[GitHub] xccui commented on a change in pull request #6407: [FLINK-9877][docs] Add documentation page for different datastream joins

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xccui commented on a change in pull request #6407: [FLINK-9877][docs] Add documentation page for different datastream joins
URL: https://github.com/apache/flink/pull/6407#discussion_r205190250
 
 

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 File path: docs/dev/stream/operators/joining.md
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 @@ -0,0 +1,286 @@
+---
+title: "Joining"
+nav-id: streaming_joins
+nav-show_overview: true
+nav-parent_id: streaming
+nav-pos: 10
+---
+<!--
+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.
+-->
+
+* toc
+{:toc}
+
+# Window Join
+A window join will join the elements of two streams that share a common key and lie in the same window. These windows can be defined by using a [window assigner]({{ site.baseurl}}/dev/stream/operators/windows.html#window-assigners) and are evaluated on a union of both streams. This is especially important for session window joins, which will be demonstrated below.
+
+The joined elements are then passed to a user-defined `JoinFunction` or `FlatJoinFunction` where the user can perform transformations on the joined elements.
+
+The general usage always looks like the followning:
+
+```java
+stream.join(otherStream)
+    .where(<KeySelector>)
+    .equalTo(<KeySelector>)
+    .window(<WindowAssigner>)
+    .apply(<JoinFunction>)
+```
+
+Some notes on semantics:
+- The creation of pairwise combinations of elements of the two streams behaves like an inner-join, meaning elements from one stream will not be emitted if they don't have a corresponding element from the other stream to be joined with.
+- Those elements that do get joined will have as their timestamp the largest timestamp that still lies in the respective window. For example a window with `[5, 10)` as its boundaries would result in the joined elements having nine as their timestamp.
+
+In the following section we are going to give an overview over how different kinds of windows can be used for a window join and what the results of those joins would look like using examplary scenarios.
+
+## Tumbling Window
+When performing a tumbling window join, all elements with a common key and a common tumbling window are joined as pairwise combinations and passed on to the user-defined function. Because this behaves like an inner join, elements of one stream that do not have elements from another stream in their tumbling window are not emitted!
+
+### Example
+<img src="{{ site.baseurl }}/fig/tumbling-window-join.svg" class="center" style="width: 80%;" />
+
+In our example we are defining a tumbling window with the size of 2 milliseconds, which results in windows of the form `[0,1], [2,3], ...`. The image shows the pairwise combinations of all elements in each window which will be passed on to the user-defined function. You can also see how in the tumbling window `[6,7]` nothing is emitted because no elements from the green stream exist to be joined with the orange elements ⑥ and ⑦.
+
+<div class="codetabs" markdown="1">
+<div data-lang="java" markdown="1">
+
+```java
+import org.apache.flink.api.java.functions.KeySelector;
+import org.apache.flink.streaming.api.windowing.assigners.TumblingEventTimeWindows;
+import org.apache.flink.streaming.api.windowing.time.Time;
+ 
+...
+
+DataStream<Integer> orangeStream = ...
+DataStream<Integer> greenStream = ...
+
+orangeStream.join(greenStream)
+    .where(<KeySelector>)
+    .equalTo(<KeySelector>)
+    .window(TumblingEventTimeWindows.of(Time.seconds(2)))
+    .apply (new JoinFunction<Integer, Integer, String> () {
+        @Override
+        public String join(Integer first, Integer second) {
+            return first + "," + second;
+        }
+    });
+```
+</div>
+<div data-lang="scala" markdown="1">
+
+```scala
+import org.apache.flink.streaming.api.windowing.assigners.SlidingEventTimeWindows;
+import org.apache.flink.streaming.api.windowing.time.Time;
+
+...
+
+val orangeStream: DataStream[Integer] = ...
+val greenStream: DataStream[Integer] = ...
+
+orangeStream.join(greenStream)
+    .where(elem => /* select key */)
+    .equalTo(elem => /* select key */)
+    .window(TumblingEventTimeWindows.of(Time.milliseconds(2)))
+    .apply { (e1, e2) => e1 + "," + e2 }
+```
+
+</div>
+</div>
+
+## Sliding Window Join
+When performing a sliding window join, all elements with a common key and common sliding window are joined are pairwise combinations and passed on to the user-defined function. Elements of one stream that do not have elements from the other stream in the current sliding window are not emitted! Note that some elements might be joined in one sliding window but not in another!
+
+<img src="{{ site.baseurl }}/fig/sliding-window-join.svg" class="center" style="width: 80%;" />
+
+In this example we are using sliding windows with a duration of two milliseconds and slide them by one millisecond, resulting in the sliding windows `[-1, 0],[0,1],[1,2],[2,3], …`.<!-- TODO: Can -1 actually exist?--> The joined elements below the x-axis are the ones that are passed to the user-defined function for each sliding window. Here you can also see how for example the orange ② is joined with the green ③ in the window `[2,3]`, but is not joined with anything in the window `[1,2]`.
 
 Review comment:
   1. Replace the word "duration" with "size" 
   2. Use `JoinFunction` instead of user-defined function.
   3. Extend the window size a little bit.
   4. Would be better if you could make the elements from different sides distinguishable.

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