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https://issues.apache.org/jira/browse/FLINK-9574?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16693186#comment-16693186
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ASF GitHub Bot commented on FLINK-9574:
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igalshilman commented on a change in pull request #7124: [FLINK-9574] [doc]
Rework documentation for custom state serializers and state evolution
URL: https://github.com/apache/flink/pull/7124#discussion_r234973231
##########
File path: docs/dev/stream/state/custom_serialization.md
##########
@@ -66,125 +69,166 @@ checkpointedState =
getRuntimeContext.getListState(descriptor)
</div>
</div>
-Note that Flink writes state serializers along with the state as metadata. In
certain cases on restore (see following
-subsections), the written serializer needs to be deserialized and used.
Therefore, it is recommended to avoid using
-anonymous classes as your state serializers. Anonymous classes do not have a
guarantee on the generated classname,
-which varies across compilers and depends on the order that they are
instantiated within the enclosing class, which can
-easily cause the previously written serializer to be unreadable (since the
original class can no longer be found in the
-classpath).
-
-### 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 provided to the _new_ serializer of the same state via the counterpart
method, `ensureCompatibility`, to verify
-compatibility of the new serializer. This method serves as a check for whether
or not the new serializer is compatible,
-as well as a hook to possibly reconfigure the new serializer in the case that
it is incompatible.
+## State serializers and schema evolution
-Note that Flink's own serializers are implemented such that they are at least
compatible with themselves, i.e. when the
-same serializer is used for the state in the restored job, the serializer's
will reconfigure themselves to be compatible
-with their previous configuration.
+This section explains the user-facing abstractions related to state
serialization and schema evolution, and necessary
+internal details about how Flink interacts with these abstractions.
-The following subsections illustrate guidelines to implement these two methods
when using custom serializers.
+When restoring from savepoints, Flink allows changing the serializers used to
read and write prior registered state,
+so that users are not locked in to any specific serialization schema. When
state is restored, a new serializer will be
+registered for the state (i.e., the serializer that comes with the
`StateDescriptor` used to access the state in the
+restored job). This new serializer may have a different schema than that of
the prior serializer. Therefore, when
+implementing state serializers, besides the basic logic of reading / writing
data, another important thing to keep in
+mind is how the serialization schema can be changed in the future.
-#### Implementing the `snapshotConfiguration` method
+When speaking of *schema*, in this context the term is interchangeable between
referring to the *data model* of a state
+type and the *serialized binary format* of a state type. The schema, generally
speaking, can change for a few cases:
-The serializer's configuration snapshot should capture enough information such
that on restore, the information
-carried over to the new serializer for the state is sufficient for it to
determine whether or not it is compatible.
-This could typically contain information about the serializer's parameters or
binary format of the serialized data;
-generally, anything that allows the new serializer to decide whether or not it
can be used to read previous serialized
-bytes, and that it writes in the same binary format.
+ 1. Data schema of the state type has evolved, i.e. adding or removing a field
from a POJO that is used as state.
+ 2. Following the above, generally speaking, the serialization format of the
serializer is intended to be upgraded.
+ 3. Configuration of the serializer has changed, i.e. Kryo types are
registered in a different order than the
Review comment:
suggestion: remove Kryo from here since it might confuse the reader (the
page deals with custom serializers, and we support Kryo)
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> Add a dedicated documentation page for state evolution
> ------------------------------------------------------
>
> Key: FLINK-9574
> URL: https://issues.apache.org/jira/browse/FLINK-9574
> Project: Flink
> Issue Type: Sub-task
> Components: Documentation, State Backends, Checkpointing, Type
> Serialization System
> Reporter: Tzu-Li (Gordon) Tai
> Assignee: Tzu-Li (Gordon) Tai
> Priority: Critical
> Labels: pull-request-available
> Fix For: 1.8.0
>
>
> Currently, the only bit of documentation about serializer upgrades / state
> evolution, is
> [https://ci.apache.org/projects/flink/flink-docs-release-1.5/dev/stream/state/custom_serialization.html#handling-serializer-upgrades-and-compatibility.],
> which only explains things at an API level.
> State evolution over the time has proved to be a rather complex topic that is
> often overlooked by users. Users would probably benefit from a actual
> full-grown dedicated page that covers both API, some necessary internal
> details regarding interplay of state serializers, best practices, and caution
> notices.
> I propose to add this documentation as a subpage under Streaming/State &
> Fault-Tolerance/.
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