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https://issues.apache.org/jira/browse/FLINK-9877?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16558259#comment-16558259
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ASF GitHub Bot commented on FLINK-9877:
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florianschmidt1994 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_r205439601
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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.
Review comment:
I agree that the example is a little contrived! I created some drafts using
"real" timestamps instead of starting from zero, but in the end they ended up
being muss less clear to explain and reason about
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> Add separate docs page for different join types in DataStream API
> -----------------------------------------------------------------
>
> Key: FLINK-9877
> URL: https://issues.apache.org/jira/browse/FLINK-9877
> Project: Flink
> Issue Type: Sub-task
> Components: Documentation
> Reporter: Florian Schmidt
> Assignee: Florian Schmidt
> Priority: Major
> Labels: pull-request-available
> Fix For: 1.6.0
>
>
> https://github.com/apache/flink/pull/6407
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