[jira] [Commented] (FLINK-4067) Add version header to savepoints

["ASF GitHub Bot (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/FLINK-4067?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15360615#comment-15360615 ] 

ASF GitHub Bot commented on FLINK-4067:
---------------------------------------

GitHub user uce opened a pull request:

    https://github.com/apache/flink/pull/2194

    [FLINK-4067] [runtime] Add savepoint headers

    Savepoints were previously persisted without any meta data using default Java serialization of a `CompletedCheckpoint`. This PR introduces a savepoint class with version-specific serializers and stores savepoints with meta data.
    
    Savepoints expose a version number and a `Collection<TaskState>` for savepoint restore.
    
    Currently, there are two savepoint versions:
    
    - `SavepointV0` (Flink 1.0): This is a wrapper around the Flink 1.0 `CompletedCheckpoint`. Because this class changed since Flink 1.0 I had to re-introduce it with this PR in order to allow restoring from Flink 1.0 savepoints (otherwise the old `StateForTask` state can not be deserialized via default Java serialization). Therefore, I've renamed the Flink 1.1 `CompletedCheckpoint` to `Checkpoint`. I've annotated the re-introduced classes with `@Deprecated`. We can remove those with Flink 1.2.
    - `SavepointV1` (Flink 1.1): This is the current savepoint version, which holds a reference to the `Checkpoint` task state collection, but is serialized with a custom serializater not relying on default Java serialization. Therefore, it should not happen again that we need to stick to certain classes in future Flink versions.
    
    The savepoints are stored in `FsSavepointStore` with the following format:
    
    ```
    MagicNumber SavepointVersion Savepoint
      - MagicNumber => int
      - SavepointVersion => int (returned by Savepoint#getVersion())
      - Savepoint => bytes (serialized via version-specific SavepointSerializer)
    ```
    
    The header is minimal (magic number, version). All savepoint-specific meta data can be moved to the savepoint itself. This is also were we would have to add new meta data in future versions, allowing us to differentiate between different savepoint versions when we change the serialization stack etc.
    
    I've tested both manually and via a unit test that it works to trigger a savepoint via Flink 1.0 and resume from it with Flink 1.1.
    
    All savepoint related classes have been moved from `checkpoint` to a new sub package `checkpoint.savepoint`.
    
    The main classes to look at to review this PR are `Savepoint` and subclasses, `SavepointSerializer` and subclasses, and `FsSavepointStore`. @tillrohrmann do you have time to do this?

You can merge this pull request into a Git repository by running:

    $ git pull https://github.com/uce/flink 4067-savepoint_header

Alternatively you can review and apply these changes as the patch at:

    https://github.com/apache/flink/pull/2194.patch

To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:

    This closes #2194
    
----
commit af0d418ba2bc3591f0526476d1ad7dfc4160c205
Author: Ufuk Celebi <uc...@apache.org>
Date:   2016-06-30T14:16:41Z

    [FLINK-4067] [runtime] Add savepoint headers

----


> Add version header to savepoints
> --------------------------------
>
>                 Key: FLINK-4067
>                 URL: https://issues.apache.org/jira/browse/FLINK-4067
>             Project: Flink
>          Issue Type: Improvement
>    Affects Versions: 1.0.3
>            Reporter: Ufuk Celebi
>            Assignee: Ufuk Celebi
>             Fix For: 1.1.0
>
>
> Adding a header with version information to savepoints ensures that we can migrate savepoints between Flink versions in the future (for example when changing internal serialization formats between versions).
> After talking with Till, we propose to add the following meta data:
> - Magic number (int): identify data as savepoint
> - Version (int): savepoint version (independent of Flink version)
> - Data Offset (int): specifies at which point the actual savepoint data starts. With this, we can allow future Flink versions to add fields to the header without breaking stuff, e.g. Flink 1.1 could read savepoints of Flink 2.0.
> For Flink 1.0 savepoint support, we have to try reading the savepoints without a header before failing if we don't find the magic number.



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