Github user tzulitai commented on a diff in the pull request:
https://github.com/apache/flink/pull/4006#discussion_r118906667
--- Diff: docs/dev/stream/state.md ---
@@ -429,3 +429,120 @@ public static class CounterSource
Some operators might need the information when a checkpoint is fully
acknowledged by Flink to communicate that with the outside world. In this case
see the `org.apache.flink.runtime.state.CheckpointListener` interface.
+## Custom serialization for managed state
+
+This section is targeted as a guideline for users who require using custom
serialization for their state, covering how
+to provide a custom serializer and how to handle upgrades to the
serializer for compatibility. If you're simply using
+Flink's own serializers, this section is irrelevant and can be skipped.
+
+### Using custom serializers
+
+As demonstrated in the above examples, when registering a managed operator
or keyed state, a `StateDescriptor` is required
+to specify the state's name, as well as information about the type of the
state. The type information is used by Flink's
+[type serialization framework](../types_serialization.html) to create
appropriate serializers for the state.
+
+It is also possible to completely bypass this and let Flink use your own
custom serializer to serialize managed states,
+simply by directly instantiating the `StateDescriptor` with your own
`TypeSerializer` implementation:
+
+{% highlight java %}
+ListStateDescriptor<Tuple2<String, Integer>> descriptor =
+ new ListStateDescriptor<>(
+ "state-name",
+ new TypeSerializer<> {...});
+
+checkpointedState = getRuntimeContext().getListState(descriptor);
+{% endhighlight %}
+
+### Handling serializer upgrades and compatibility
+
+Flink allows changing the serializers used to read and write managed
state, so that users are not locked in to any
+specific serialization. When state is restored, the new serializer
registered for the state (i.e., the serializer
+that comes with the `StateDescriptor` used to access the state in the
restored job) will be checked for compatibility,
+and is replaced as the new serializer for the state.
+
+A compatible serializer would mean that the serializer is capable of
reading previous serialized bytes of the state,
+and the written binary format of the state also remains identical. The
means to check the new serializer's compatibility
+is provided through the following two methods of the `TypeSerializer`
interface:
+
+{% highlight java %}
+public abstract TypeSerializerConfigSnapshot snapshotConfiguration();
+public abstract CompatibilityResult
ensureCompatibility(TypeSerializerConfigSnapshot configSnapshot);
+{% endhighlight %}
+
+Briefly speaking, every time a checkpoint is performed, the
`snapshotConfiguration` method is called to create a
+point-in-time view of the state serializer's configuration. The returned
configuration snapshot is stored along with the
+checkpoint as the state's metadata. When the checkpoint is used to restore
a job, that serializer configuration snapshot
+will be used to confront the _new_ serializer of the same state via the
counterpart method, `ensureCompatibility`. This
--- End diff --
It was just to explain that the checkpointed serializer config snapshot
will be provided to the new serializer to verify its compatibility. I'll use
"provide" instead.
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