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Re: How to make savepoints more robust in the face of refactorings ?

Posted by Tzu-Li (Gordon) Tai on
URL: http://deprecated-apache-flink-user-mailing-list-archive.369.s1.nabble.com/How-to-make-savepoints-more-robust-in-the-face-of-refactorings-tp18035p18043.html

Hi,

In the Scala API, type serializers may be anonymous classes generated by Scala macros, and would therefore contain a reference to the wrapping class (i.e., your `Operators` class).
Since Flink currently serializes serializers into the savepoint to be used for deserialization on restore, and the fact that they must be present at restore time, changing the `Operators` classname would result in the previous anonymous class serializer to no longer be in the classpath and therefore fails the deserialization of the written serializer.
This is a limitation caused by how registering serializers for written state currently works in Flink.

Generally speaking, to overcome this, you would need to have the previous serializer class still around in the classpath when restoring, and can only be completely removed from user code once the migration is completed.

One thing that I’m not completely certain with yet, is where in your demonstrated code a anonymous-classed serializer is generated for some type.
From what I see, there shouldn’t be any anonymous-class serializers for the code. Is the code you provided a “simplified” version of the actual code in which you observed the restore error?

Cheers,
Gordon


On 28 January 2018 at 6:00:32 PM, jelmer ([hidden email]) wrote:

Changing the class operators are nested in can break compatibility with existing savepoints. The following piece of code demonstrates this


If I change Operators in this file to Operators2  i will not be able to recover from a savepoint that was made  when this class still had its old name.

The error in the flink ui will be 

java.lang.IllegalStateException: Could not initialize keyed state backend.
at org.apache.flink.streaming.api.operators.AbstractStreamOperator.initKeyedState(AbstractStreamOperator.java:293)
at org.apache.flink.streaming.api.operators.AbstractStreamOperator.initializeState(AbstractStreamOperator.java:225)
at org.apache.flink.streaming.runtime.tasks.StreamTask.initializeOperators(StreamTask.java:692)
at org.apache.flink.streaming.runtime.tasks.StreamTask.initializeState(StreamTask.java:679)
at org.apache.flink.streaming.runtime.tasks.StreamTask.invoke(StreamTask.java:253)
at org.apache.flink.runtime.taskmanager.Task.run(Task.java:718)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException
at org.apache.flink.util.Preconditions.checkNotNull(Preconditions.java:58)
at org.apache.flink.runtime.state.RegisteredKeyedBackendStateMetaInfo.<init>(RegisteredKeyedBackendStateMetaInfo.java:53)
at org.apache.flink.contrib.streaming.state.RocksDBKeyedStateBackend$RocksDBFullRestoreOperation.restoreKVStateMetaData(RocksDBKeyedStateBackend.java:1216)
at org.apache.flink.contrib.streaming.state.RocksDBKeyedStateBackend$RocksDBFullRestoreOperation.restoreKeyGroupsInStateHandle(RocksDBKeyedStateBackend.java:1153)
at org.apache.flink.contrib.streaming.state.RocksDBKeyedStateBackend$RocksDBFullRestoreOperation.doRestore(RocksDBKeyedStateBackend.java:1139)
at org.apache.flink.contrib.streaming.state.RocksDBKeyedStateBackend.restore(RocksDBKeyedStateBackend.java:1034)
at org.apache.flink.streaming.runtime.tasks.StreamTask.createKeyedStateBackend(StreamTask.java:773)
at org.apache.flink.streaming.api.operators.AbstractStreamOperator.initKeyedState(AbstractStreamOperator.java:283)
... 6 more

But the real reason is found in the task manager logs


2018-01-28 17:03:58,830 WARN  org.apache.flink.api.common.typeutils.TypeSerializerSerializationUtil  - Deserialization of serializer errored; replacing with null.
java.io.IOException: Unloadable class for type serializer.
at org.apache.flink.api.common.typeutils.TypeSerializerSerializationUtil$TypeSerializerSerializationProxy.read(TypeSerializerSerializationUtil.java:463)
at org.apache.flink.api.common.typeutils.TypeSerializerSerializationUtil.tryReadSerializer(TypeSerializerSerializationUtil.java:189)
at org.apache.flink.api.common.typeutils.TypeSerializerSerializationUtil.tryReadSerializer(TypeSerializerSerializationUtil.java:162)
at org.apache.flink.api.common.typeutils.TypeSerializerSerializationUtil.readSerializersAndConfigsWithResilience(TypeSerializerSerializationUtil.java:282)
at org.apache.flink.runtime.state.KeyedBackendStateMetaInfoSnapshotReaderWriters$KeyedBackendStateMetaInfoReaderV3.readStateMetaInfo(KeyedBackendStateMetaInfoSnapshotReaderWriters.java:200)
at org.apache.flink.runtime.state.KeyedBackendSerializationProxy.read(KeyedBackendSerializationProxy.java:152)
at org.apache.flink.contrib.streaming.state.RocksDBKeyedStateBackend$RocksDBFullRestoreOperation.restoreKVStateMetaData(RocksDBKeyedStateBackend.java:1175)
at org.apache.flink.contrib.streaming.state.RocksDBKeyedStateBackend$RocksDBFullRestoreOperation.restoreKeyGroupsInStateHandle(RocksDBKeyedStateBackend.java:1153)
at org.apache.flink.contrib.streaming.state.RocksDBKeyedStateBackend$RocksDBFullRestoreOperation.doRestore(RocksDBKeyedStateBackend.java:1139)
at org.apache.flink.contrib.streaming.state.RocksDBKeyedStateBackend.restore(RocksDBKeyedStateBackend.java:1034)
at org.apache.flink.streaming.runtime.tasks.StreamTask.createKeyedStateBackend(StreamTask.java:773)
at org.apache.flink.streaming.api.operators.AbstractStreamOperator.initKeyedState(AbstractStreamOperator.java:283)
at org.apache.flink.streaming.api.operators.AbstractStreamOperator.initializeState(AbstractStreamOperator.java:225)
at org.apache.flink.streaming.runtime.tasks.StreamTask.initializeOperators(StreamTask.java:692)
at org.apache.flink.streaming.runtime.tasks.StreamTask.initializeState(StreamTask.java:679)
at org.apache.flink.streaming.runtime.tasks.StreamTask.invoke(StreamTask.java:253)
at org.apache.flink.runtime.taskmanager.Task.run(Task.java:718)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.io.InvalidClassException: failed to read class descriptor
at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1611)
at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1521)
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1781)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1353)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:373)
at org.apache.flink.api.common.typeutils.TypeSerializerSerializationUtil$TypeSerializerSerializationProxy.read(TypeSerializerSerializationUtil.java:454)
... 17 more
Caused by: java.lang.ClassNotFoundException: com.ecg.foo.Main$Operators$$anon$3$$anon$1
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:348)
at org.apache.flink.util.InstantiationUtil$ClassLoaderObjectInputStream.resolveClass(InstantiationUtil.java:64)
at org.apache.flink.api.common.typeutils.TypeSerializerSerializationUtil$FailureTolerantObjectInputStream.readClassDescriptor(TypeSerializerSerializationUtil.java:110)
at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1609)
... 22 more



Is there any way to make this code more robust ? Using java serialization in this way feels very brittle in the face of refactorings. 
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