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https://issues.apache.org/jira/browse/FLINK-9808?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16654972#comment-16654972
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ASF GitHub Bot commented on FLINK-9808:
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tzulitai opened a new pull request #6875: [FLINK-9808] [state backends] Migrate
state when necessary in state backends
URL: https://github.com/apache/flink/pull/6875
## What is the purpose of the change
This PR adds the procedure of migrating state (i.e., reading state bytes
with the restored prior serializer, and then re-writing the deserialized object
with the new serializer).
This should only ever occur in the RocksDB state backend, since the restore
/ snapshotting of heap-based backends are already a migration process by nature.
The requirement to perform a state migration is decided by the
`CompatibilityResult` returned from compatibility checks against the new
serializer using its prior serializer's snapshot.
## Brief change log
- Add `migrateSerializedValue` to `AbstractRocksDBStateValue`
- Use the new method to iterate through RocksDB bytes when state migration
is necessary
- Remove checks that are no longer relevant in the heap backends, since it
is inherently already migrating state bytes as part of its restore / snapshot
process
- Add a `StateMigrationTestBase` that verifies serializers are being used as
expected for a restore procedure
## Verifying this change
The main new test coverage that covers this is in the
`StateMigrationTestBase` class.
## Does this pull request potentially affect one of the following parts:
- Dependencies (does it add or upgrade a dependency): (yes / **no**)
- The public API, i.e., is any changed class annotated with
`@Public(Evolving)`: (yes / **no**)
- The serializers: (yes / **no** / don't know)
- The runtime per-record code paths (performance sensitive): (**yes** / no
/ don't know)
- Anything that affects deployment or recovery: JobManager (and its
components), Checkpointing, Yarn/Mesos, ZooKeeper: (**yes** / no / don't know)
- The S3 file system connector: (yes / **no** / don't know)
## Documentation
- Does this pull request introduce a new feature? (**yes** / no)
- If yes, how is the feature documented? (**not yet documented**)
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> Implement state conversion procedure in state backends
> ------------------------------------------------------
>
> Key: FLINK-9808
> URL: https://issues.apache.org/jira/browse/FLINK-9808
> Project: Flink
> Issue Type: Sub-task
> Components: State Backends, Checkpointing
> Reporter: Tzu-Li (Gordon) Tai
> Assignee: Aljoscha Krettek
> Priority: Critical
> Labels: pull-request-available
> Fix For: 1.7.0
>
>
> With FLINK-9377 in place and that config snapshots serve as the single source
> of truth for recreating restore serializers, the next step would be to
> utilize this when performing a full-pass state conversion (i.e., read with
> old / restore serializer, write with new serializer).
> For Flink's heap-based backends, it can be seen that state conversion
> inherently happens, since all state is always deserialized after restore with
> the restore serializer, and written with the new serializer on snapshots.
> For the RocksDB state backend, since state is lazily deserialized, state
> conversion needs to happen for per-registered state on their first access if
> the registered new serializer has a different serialization schema than the
> previous serializer.
> This task should consist of three parts:
> 1. Allow {{CompatibilityResult}} to correctly distinguish between whether the
> new serializer's schema is a) compatible with the serializer as it is, b)
> compatible after the serializer has been reconfigured, or c) incompatible.
> 2. Introduce state conversion procedures in the RocksDB state backend. This
> should occur on the first state access.
> 3. Make sure that all other backends no longer do redundant serializer
> compatibility checks. That is not required because those backends always
> perform full-pass state conversions.
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