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https://issues.apache.org/jira/browse/FLINK-9574?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16693187#comment-16693187
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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_r234974424
##########
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
+ previous execution, leading to mismatching registration ids for types.
+
+In order for the new execution to have information about the *written schema*
of state and detect whether or not the
+schema has changed, upon taking a savepoint of an operator's state, a
*snapshot* of the state serializer needs to be
+written along with the state bytes. This is abstracted a
`TypeSerializerSnapshot`, explained in the next subsection.
-How the serializer's configuration snapshot is written to and read from
checkpoints is fully customizable. The below
-is the base class for all serializer configuration snapshot implementations,
the `TypeSerializerConfigSnapshot`.
+### The `TypeSerializerSnapshot` abstraction
+<div data-lang="java" markdown="1">
{% highlight java %}
-public abstract TypeSerializerConfigSnapshot extends
VersionedIOReadableWritable {
- public abstract int getVersion();
- public void read(DataInputView in) {...}
- public void write(DataOutputView out) {...}
+public interface TypeSerializerSnapshot<T> {
+ int getCurrentVersion();
+ void writeSnapshot(DataOuputView out) throws IOException;
+ void readSnapshot(int readVersion, DataInputView in, ClassLoader
userCodeClassLoader) throws IOException;
+ TypeSerializerSchemaCompatibility<T>
resolveSchemaCompatibility(TypeSerializer<T> newSerializer);
+ TypeSerializer<T> restoreSerializer();
}
{% endhighlight %}
+</div>
-The `read` and `write` methods define how the configuration is read from and
written to the checkpoint. The base
-implementations contain logic to read and write the version of the
configuration snapshot, so it should be extended and
-not completely overridden.
-
-The version of the configuration snapshot is determined through the
`getVersion` method. Versioning for the serializer
-configuration snapshot is the means to maintain compatible configurations, as
information included in the configuration
-may change over time. By default, configuration snapshots are only compatible
with the current version (as returned by
-`getVersion`). To indicate that the configuration is compatible with other
versions, override the `getCompatibleVersions`
-method to return more version values. When reading from the checkpoint, you
can use the `getReadVersion` method to
-determine the version of the written configuration and adapt the read logic to
the specific version.
-
-<span class="label label-danger">Attention</span> The version of the
serializer's configuration snapshot is **not**
-related to upgrading the serializer. The exact same serializer can have
different implementations of its
-configuration snapshot, for example when more information is added to the
configuration to allow more comprehensive
-compatibility checks in the future.
-
-One limitation of implementing a `TypeSerializerConfigSnapshot` is that an
empty constructor must be present. The empty
-constructor is required when reading the configuration snapshot from
checkpoints.
-
-#### Implementing the `ensureCompatibility` method
-
-The `ensureCompatibility` method should contain logic that performs checks
against the information about the previous
-serializer carried over via the provided `TypeSerializerConfigSnapshot`,
basically doing one of the following:
-
- * Check whether the serializer is compatible, while possibly reconfiguring
itself (if required) so that it may be
- compatible. Afterwards, acknowledge with Flink that the serializer is
compatible.
-
- * Acknowledge that the serializer is incompatible and that state migration
is required before Flink can proceed with
- using the new serializer.
-
-The above cases can be translated to code by returning one of the following
from the `ensureCompatibility` method:
-
- * **`CompatibilityResult.compatible()`**: This acknowledges that the new
serializer is compatible, or has been reconfigured to
- be compatible, and Flink can proceed with the job with the serializer as
is.
-
- * **`CompatibilityResult.requiresMigration()`**: This acknowledges that the
serializer is incompatible, or cannot be
- reconfigured to be compatible, and requires a state migration before the
new serializer can be used. State migration
- is performed by using the previous serializer to read the restored state
bytes to objects, and then serialized again
- using the new serializer.
-
- * **`CompatibilityResult.requiresMigration(TypeDeserializer
deserializer)`**: This acknowledgement has equivalent semantics
- to `CompatibilityResult.requiresMigration()`, but in the case that the
previous serializer cannot be found or loaded
- to read the restored state bytes for the migration, a provided
`TypeDeserializer` can be used as a fallback resort.
-
-<span class="label label-danger">Attention</span> Currently, as of Flink 1.3,
if the result of the compatibility check
-acknowledges that state migration needs to be performed, the job simply fails
to restore from the checkpoint as state
-migration is currently not available. The ability to migrate state will be
introduced in future releases.
-
-### Managing `TypeSerializer` and `TypeSerializerConfigSnapshot` classes in
user code
-
-Since `TypeSerializer`s and `TypeSerializerConfigSnapshot`s are written as
part of checkpoints along with the state
-values, the availability of the classes within the classpath may affect
restore behaviour.
-
-`TypeSerializer`s are directly written into checkpoints using Java Object
Serialization. In the case that the new
-serializer acknowledges that it is incompatible and requires state migration,
it will be required to be present to be
-able to read the restored state bytes. Therefore, if the original serializer
class no longer exists or has been modified
-(resulting in a different `serialVersionUID`) as a result of a serializer
upgrade for the state, the restore would
-not be able to proceed. The alternative to this requirement is to provide a
fallback `TypeDeserializer` when
-acknowledging that state migration is required, using
`CompatibilityResult.requiresMigration(TypeDeserializer deserializer)`.
+<div data-lang="java" markdown="1">
+{% highlight java %}
+public abstract class TypeSerializer<T> {
+
+ // ...
+
+ public abstract TypeSerializerSnapshot<T> snapshotConfiguration();
+}
+{% endhighlight %}
+</div>
-The class of `TypeSerializerConfigSnapshot`s in the restored checkpoint must
exist in the classpath, as they are
-fundamental components to compatibility checks on upgraded serializers and
would not be able to be restored if the class
-is not present. Since configuration snapshots are written to checkpoints using
custom serialization, the implementation
-of the class is free to be changed, as long as compatibility of the
configuration change is handled using the versioning
-mechanisms in `TypeSerializerConfigSnapshot`.
+A serializer's `TypeSerializerSnapshot` is a piece of information that serves
as the single source of truth about
Review comment:
suggestion: ..`TypeSerializerSnapshot` is a point-in-time configuration of
the serializer, which includes the writer schema, and any additional
information that is mandatory to restore a serializer that would be identical
to this point-in-time. The logic about ...
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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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