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https://issues.apache.org/jira/browse/FLINK-4062?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15352757#comment-15352757
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ASF GitHub Bot commented on FLINK-4062:
---------------------------------------
Github user kl0u commented on a diff in the pull request:
https://github.com/apache/flink/pull/2154#discussion_r68732411
--- Diff: docs/apis/streaming/windows.md ---
@@ -24,1023 +24,593 @@ specific language governing permissions and
limitations
under the License.
-->
+Flink uses a concept called *windows* to divide a (potentially) infinite
`DataStream` into slices
+based on the timestamps of elements or other criteria. This division is
required when working
+with infinite streams of data and performing transformations that
aggregate elements.
+
+<span class="label label-info">Info</span> We will mostly talk about
*keyed windowing* here, this
+means that the elements are subdivided based on both window and key before
being given to
+a user function. Keyed windows have the advantage that work can be
distributed across the cluster
+because the elements for different keys can be processed in isolation. If
you absolutely must,
+you can check out [non-keyed windowing](#non-keyed-windowing) where we
describe how non-keyed
+windows work.
+
* This will be replaced by the TOC
{:toc}
-## Windows on Keyed Data Streams
-
-Flink offers a variety of methods for defining windows on a `KeyedStream`.
All of these group elements *per key*,
-i.e., each window will contain elements with the same key value.
+## Basics
-### Basic Window Constructs
+For a windowed transformations you must at least specify a *key* (usually
in the form of a
+`KeySelector`) a *window assigner* and a *window function*. The *key*
specifies how elements are
+put into groups. The *window assigner* specifies how the infinite stream
is divided into windows.
--- End diff --
The keyslector divides the infinite, non-keyed stream into multiple per-key
infinite streams, while the window assigner divides the per-key infinite
streams into per-key finite windows.
> Update Windowing Documentation
> ------------------------------
>
> Key: FLINK-4062
> URL: https://issues.apache.org/jira/browse/FLINK-4062
> Project: Flink
> Issue Type: Sub-task
> Components: Documentation
> Affects Versions: 1.1.0
> Reporter: Aljoscha Krettek
> Assignee: Aljoscha Krettek
>
> The window documentation could be a bit more principled and also needs
> updating with the new allowed lateness setting.
> There is also essentially no documentation about how to write a custom
> trigger.
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