造函数，其参数为Time maxOutOfOrderness，意思是最大可以延迟多久，超过这个时间的则被ignore。
具体使用如下：
DataStream<MyEvent> stream = ...
DataStream<MyEvent> withTimestampsAndWatermarks =
stream.assignTimestampsAndWatermarks(new BoundedOutOfOrdernessTimestampExtractor<MyEvent>(Time.seconds(10)) {
@Override
public long extractTimestamp(MyEvent element) {
return element.getCreationTime();
}
});
参考：
https://ci.apache.org/projects/flink/flink-docs-release-1.7/dev/event_timestamp_extractors.html
这里的Assigners allowing a fixed amount of lateness，amount表示时间，而不是具体的数目
