Github user dyanarose commented on a diff in the pull request:

    https://github.com/apache/flink/pull/5295#discussion_r161807837
  
    --- Diff: 
flink-streaming-java/src/main/java/org/apache/flink/streaming/api/windowing/assigners/DynamicEventTimeSessionWindows.java
 ---
    @@ -0,0 +1,101 @@
    +/*
    + * Licensed to the Apache Software Foundation (ASF) under one
    + * or more contributor license agreements.  See the NOTICE file
    + * distributed with this work for additional information
    + * regarding copyright ownership.  The ASF licenses this file
    + * to you under the Apache License, Version 2.0 (the
    + * "License"); you may not use this file except in compliance
    + * with the License.  You may obtain a copy of the License at
    + *
    + * http://www.apache.org/licenses/LICENSE-2.0
    + *
    + * Unless required by applicable law or agreed to in writing, software
    + * distributed under the License is distributed on an "AS IS" BASIS,
    + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    + * See the License for the specific language governing permissions and
    + * limitations under the License.
    + */
    +
    +package org.apache.flink.streaming.api.windowing.assigners;
    +
    +import org.apache.flink.api.common.ExecutionConfig;
    +import org.apache.flink.api.common.typeutils.TypeSerializer;
    +import 
org.apache.flink.streaming.api.environment.StreamExecutionEnvironment;
    +import org.apache.flink.streaming.api.windowing.triggers.Trigger;
    +import 
org.apache.flink.streaming.api.windowing.triggers.TypedEventTimeTrigger;
    +import org.apache.flink.streaming.api.windowing.windows.TimeWindow;
    +
    +import java.util.Collection;
    +import java.util.Collections;
    +
    +/**
    + * A {@link WindowAssigner} that windows elements into sessions based on 
the timestamp of the
    + * elements. Windows cannot overlap.
    + *
    + * <p>For example, in order to window into windows with a dynamic time gap:
    + * <pre> {@code
    + * DataStream<Tuple2<String, Integer>> in = ...;
    + * KeyedStream<String, Tuple2<String, Integer>> keyed = in.keyBy(...);
    + * WindowedStream<Tuple2<String, Integer>, String, TimeWindows> windowed =
    + *   keyed.window(DynamicEventTimeSessionWindows.withDynamicGap({@link 
SessionWindowTimeGapExtractor }));
    + * } </pre>
    + *
    + * @param <T> The type of the input elements
    + */
    +public class DynamicEventTimeSessionWindows<T> extends 
MergingWindowAssigner<T, TimeWindow> {
    +   private static final long serialVersionUID = 1L;
    +
    +   protected SessionWindowTimeGapExtractor<T> 
sessionWindowTimeGapExtractor;
    +
    +   protected 
DynamicEventTimeSessionWindows(SessionWindowTimeGapExtractor<T> 
sessionWindowTimeGapExtractor) {
    +           this.sessionWindowTimeGapExtractor = 
sessionWindowTimeGapExtractor;
    +   }
    +
    +   @Override
    +   public Collection<TimeWindow> assignWindows(T element, long timestamp, 
WindowAssignerContext context) {
    +           long sessionTimeout = 
sessionWindowTimeGapExtractor.extract(element, timestamp, context);
    +           if (sessionTimeout <= 0) {
    +                   throw new IllegalArgumentException("Dynamic session 
time gap must satisfy 0 < gap");
    +           }
    +           return Collections.singletonList(new TimeWindow(timestamp, 
timestamp + sessionTimeout));
    +   }
    +
    +   @Override
    +   public Trigger<T, TimeWindow> 
getDefaultTrigger(StreamExecutionEnvironment env) {
    +           return TypedEventTimeTrigger.create();
    --- End diff --
    
    as above


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