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https://issues.apache.org/jira/browse/FLINK-9377?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16629471#comment-16629471
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ASF GitHub Bot commented on FLINK-9377:
---------------------------------------
StephanEwen commented on a change in pull request #6711: [FLINK-9377] [core,
state backends] Remove serializers from checkpoints
URL: https://github.com/apache/flink/pull/6711#discussion_r220734142
##########
File path:
flink-core/src/main/java/org/apache/flink/api/common/typeutils/BackwardsCompatibleConfigSnapshot.java
##########
@@ -0,0 +1,134 @@
+/*
+ * 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
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.api.common.typeutils;
+
+import org.apache.flink.annotation.Internal;
+import org.apache.flink.annotation.VisibleForTesting;
+import org.apache.flink.core.memory.DataInputView;
+import org.apache.flink.core.memory.DataOutputView;
+import org.apache.flink.util.Preconditions;
+
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+import java.io.IOException;
+import java.util.Objects;
+
+/**
+ * A utility {@link TypeSerializerConfigSnapshot} that is used for backwards
compatibility purposes.
+ *
+ * <p>In older versions of Flink (<= 1.6), we used to write state serializers
into checkpoints, along
+ * with the serializer's configuration snapshot. Since 1.7.0, we no longer
wrote the serializers, but
+ * instead used the configuration snapshot as a factory to instantiate
serializers for restoring state.
+ * However, since some outdated implementations of configuration snapshots did
not contain sufficient
+ * information to serve as a factory, the backwards compatible path for
restoring from these older
+ * savepoints would be to just use the written serializer.
+ *
+ * <p>Therefore, when restoring from older savepoints which still contained
both the config snapshot
+ * and the serializer, they are both wrapped within this utility class. When
the caller intends
+ * to instantiate a restore serializer, we simply return the wrapped
serializer instance.
+ *
+ * @param <T> the data type that the wrapped serializer instance serializes.
+ */
+@Internal
+public class BackwardsCompatibleConfigSnapshot<T> extends
TypeSerializerConfigSnapshot<T> {
+
+ /**
+ * The actual serializer config snapshot. This may be {@code null} when
reading a
+ * savepoint from Flink <= 1.2.
+ */
+ @Nullable
+ private TypeSerializerConfigSnapshot<?> wrappedConfigSnapshot;
+
+ /**
+ * The serializer instance written in savepoints.
+ */
+ @Nonnull
+ private TypeSerializer<T> serializerInstance;
+
+ public BackwardsCompatibleConfigSnapshot(
+ @Nullable TypeSerializerConfigSnapshot<?>
wrappedConfigSnapshot,
+ TypeSerializer<T> serializerInstance) {
+
+ this.wrappedConfigSnapshot = wrappedConfigSnapshot;
+ this.serializerInstance =
Preconditions.checkNotNull(serializerInstance);
+ }
+
+ @Override
+ public void write(DataOutputView out) throws IOException {
+ throw new UnsupportedOperationException(
+ "This is a dummy config snapshot used only for
backwards compatibility.");
+ }
+
+ @Override
+ public void read(DataInputView in) throws IOException {
+ throw new UnsupportedOperationException(
+ "This is a dummy config snapshot used only for
backwards compatibility.");
+ }
+
+ @Override
+ public int getVersion() {
+ throw new UnsupportedOperationException(
+ "This is a dummy config snapshot used only for
backwards compatibility.");
+ }
+
+ @Override
+ public TypeSerializer<T> restoreSerializer() {
+ return serializerInstance;
+ }
+
+ @Override
+ @SuppressWarnings("unchecked")
+ public TypeSerializerSchemaCompatibility<T>
resolveSchemaCompatibility(TypeSerializer<?> newSerializer) {
+ if (wrappedConfigSnapshot != null) {
+ return (TypeSerializerSchemaCompatibility<T>)
wrappedConfigSnapshot.resolveSchemaCompatibility(newSerializer);
+ } else {
+ // if there is no configuration snapshot to check
against,
+ // then we can only assume that the new serializer is
compatible as is
+ return
TypeSerializerSchemaCompatibility.compatibleAsIs();
+ }
+ }
+
+ @Override
+ public int hashCode() {
Review comment:
Do all config snapshots in general define their equals and hashCode?
Do we assume semantic equality of snapshots at any point, and is it well
defined? If not, I would suggest to remove these methods, they may be
misleading.
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> Remove writing serializers as part of the checkpoint meta information
> ---------------------------------------------------------------------
>
> Key: FLINK-9377
> URL: https://issues.apache.org/jira/browse/FLINK-9377
> Project: Flink
> Issue Type: Sub-task
> Components: State Backends, Checkpointing
> Reporter: Tzu-Li (Gordon) Tai
> Assignee: Tzu-Li (Gordon) Tai
> Priority: Critical
> Labels: pull-request-available
> Fix For: 1.7.0
>
>
> When writing meta information of a state in savepoints, we currently write
> both the state serializer as well as the state serializer's configuration
> snapshot.
> Writing both is actually redundant, as most of the time they have identical
> information.
> Moreover, the fact that we use Java serialization to write the serializer
> and rely on it to be re-readable on the restore run, already poses problems
> for serializers such as the {{AvroSerializer}} (see discussion in FLINK-9202)
> to perform even a compatible upgrade.
> The proposal here is to leave only the config snapshot as meta information,
> and use that as the single source of truth of information about the schema of
> serialized state.
> The config snapshot should be treated as a factory (or provided to a
> factory) to re-create serializers capable of reading old, serialized state.
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