Github user alpinegizmo commented on a diff in the pull request:
https://github.com/apache/flink/pull/4441#discussion_r131087040
--- Diff: docs/dev/stream/state/index.md ---
@@ -0,0 +1,56 @@
+---
+title: "State & Fault Tolerance"
+nav-id: streaming_state
+nav-title: "State & Fault Tolerance"
+nav-parent_id: streaming
+nav-pos: 3
+nav-show_overview: true
+---
+<!--
+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
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+ http://www.apache.org/licenses/LICENSE-2.0
+
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+KIND, either express or implied. See the License for the
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+-->
+
+Stateful functions and operators store data across the processing of
individual elements/events, making state a critical building block for
+any type of more elaborate operation.
+
+For example:
+
+ - When an application searches for certain event patterns, the state
will store the sequence of events encountered so far.
+ - When aggregating events per minute/hour/day, the state holds the
pending aggregates.
+ - When training a machine learning model over a stream of data points,
the state holds the current version of the model parameters.
+ - When historic data needs to be managed, the state allows efficient
access to events occured in the past.
+
+Flink needs to be aware of the state in order to make state fault tolerant
using [checkpoints](checkpointing.html) and allow [savepoints]({{ site.baseurl
}}/ops/state/savepoints.html) of streaming applications.
--- End diff --
"and to allow [savepoints]"
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