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https://issues.apache.org/jira/browse/FLINK-6742?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16027493#comment-16027493
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Chesnay Schepler commented on FLINK-6742:
-----------------------------------------
Well, alright, technically you can add operators, so long as they don't modify
chains.
The internal structure of savepoints was changed in 1.3. A 1.2 savepoint
contains the state of tasks, as a list of states of the contained operators. In
1.3 a savepoint only contains the states of operators, the notion of tasks was
removed. In order to map an old savepoint to a new one we have to map the state
of a task to the individual operators. For non-chains this is easy, but for
chains this can only be done in a reliable way if the chains don't change, i.e
no operator removed nor added.
The problem is that we don't know what happened to the missing task. It may
very well be that the task was removed on purpose and the state should be lost.
But it could also be that a user read that you can modify chains in 1.3 and did
so before migrating the savepoint, this however only works after migration.
This isn't a technical hurdle, but a safety precaution.
> Savepoint conversion might fail if operators change
> ---------------------------------------------------
>
> Key: FLINK-6742
> URL: https://issues.apache.org/jira/browse/FLINK-6742
> Project: Flink
> Issue Type: Bug
> Components: State Backends, Checkpointing
> Affects Versions: 1.3.0
> Reporter: Gyula Fora
> Priority: Critical
>
> Caused by: java.lang.NullPointerException
> at
> org.apache.flink.runtime.checkpoint.savepoint.SavepointV2.convertToOperatorStateSavepointV2(SavepointV2.java:171)
> at
> org.apache.flink.runtime.checkpoint.savepoint.SavepointLoader.loadAndValidateSavepoint(SavepointLoader.java:75)
> at
> org.apache.flink.runtime.checkpoint.CheckpointCoordinator.restoreSavepoint(CheckpointCoordinator.java:1090)
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