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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLINK-5820) Extend State Backend Abstraction to support Global Cleanup Hooks
Date Thu, 01 Feb 2018 16:02:00 GMT

    [ https://issues.apache.org/jira/browse/FLINK-5820?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16348805#comment-16348805

ASF GitHub Bot commented on FLINK-5820:

Github user StephanEwen commented on the issue:

    Merged in 31e97e57ceeaf37264ab6db078552b73ee5121bf

> Extend State Backend Abstraction to support Global Cleanup Hooks
> ----------------------------------------------------------------
>                 Key: FLINK-5820
>                 URL: https://issues.apache.org/jira/browse/FLINK-5820
>             Project: Flink
>          Issue Type: Improvement
>          Components: State Backends, Checkpointing
>    Affects Versions: 1.2.0
>            Reporter: Stephan Ewen
>            Assignee: Stephan Ewen
>            Priority: Blocker
>             Fix For: 1.5.0
> The current state backend abstraction has the limitation that each piece of state is
only meaningful in the context of its state handle. There is no possibility of a view onto
"all state associated with checkpoint X".
> That causes several issues
>   - State might not be cleaned up in the process of failures. When a TaskManager hands
over a state handle to the JobManager and either of them has a failure, the state handle may
be lost and state lingers.
>   - State might also linger if a cleanup operation failed temporarily, and the checkpoint
metadata was already disposed
>   - State cleanup is more expensive than necessary in many cases. Each state handle is
individually released. For large jobs, this means 1000s of release operations (typically file
deletes) per checkpoint, which can be expensive on some file systems.
>   - It is hard to guarantee cleanup of parent directories with the current architecture.
> The core changes proposed here are:
>   1. Each job has one core {{StateBackend}}. In the future, operators may have different
{{KeyedStateBackends}} and {{OperatorStateBackends}} to mix and match for example RocksDB
storabe and in-memory storage.
>   2. The JobManager needs to be aware of the {{StateBackend}}.
>   3. Storing checkpoint metadata becomes responsibility of the state backend, not the
"completed checkpoint store". The later only stores the pointers to the available latest checkpoints
(either in process or in ZooKeeper).
>   4. The StateBackend may optionally have a hook to drop all checkpointed state that
belongs to only one specific checkpoint (shared state comes as part of incremental checkpointing).
>   5.  The StateBackend needs to have a hook to drop all checkpointed state up to a specific
checkpoint (for all previously discarded checkpoints).
>   6. In the future, this must support periodic cleanup hooks that track orphaned shared
state from incremental checkpoints.
> For the {{FsStateBackend}}, which stores most of the checkpointes state currently (transitively
for RocksDB as well), this means a re-structuring of the storage directories as follows:
> {code}
> ../<flink-checkpoints>/job1-id/
>                               /shared/    <-- shared checkpoint data
>                               /chk-1/...  <-- data exclusive to checkpoint 1
>                               /chk-2/...  <-- data exclusive to checkpoint 2
>                               /chk-3/...  <-- data exclusive to checkpoint 3
> ../<flink-checkpoints>/job2-id/
>                               /shared/...
>                               /chk-1/...
>                               /chk-2/...
>                               /chk-3/...
> ../<flink-savepoints>/savepoint-1/savepoint-root
>                                  /file-1-uid
>                                  /file-2-uid
>                                  /file-3-uid
>                      /savepoint-2/savepoint-root
>                                  /file-1-uid
>                                  /file-2-uid
>                                  /file-3-uid
> {code}
> This is the umbrella issue for the individual steps needed to address this.

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