[FLINK-29807] Drop TypeSerializerConfigSnapshot and savepoint support from Flink versions < 1.8.0

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 1.17.0
• Component/s: Runtime / Checkpointing
• Labels: pull-request-available

Release Note

Savepoints using `TypeSerializerConfigSnapshot` are no longer supported. That means, all savepoints from Flink < 1.8.0 are no longer supported. Furthermore, savepoints from Flink < 1.17.0 created with custom serialiser using deprecated since Flink 1.8.0 class `TypeSerializerConfigSnapshot` are also no longer supported.

If you are only using built-in serialisers (Pojo, Kryo, Avro, Tuple, ...), and your savepoint is from 1.8.0 <= Flink, you don't have to do anything.
If you are only using built-in serialisers (Pojo, Kryo, Avro, Tuple, ...), and your savepoint is from Flink < 1.8.0, please first upgrade your job to 1.8.x <= Flink <= 1.16.x, before upgrading in the second step to Flink >= 1.17.x.
If previously you were using a custom serialiser that depends on `TypeSerializerConfigSnapshot`, please first while still using 1.8.x <= Flink <= 1.16.x upgrade your serialiser to `TypeSerializerSnapshot`, take a savepoint and restore from that savepoint in Flink >= 1.7.0

• Assignee: Piotr Nowojski
• Reporter: Piotr Nowojski
• Created: 31/Oct/22 09:54
• Updated: 14/Dec/23 15:15
• Resolved: 04/Nov/22 16:46

Description

The motivation behind this move is two fold. One reason is that it complicates our code base unnecessarily and creates confusion on how to actually implement custom serializers. The immediate reason is that I wanted to clean up Flink's configuration stack a bit and refactor the ExecutionConfig class FLINK-29379. This refactor would keep the API compatibility of the ExecutionConfig, but it would break savepoint compatibility with snapshots written with some of the old serializers, which had ExecutionConfig as a field and were serialized in the snapshot. This issue has been resolved by the introduction of TypeSerializerSnapshot in Flink 1.7 FLINK-9377, where serializers are no longer part of the snapshot.

TypeSerializerConfigSnapshot has been deprecated and no longer used by built-in serializers since Flink 1.8 FLINK-9376 and FLINK-11323. Users were encouraged to migrate to TypeSerializerSnapshot since then with their own custom serializers. That has been plenty of time for the migration.

This proposal would have the following impact for the users:
1. we would drop support for recovery from savepoints taken with Flink < 1.7.0 for all built in types serializers
2. we would drop support for recovery from savepoints taken with Flink < 1.8.0 for built in kryo serializers
3. we would drop support for recovery from savepoints taken with Flink < 1.17 for custom serializers using deprecated TypeSerializerConfigSnapshot

1. and 2. would have a simple migration path. Users migrating from those old savepoints would have to first start his job using a Flink version from the [1.8, 1.16] range, and take a new savepoint that would be compatible with Flink 1.17.
3. This is a bit more problematic, because users would have to first migrate their own custom serializers to use TypeSerializerSnapshot (using a Flink version from the [1.8, 1.16]), take a savepoint, and only then migrate to Flink 1.17. However users had already 4 years to migrate, which in my opinion has been plenty of time to do so.

As discussed and vote is currently in progress: https://lists.apache.org/thread/x5d0p08pf2wx47njogsgqct0k5rpfrl4

Issue Links

• blocks: FLINK-29379 Back (most of the) ExecutionConfig and CheckpointConfig by Configuration
• is related to: FLINK-9377 Remove writing serializers as part of the checkpoint meta information
• is related to: FLINK-9376 Allow upgrading to incompatible state serializers (state schema evolution)
• is related to: FLINK-11323 Upgrade KryoSerializer snapshot to implement new TypeSerializerSnapshot interface
• relates to: FLINK-31167 Verify that no exclusions were erroneously added to the japicmp plugin
• links to: GitHub Pull Request #21201